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diff --git a/.gitattributes b/.gitattributes new file mode 100644 index 0000000..6833f05 --- /dev/null +++ b/.gitattributes @@ -0,0 +1,3 @@ +* text=auto +*.txt text +*.md text diff --git a/36036-8.txt b/36036-8.txt new file mode 100644 index 0000000..1bb7bf9 --- /dev/null +++ b/36036-8.txt @@ -0,0 +1,4264 @@ +The Project Gutenberg EBook of Metabolic Adaptation to Climate and +Distribution of the Raccoon Procyon Lotor and Other Procyonidae, by John N. Mugaas and John Seidensticker and Kathleen P. Mahlke-Johnson + +This eBook is for the use of anyone anywhere at no cost and with +almost no restrictions whatsoever. You may copy it, give it away or +re-use it under the terms of the Project Gutenberg License included +with this eBook or online at www.gutenberg.org + + +Title: Metabolic Adaptation to Climate and Distribution of the Raccoon Procyon Lotor and Other Procyonidae + +Author: John N. Mugaas + John Seidensticker + Kathleen P. Mahlke-Johnson + +Release Date: May 5, 2011 [EBook #36036] + +Language: English + +Character set encoding: ISO-8859-1 + +*** START OF THIS PROJECT GUTENBERG EBOOK THE RACCOON PROCYON LOTOR *** + + + + +Produced by Colin Bell, Tom Cosmas, Joseph Cooper and the +Online Distributed Proofreading Team at http://www.pgdp.net + + + + + + + + + + SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY · NUMBER 542 + + + Metabolic Adaptation to Climate + and Distribution of the Raccoon + _Procyon lotor_ and Other Procyonidae + + + _John N. Mugaas, John Seidensticker, + and Kathleen P. Mahlke-Johnson_ + + + [Smithsonian Institution logo] + + + SMITHSONIAN INSTITUTION PRESS + Washington, D.C. + 1993 + + + + +ABSTRACT + + +Mugaas, J. N., J. Seidensticker, and K. Mahlke-Johnson. Metabolic +Adaptation to Climate and Distribution of the Raccoon _Procyon lotor_ +and Other Procyonidae. _Smithsonian Contributions to Zoology_, number +542, 34 pages, 8 figures, 12 tables, 1993.--Although the family +Procyonidae is largely a Neotropical group, the North American +raccoon, _Procyon lotor_, is more versatile in its use of climate, and +it is found in nearly every habitat from Panama to 60°N in Canada. We +hypothesized that most contemporary procyonids have remained in tropic +and subtropic climates because they have retained the metabolic +characteristics of their warm-adapted ancestors, whereas _Procyon +lotor_ evolved a different set of adaptations that have enabled it to +generalize its use of habitats and climates. To test this hypothesis +we compared _Procyon lotor_ with several other procyonids +(_Bassariscus astutus_, _Nasua nasua_, _Nasua narica_, _Procyon +cancrivorus_, and _Potos flavus_) with respect to (1) basal metabolic +rate ([.H]_{b}), (2) minimum wet thermal conductance (C_{mw}), (3) +diversity of diet (D_{d}), (4) intrinsic rate of natural increase +(r_{max}), and, where possible, (5) capacity for evaporative cooling +(E_{c}). We measured basal and thermoregulatory metabolism, +evaporative water loss, and body temperature of both sexes of _Procyon +lotor_ from north central Virginia, in summer and winter. Metabolic +data for other procyonids were from literature, as were dietary and +reproductive data for all species. + +Procyon lotor differed from other procyonids in all five +variables. (1) _Procyon lotor_'s mass specific [.H]_{b} (0.46 +mL O_{2}·g^{-1}·h^{-1}) was 1.45 to 1.86 times greater than values for +other procyonids. (2) Because of its annual molt, _Procyon lotor_'s +C_{mw} was about 49% higher in summer than winter, 0.0256 and 0.0172 +mL O_{2}·g^{-1}·h^{-1}·°C^{-1}, respectively. The ratio of measured to +predicted C_{mw} for _Procyon lotor_ in winter (1.15) was similar to +values calculated for _Potos flavus_ (1.02) and _Procyon cancrivorus_ +(1.25). Values for other procyonids were higher than this, but less +than the value for _Procyon lotor_ (1.76) in summer. On a mass +specific basis, _Bassariscus astutus_ had the lowest C_{mw} with a +ratio of 0.85. (3) _Procyon lotor_ utilized three times as many food +categories as _Procyon cancrivorus_, _Nasua nasua_, and _Bassariscus +astutus_; about two times as many as _Nasua narica_; and nine times as +many as _Potos flavus_. (4) Intrinsic rate of natural increase +correlated positively with [.H]_{b}. _Procyon lotor_ had the highest +r_{max} (2.52 of expected) and _Potos flavus_ the lowest (0.48 of +expected). The other procyonids examined also had low [.H]_{b}, but +their r_{max}'s were higher than predicted (1.11-1.32 of expected). +Early age of first female reproduction, fairly large litter size, long +life span, high-quality diet, and, in one case, female social +organization all compensated for low [.H]_{b} and elevated r_{max}. +(5) Although data on the capacity for evaporative cooling were +incomplete, this variable appeared to be best developed in _Procyon +lotor_ and _Bassariscus astutus_, the two species that have been most +successful at including temperate climates in their distributions. + +These five variables are functionally interrelated, and have +co-evolved in each species to form a unique adaptive unit that +regulates body temperature and energy balance throughout each annual +cycle. The first four variables were converted into normalized +dimensionless numbers, which were used to derive a composite score +that represented each species' adaptive unit. _Procyon lotor_ had the +highest composite score (1.47) and _Potos flavus_ the lowest (0.39). +Scores for the other procyonids were intermediate to these extremes +(0.64-0.79). There was a positive correlation between the number of +climates a species occupies and the magnitude of its composite score. +Linear regression of this relationship indicated that 89% of the +variance in climatic distribution was attributed to the composite +scores. Differences in metabolic adaptation, therefore, have played a +role in delimiting climatic distribution of these species. + +It was clear that _Procyon lotor_ differed from the other procyonids +with respect to thermoregulatory ability, diet, and reproductive +potential. These differences have enabled it to become a highly +successful climate generalist, and its evolution of an [.H]_{b} that +is higher than the procyonid norm appears to be the cornerstone of its +success. + + +OFFICIAL PUBLICATION DATE is handstamped in a limited number of +initial copies and is recorded in the Institution's annual report, +_Smithsonian Year_. SERIES COVER DESIGN: The coral _Montastrea +cavernosa_ (Linnaeus). + + + Library of Congress Cataloging-in-Publication Data + + Mugaas, John N. + + Metabolic adaptation to climate and distribution of the raccoon + Procyon lotor and other Procyonidae / John N. Mugaas, John + Seidensticker, and Kathleen P. Mahlke-Johnson. + + p. cm.--(Smithsonian contributions to zoology; no. 542) + + Includes bibliographical references (p. ) + + 1. Raccoons-Metabolism-Climatic factors. + 2. Procyonidae-Metabolism-Climatic factors. + 3. Raccoons-Geographical distribution. + 4. Procyonidae-Geographical distribution. + I. Seidensticker, John. + II. Mahlke-Johnson, Kathleen. + III. Title. + IV. Series. + + QL1.S54 no. 542 [QL737.C26] 591 s-dc20 [599.74´443´04542] 93-3119 + + +[permanent paper symbol] The paper used in this publication meets the +minimum requirements of the American National Standard for Permanence +of Paper for Printed Library Materials z39.48--1984. + + + + +Contents + + + _Page_ + Introduction 1 + Defining the Problem 1 + Procyonid Origins 1 + Typical Procyonids 2 + The Atypical Procyonid 3 + The Hypothesis 4 + Hypothesis Testing 4 + Adaptive Significance of the Variables 4 + Basal Metabolic Rate and Intrinsic Rate of Natural Increase 4 + Minimum Thermal Conductance 4 + Capacity for Evaporative Cooling 5 + Diet 5 + Experimental Design and Summary 5 + Acknowledgments 5 + + Materials and Methods 6 + Live-trapping 6 + Metabolic Studies 6 + Basal and Thermoregulatory Metabolism 6 + Evaporative Water Loss 7 + Body Temperature 7 + Calibrations 7 + Calorimeter 7 + Body Temperature Transmitters 8 + Statistical Methods 8 + Estimating Intrinsic Rate of Natural Increase 8 + Comparison of Adaptive Units 8 + + Results 8 + Body Mass 8 + Basal Metabolic Rate 9 + Minimum Thermal Conductance 9 + Evaporative Water Loss 11 + Thermoregulation at Low Temperatures 12 + Body Temperature 12 + Summer 14 + Winter 14 + Thermoregulation at High Temperatures 16 + Body Temperature 16 + Summer 16 + Winter 16 + Daily Cycle of Body Temperature 16 + + Discussion 16 + Basal Metabolic Rate 16 + Background 16 + Captive versus Wild Raccoons 17 + Seasonal Metabolism of Raccoons 17 + Comparison of _Procyon lotor_ with Other Procyonids 17 + Influence of Diet on Basal Metabolism 18 + Background 18 + Food Habits of Procyonids 18 + Food Habits and Basal Metabolism 19 + Summary 19 + Basal Metabolism and Intrinsic Rate of Natural Increase 19 + Background 19 + _Procyon lotor_ 19 + _Bassariscus astutus_ 19 + _Nasua narica_ 19 + _Nasua nasua_ 20 + _Procyon cancrivorus_ 20 + _Potos flavus_ 20 + Summary 20 + Basal Metabolism and Climatic Distribution 21 + _Procyon lotor_ 21 + Other Procyonids 21 + Minimum Thermal Conductance 21 + Background 21 + Effect of Molt on Thermal Conductance 21 + Comparison of Thermal Conductances 22 + _Procyon lotor_ versus Tropical Procyonids 22 + _Bassariscus astutus_ 22 + Thermoregulation and Use of Stored Fat at Low Temperatures 22 + Background 22 + Thermoregulation 22 + Stored Fat 23 + Thermal Model of the Raccoon and Its Den 23 + Metabolic Advantage of the Den 23 + Thermoregulation at High Temperatures 24 + Background 24 + Comparison of Procyonid Responses to Heat Stress 24 + _Potos flavus_ 24 + _Nasua nasua and Nasua narica_ 24 + _Bassariscus astutus_ 24 + _Procyon lotor_ 24 + _Procyon cancrivorus_ 24 + Summary 24 + Composite Scores of Adaptive Units and Geographic Distribution 25 + Evolution of Metabolic Adaptations 26 + Evolution of Low Basal Metabolic Rate 26 + Evolution of High Basal Metabolic Rate 27 + Summary 28 + + Appendix: List of Symbols 29 + + Literature Cited 30 + + + + + Metabolic Adaptation to Climate + and Distribution of the Raccoon + _Procyon lotor_ and Other Procyonidae + + + _John N. Mugaas, John Seidensticker, + and Kathleen P. Mahlke-Johnson_ + + + _John N. Mugaas, Department of Physiology, Division of Functional + Biology, West Virginia School of Osteopathic Medicine, Lewisburg, + West Virginia 24901. John Seidensticker and Kathleen P. + Mahlke-Johnson, National Zoological Park, Smithsonian Institution, + Washington, D.C. 20008._ + + + + +$Introduction$ + + + +DEFINING THE PROBLEM + + +_Procyonid Origins_ + +The major carnivore radiations took place about 40 million years +before present (MYBP) in the late Eocene and early Oligocene (Ewer, +1973:363; Wayne et al., 1989). Between 30 and 40 MYBP, a progenitor +split into the ursid and procyonid lineages, which evolved into +present-day bears, pandas, and raccoons (Wayne et al., 1989). The +taxonomic relatedness of pandas to bears and raccoons has been tested +extensively and a number of authors have summarized current thinking +on the problem (Martin, 1989; Wayne et al., 1989; Wozencraft, 1989a, +1989b; Decker and Wozencraft, 1991). Davis (1964:322-327) and others +(Leone and Wiens, 1956; Todd and Pressman, 1968; Sarich, 1976; O'Brien +et al., 1985) place the giant panda, _Ailuropoda melanoleuca_, with +the ursids. The taxonomic status of the red panda, _Ailurus fulgens_, +appears to be less certain. Some current investigations align the red +panda with bears (Segall, 1943; Todd and Pressman, 1968; Hunt, 1974; +Ginsburg, 1982; Wozencraft, 1984:56-110; 1989a), whereas others place +them intermediate to procyonids and bears (Wurster and Benirschke, +1968; Sarich, 1976; O'Brien et al., 1985), or in close relationship to +the giant panda (Tagle et al., 1986). + +The procyonid radiation took place in North America and produced forms +that were mostly arboreal and omnivorous (Eisenberg, 1981:122; Martin, +1989). The center of this diversification occurred in Middle America +(Baskin, 1982; Webb, 1985b) during the Miocene (Darlington, 1963:367; +Webb, 1985b). Fossil procyonids from the late Miocene are represented +in Florida, California, Texas, Nebraska, Kansas, and South Dakota +(Baskin, 1982; Martin, 1989) and include such genera as _Bassariscus_, +_Arctonasua_, _Cyonasua_, _Paranasua_, _Nasua_, and _Procyon_ (Baskin, +1982; Webb, 1985b). During the Miocene procyonids underwent a modest +radiation within tropical and subtropical climates of North America's +central and middle latitudes. _Cyonasua_, which has close affinities +to _Arctonasua_ (Baskin, 1982), appears in tropical South America in +the late Miocene and immigrated there either by rafting across the +Bolivar Trough or by island-hopping through the Antilles archipelagoes +(Marshall et al., 1982; Marshall, 1988). Thus, procyonids were found +on both continents prior to formation of the Panamanian land bridge +(Darlington, 1963:367, 395; Marshall et al., 1982; Marshall, 1988). +Origins of _Bassaricyon_ and _Potos_ are obscure but probably occurred +in tropical rainforests of Middle America (Baskin, 1982; Webb, 1985b). +A subsequent Pleistocene dispersal carried several modern genera +(Table 1) across the Panamanian land bridge into South America (Webb, +1985b). _Bassariscus_ and _Bassaricyon_ represent the most primitive +genera in Procyoninae and Potosinae subfamilies, respectively (Table 1; +Wozencraft, 1989a; Decker and Wozencraft, 1991). + +In the early Tertiary, mid-latitudes of North America were much warmer +than they are now, but not fully tropical, and temperate deciduous +forests, associated with strongly seasonal climates, occurred only in +the far north (Barghoorn, 1953; Colbert, 1953; Darlington, 1963:589, +590). Major climatic deteriorations, with their attendant cooling of +northern continents, occurred during the Eo-Oligocene transition, in +the middle Miocene, at the end of the Miocene, and at about 3 MYBP +(late Pliocene). This last deterioration corresponds with closure of +the Panamanian isthmus (Berggren, 1982; Webb, 1985a). Climatic +deterioration went on at an accelerating rate during the late +Tertiary, with glacial conditions developing at the poles by the +mid-Pliocene (Barghoorn, 1953). Therefore, throughout the Tertiary, as +continents cooled, northern climate zones moved toward the tropics +(Barghoorn, 1953; Colbert, 1953; Darlington, 1963:589, 590, 594, 595; +Webb, 1985a). + + + TABLE 1.--Classification of recent Procyonidae after Wozencraft + (1989a) and Decker and Wozencraft (1991). Information in + parenthesis indicates general geographic distribution (modified from + Kortlucke and Ramirez-Pulido (1982) and Poglayen-Neuwall (1975)): + S.A. = South America; C.A. = Central America; M. = Mexico; + U.S. = United States; C. = Canada. Lower case letters preceding + geographic areas signify north (n), south (s), and west (w). + + Order CARNIVORA Bowdich, 1821 + Suborder CANIFORMIA Kretzoi, 1945 + Family PROCYONIDAE Gray, 1825 + Subfamily POTOSINAE Trouessart, 1904 + Genus _Potos_ E. Geoffroy and G. Cuvier, 1795 + _P. flavus_ (S.A., C.A., M.) + Genus _Bassaricyon_ Allen, 1876 + _B. alleni_[a] (S.A.) + _B. beddardi_[a] (S.A.) + _B. gabbii_[a] (nS.A., C.A.) + _B. lasius_[a] (C.A.) + _B. pauli_[a] (C.A.) + Subfamily PROCYONINAE Gray, 1825 + Genus _Bassariscus_ Coues, 1887 + _B. astutus_ (M., wU.S.) + _B. sumichrasti_ (C.A., M.) + Genus _Nasua_ Storr, 1780 + _N. narica_[b] (nS.A., C.A., M., swU.S.) + _N. nasua_[b] (S.A., sC.A.) + Genus _Nasuella_ Hollister, 1915 + _N. olivacea_ (S.A.) + Genus _Procyon_ Storr, 1780 + _P. cancrivorus_ (S.A., sC.A.) + _P. gloveralleni_[c] (Barbados) + _P. insularis_[c] (Maria Madre Is., Maria Magdalene Is.) + _P. lotor_[c] (C.A., M., U.S., sC.) + _P. maynardi_[c] (Bahamas, New Providence Is.) + _P. minor_[c] (Guadeloupe Is.) + _P. pygmaeus_[c] (M., Quintana Roo, Cozumel Is.) + + [a] The several named forms of _Bassaricyon_ are a single + species, _Bassaricyon gabbii_ (Wozencraft, 1989a). + + [b] These are considered conspecific in some current + taxonomies (Kortlucke and Ramirez-Pulido, 1982); however, + the scheme followed here maintains them as separate species + (Decker, 1991). + + [c] Several named forms of _Procyon_ are a single species, + _Procyon lotor_ (Wozencraft, 1989a). + + +During the late Miocene, late Pliocene, and Pleistocene, the Bering +land bridge between North America and Asia formed periodically, +offering an avenue for dispersal between northern continents +(Darlington, 1963:366; Webb, 1985a). However, by the late Tertiary, +northern continents had cooled to the extent that climate, with its +attendant sharply defined vegetative zones, became the major factor +limiting dispersal by this route (Darlington, 1963:366; Webb, 1985a). +Those Holarctic mammals that did cross the Bering land bridge in the +late Tertiary were "cold-adapted" species associated with relatively +cool, but not alpine, climates (Darlington, 1963:366; Ewer, 1973:369). +Among carnivores this included some canids, ursids, mustelids, and +felids (Darlington, 1963:393-395, 397; Webb, 1985a). Procyonids, +however, did not cross the Bering land bridge into Asia, and Ewer +(1973:369) ascribes this to their being an "essentially tropical +group." Miocene radiation of procyonids occurred at a time when two of +the four major climatic deteriorations (middle and late Miocene) were +taking place (Webb, 1985a, 1985b). These deteriorations had the effect +of cooling the middle latitudes to the extent that temperate forest +forms began to appear in mid-latitude floras, along with a rapid +influx of herbaceous plants (Barghoorn, 1953). The procyonid radiation +did not penetrate beyond these climatically changing middle latitudes, +which implies that these animals were "warm-adapted," and were, +therefore, physiologically excluded from reaching the Bering land +bridge. Today, three of the six genera and over half of the 18 species +that comprise Procyonidae (Table 1; Wozencraft, 1989b) remain confined +to tropical regions of North and South America (Hall and Kelson, +1959:878-897; Poglayen-Neuwall, 1975; Kortlucke and Ramirez-Pulido, +1982; Nowak and Paradiso, 1983:977-985). + + +_Typical Procyonids_ + +McNab (1988a) contends that basal metabolism is a highly plastic +character in evolution, and he has amply shown that ecologically +uniform species are more apt to share common metabolic rates than +taxonomically allied species from drastically different environments +(McNab, 1984a, 1986a, 1986b, 1988a). Procyonids represent a +taxonomically allied group that shared a common ecological situation +for millions of years; consequently, members of this family might be +expected to show some uniformity in their [.H]_{b}. Basal and +thermoregulatory metabolism of several procyonids have been measured: +kinkajou, _Potos flavus_ (Müller and Kulzer, 1977; McNab, 1978a; +Müller and Rost, 1983), coatis, _Nasua nasua_ (Chevillard-Hugot et +al., 1980; Mugaas et al., in prep.), and _Nasua narica_ (Scholander et +al., 1950c; Mugaas et al., in prep.), ringtail, _Bassariscus astutus_ +(Chevalier, 1985), and crab-eating raccoon, _Procyon cancrivorus_ +(Scholander et al., 1950c). In general, these species have [.H]_{b}'s +that are 40%-80% of the values predicted for them by the Kleiber +(1961:206) equation. Lower than predicted [.H]_{b} is viewed as an +energy-saving adaptation for procyonids living in relatively stable +tropical climates (Müller and Kulzer, 1977; Chevillard-Hugot et al., +1980; Müller and Rost, 1983). This implies that lower than predicted +[.H]_{b} is a general procyonid condition and that it represents a +characteristic that evolved in response to the family's long +association with tropical and subtropical forest environments. + + +_The Atypical Procyonid_ + +Although most procyonids are found in only tropical to subtropical +climates, the North American raccoon, _Procyon lotor_, (Figure 1) has +a much broader distribution that extends from tropical Panama (8°N) to +southern Canada. In Alberta, Canada, its range reaches the edge of the +Hudsonian Life Zone at 60°N (for distribution maps see Hall and +Kelson, 1959:878-897, and Poglayen-Neuwall, 1975). Range extensions +and an increase in numbers have been noted in Canada and in parts of +the United States since the 19th century (Lotze and Anderson, 1979; +Kaufmann, 1982; Nowak and Paradiso, 1983:977-985). Thus, _Procyon +lotor_ is more complex ecologically than other procyonids, +particularly when one takes into account its highly generalized food +habits (Hamilton, 1936; Stuewer, 1943; Stains, 1956:39-51; Greenwood, +1981) and the wide range of habitat types (forest, prairie, desert, +mountain, coastal marsh, freshwater marsh) and climates (tropical to +north temperate) in which it is successful (Whitney and Underwood, +1952:1; Hall and Kelson, 1959:885; Lotze and Anderson, 1979; +Kaufmann, 1982). On this basis it is clear that _Procyon lotor_ has +deviated from the typical procyonid portrait and has become the +consummate generalist of the Procyonidae. + + [Illustration: FIGURE 1.--North American raccoon, _Procyon lotor_.] + + +_The Hypothesis_ + +Our general hypothesis was that whereas most contemporary procyonids +have retained the metabolic characteristics of their warm-adapted +ancestors, _Procyon lotor_ possesses a different set of adaptations, +which either evolved as characteristics unique to this species or were +acquired from its ancestral stock. In either case, its unique +adaptations have given _Procyon lotor_ the physiological flexibility +to generalize its use of habitats and climates and expand its +geographic distribution to a much greater extent than other +procyonids. + + +_Hypothesis Testing_ + +We tested our hypothesis by comparing _Procyon lotor_ with several +other procyonids (_Bassariscus astutus_, _Nasua nasua_, _Nasua +narica_, _Procyon cancrivorus_, and _Potos flavus_) on the basis of +their (1) basal metabolic rate ([.H]_{b}), (2) minimum wet thermal +conductance (C_{mw}), (3) diversity of diet (D_{d}), (4) intrinsic +rate of natural increase (r_{max}), and, when data were available, (5) +capacity for evaporative cooling (E_{c}). In a genetic sense each one +of these variables is a complex adaptive characteristic, expression of +which is determined by the interaction of several genes (Prosser, +1986:110-165). Experience has shown that a given species will express +each one of these variables in a specific manner that is relevant to +its mass, physiology, behavior, and environmental circumstance. Thus, +different expressions of these variables may represent specific +climatic adaptations (Prosser, 1986:16) that have been selected-for by +evolutionary process. Because these variables are interrelated with +respect to regulation of body temperature and energy balance, they +have co-evolved in each species to form an adaptive unit. For each +species, measured and calculated values for the first four variables +were converted into dimensionless numbers and used to derive a +composite score that represented its adaptive unit. Climatic +distributions of these species were then compared relative to their +composite scores. + + + +ADAPTIVE SIGNIFICANCE OF THE VARIABLES + + +_Basal Metabolic Rate and Intrinsic Rate of Natural Increase_ + +Basal metabolic rate represents the minimum energy required by an +animal to maintain basic homeostasis (Lusk, 1917:141; Kleiber, 1932, +1961:251; Benedict, 1938; Brody, 1945:59; Robbins, 1983:105-111). For +mammals, [.H]_b appears to be determined by complex interactions +between their body size (Kleiber, 1932, 1961:206; Benedict, 1938; +Brody, 1945:368-374; Hemmingsen, 1960:15-36; McNab, 1983b; Calder, +1987), the climate in which they live (Scholander et al., 1950c; McNab +and Morrison, 1963; Hulbert and Dawson, 1974; Shkolnik and +Schmidt-Nielsen, 1976; McNab, 1979a; Vogel, 1980), their food habits +(McNab, 1978a, 1978b, 1980a, 1983a, 1984a, 1986a, 1986b, 1988a, 1989), +and their circadian period (Aschoff and Pohl, 1970; Prothero, 1984). +Some species have higher mass-specific [.H]_{b} than others, and this +variation appears to be tied to ecological circumstances rather than +taxonomic affinities (McNab, 1988a, 1989). Basal metabolic rate is +important ecologically because it serves as a measure of a species' +minimum "obligatory" energy requirement, and under many circumstances, +it represents the largest energy demand associated with a daily energy +budget (King, 1974:38-55; McNab, 1980a; Mugaas and King, 1981:37-40). +Recently it also has been implicated as a permissive factor with +respect to r_{max} of mammals (Hennemann, 1983; Lillegraven et al., +1987; Nicoll and Thompson, 1987; Thompson, 1987) via its direct effect +on their rates of development and fecundity (McNab, 1980a, 1983a, +1986b; Hennemann, 1983; Schmitz and Lavigne, 1984; Glazier, 1985a, +1985b). The implication of this latter point is that those species +with higher [.H]_{b}'s also have faster rates of development and +greater fecundity and hence enjoy the competitive advantage of a +higher r_{max}. Basal metabolism is, therefore, "a highly plastic +character in the course of evolution" (McNab, 1988a:25) that has a +profound influence on each species' life history. + + +_Minimum Thermal Conductance_ + +Whole-body resistance to passive heat transfer is equal to tissue +resistance plus coat resistance. Within limits, these resistances can +be altered; tissue resistance can be varied by changes in blood flow, +whereas coat resistance can be changed by piloerection, molt, and +behavior. When whole-body resistance is maximized (maximum tissue and +coat resistances), passive heat transfer is minimized. The inverse of +resistance is conductance; therefore, maximum whole-body resistance is +the inverse of minimum thermal conductance (C_{m}). Minimum thermal +conductance is readily derived from metabolic chamber data, and it is +commonly used to describe an animal's capacity to minimize passive +heat transfer. Minimum thermal conductance interacts with [.H]_{b} and +body mass to set the maximum temperature differential a mammal can +maintain without increasing its basal level of heat production. The +low temperature in this differential is the lower critical temperature +(T_{lc}). + +Mass-specific C_{m} for mammals is negatively correlated with body +mass (McNab and Morrison, 1963; Herreid and Kessel, 1967; McNab, 1970, +1979b; Bradley and Deavers, 1980; Aschoff, 1981), and for any given +mass its magnitude is 52% higher during the active, rather than the +inactive, phase of the daily cycle (Aschoff, 1981). However, some +mammals have C_{m}'s that are higher or lower than would be predicted +for them on the basis of body mass and circadian phase. Seasonal +variation in C_{m} (higher values during summer than winter) has been +reported for many northern mammals that experience large annual +variations in air temperature (Scholander et al., 1950a; Irving et +al., 1955; Hart, 1956, 1957; Irving, 1972:165). Some tropical mammals +with very thin fur coats, and others with nearly hairless bodies, have +high C_{m}'s (McNab, 1984a), as do burrowing mammals (McNab, 1966, +1979b, 1984a) and the kit fox, _Vulpes macrotis_ (Golightly and +Ohmart, 1983). Some small mammals with low basal metabolic rates tend +to have lower than predicted C_{m}'s: small marsupials (McNab, 1978a), +heteromyid rodents (McNab, 1979a), several ant eaters (McNab, 1984a), +the arctic hare, _Lepus arcticus_ (Wang et al., 1973), the ringtail, +_Bassariscus astutus_ (Chevalier, 1985), and the fennec, _Fennecus +zerda_ (Noll-Banholzer, 1979). Thus, in spite of its mass dependence, +C_{m} also has been modified during the course of evolution by +selective factors in the environment and by the animal's own metabolic +characteristics. + + +_Capacity for Evaporative Cooling_ + +Latent heat loss occurs as a result of evaporation from the +respiratory tract and through the skin, and except under conditions of +heat stress, it "is a liability in thermal and osmotic homeostasis" +(Calder and King, 1974:302). E_{c}, defined as the ratio of +evaporative heat lost to metabolic heat produced, can be used to +quantify thermoregulatory effectiveness of evaporative cooling and to +make comparisons of heat tolerance between species. Thermoregulatory +effectiveness of latent heat loss is not just a function of the rate +of evaporative water loss but also of the rate of metabolic heat +production (Lasiewski and Seymour, 1972). For example, a low metabolic +rate minimizes endogenous heat load and thus conserves water, whereas +the opposite is true of high metabolic rates (Lasiewski and Seymour, +1972). Some mammals that live in arid regions have evolved low +metabolic rates and thus capitalize on this relationship to reduce +their thermoregulatory water requirement (McNab and Morrison, 1963; +McNab, 1966; MacMillen and Lee, 1970; Noll-Banholzer, 1979). What is +evident, therefore, is that an animal's capacity for increasing latent +heat loss must evolve together with its [.H]_{b} and C_{m} in response +to specific environmental demands. + + +_Diet_ + +McNab (1986a, 1988a, 1989) demonstrated that, for mammals, departures +of [.H]_{b} from the Kleiber (1961:206) "norm" are highly correlated +with diet and independent of phylogenetic relationships. McNab's +analysis indicates that for mammals that feed on invertebrates, those +species with body mass less than 100 g have [.H]_{b}'s that are equal +to or greater than values predicted by the Kleiber equation, whereas +those with body mass greater than 100 g have metabolic rates that are +lower than predicted. Grazers, vertebrate eaters, nut eaters, and +terrestrial frugivores also have [.H]_{b}'s that are equal to or +greater than predicted, whereas insectivorous bats, arboreal +folivores, arboreal frugivores, and terrestrial folivores all have +rates that are lower than predicted. McNab (1986a) found animals with +mixed diets harder to categorize, but in general he predicted that +their [.H]_{b}'s would be related to (1) a food item that is +constantly available throughout the year, (2) a food item that is most +available during the worst conditions of the year, or (3) a mix of +foods available during the worst time of the year. Although these +correlations do not establish cause and effect between food habits and +[.H]_{b}, McNab's analysis does make it clear that the relationship +between these variables has very real consequences for an animal's +physiology, ecology, and evolution. + + + +EXPERIMENTAL DESIGN AND SUMMARY + + +In this investigation we measured basal and thermoregulatory +metabolism, evaporative water loss, and body temperature of raccoons +from north central Virginia. Measurements were conducted on both sexes +in summer and winter to determine how season and sex influenced these +variables. We then compared the data for this widely distributed +generalist with data from literature for its ecologically more +restricted relatives. Dietary data for all species were taken from +literature, as were reproductive data for calculation of r_{max}. + +Our analysis demonstrated clear differences between _Procyon lotor_ +and other procyonids with respect to [.H]_{b}, C_{mw}, D_{d}, and +r_{max}. The composite score calculated from these variables for +_Procyon lotor_ was much higher than those derived for other species, +and there was a positive correlation between the number of climates a +species occupies and the magnitude of its composite score. Data on +evaporative water loss, although not complete for all species, +suggested that tropical and subtropical procyonids have less capacity +for evaporative cooling than _Procyon lotor_ or _Bassariscus astutus_. +It was clear, therefore, that with respect to its thermal physiology, +_Procyon lotor_ differed markedly from other procyonids, and we +contend that these differences have allowed this species to become a +highly successful climate generalist and to expand its distribution +into many different habitats and climates. Our analysis also suggested +that the cornerstone of _Procyon lotor_'s success as a climate +generalist is its [.H]_{b}, which is higher than the procyonid norm. + + + +ACKNOWLEDGMENTS + + +The authors would like to thank John Eisenberg and Devra Kleiman for +their support and encouragement throughout the study. This +investigation was supported by research grants from the West Virginia +School of Osteopathic Medicine (WVSOM), and Friends of the National +Zoo (FONZ). Logistic support was provided by the National Zoological +Park's Conservation and Research Center (CRC), and the departments of +Mammalogy and Zoological Research. Our ability to conduct +physiological research at CRC was made possible by the thoughtful +support and encouragement provided by Chris Wemmer. His excellent +staff at CRC, especially Jack Williams, Junior Allison, and Red +McDaniel, were very helpful in providing hospitality and logistical +support to the senior author and his family during their various +visits to the Center. The assistance of several people at the National +Zoo also is gratefully acknowledged: Mitch Bush and Lyndsay Phillips +not only provided veterinary support throughout the investigation, +but also performed surgical procedures required to implant +temperature-sensitive radio transmitters in several raccoons; Olav +Oftedal made his laboratory available to us at various times and +loaned us equipment to use at CRC; Miles Roberts and his staff +provided care for our captive raccoons in the Department of Zoological +Research during various parts of the investigation. Greg Sanders and +Ken Halama, supported by FONZ assistantships, cared for our captive +raccoons at CRC, provided assistance in the laboratory whenever +needed, and were an invaluable source of aid. Their friendship and +help is gratefully acknowledged. Ellen Broudy and Andy Meyer, +supported by WVSOM and a student work study grant, respectively, +provided assistance in the laboratory. David Brown, John Eisenberg, +Mary Etta Hight, Brian McNab, Steve Thompson, and W. Chris Wozencraft +critically reviewed various phases of the manuscript and provided many +helpful suggestions. We deeply appreciate the work of Jean B. +McConville, whose beneficial editorial suggestions helped us improve +several early versions of the manuscript. We also gratefully +acknowledge Diane M. Tyler, our editor at the Smithsonian Institution +Press, whose expertise helped us mold the manuscript into its final +form. Jill Mellon and Sriyanie Miththalapa, supported by FONZ +traineeships, assisted in measuring the daily cycle of body +temperature in raccoons. The Virginia Commission of Game and Inland +Fisheries gave us permission to use wild-caught raccoons in this +project. + + + + +$Materials and Methods$ + + + +LIVE-TRAPPING + + +Raccoons were caught from May 1980 through December 1984 on a trapping +grid of 30 to 35 stations (one or two "live traps" per station) that +covered about one-third of the National Zoological Park's Conservation +and Research Center (CRC) near Front Royal, Virginia (Seidensticker et +al., 1988; Hallett et al., 1991). Animals were trapped during 10 +consecutive days each month, and in this five-year interval 407 +raccoons were captured and marked with tattoos and ear tags. All +captured animals were individualized with respect to age, reproductive +status, physical condition, parasite load, and mass and body +dimensions. These data characterized the structure and dynamics of the +raccoon population at CRC and provided information on the annual +cycle of fattening for raccoons in north central Virginia. + +Animals used for metabolic measurements were captured at CRC about +1.5 km south of the trapping grid and thus were genetically +representative of the area. Six males were captured and measured during +the summer of 1983. These animals were kept isolated for a week before +being measured and were released later that summer at the site of their +capture. The other seven animals used in our study were from the +collection of the National Zoological Park and all of them had their +origins at CRC. + + + +METABOLIC STUDIES + + +_Basal and Thermoregulatory Metabolism_ + +Metabolic measurements, conducted at CRC, were carried out on eight +males during July and August 1983, on four females and three males +from November 1983 through March 1984, and on four females during June +and July 1984. + +Raccoons were housed throughout the study such that they were +constantly exposed to a natural cycle of temperature and photoperiod. +Weather records for the Front Royal area indicate that average +temperatures are around -0.5°C in January and 23.3°C in July +(Crockett, 1972). Light:dark (L:D) periods for the latitude of CRC +(48°55'N; United States Department of the Interior Geological Survey, +1972), calculated from duration of daylight tables (List, 1971:506-512), +were 14.9:9.1 and 9.4:14.6 hours L:D for summer and winter solstices, +respectively, and 12.2:11.8 hours L:D for vernal and autumnal equinoxes. + +Our animals were fed a measured amount of food daily, and they usually +ate most of what was provided. Occasionally these animals would eat +very little or none of their ration, and on some days they would eat +all that was given to them. We fed them either feline diet (ground +horse meat) or canned mackerel (Star-kist(R)[1]) along with +high-protein dog chow (Purina(R)). When available, fresh fruit also +was added to their diet. Water was always provided ad libitum. + + [1] _The use of product brand names in this publication is + not intended as an endorsement of the products by the + Smithsonian Institution._ + +Measurements were conducted during the raccoons' daily inactive period +(sunrise to sunset) in both summer and winter. Oxygen consumption was +measured in a flow-through metabolism chamber at 5°C intervals from +-10°C to 35°C. Animals were held at each temperature until the lowest +rate of oxygen consumption had been obtained and maintained for at +least 15 minutes. During each determination, oxygen consumption was +monitored for 30 minutes to one hour beyond a suspected minimum value +to see if an even lower reading could be obtained. Raccoons attained +minimum levels of oxygen consumption more quickly at warm (>10°C) than +at cold temperatures. Depending on the temperature, therefore, each +measurement took from two to five hours to complete. On days when two +measurements could be completed, the second trial was always at a +temperature 10°C warmer than the first. + +The metabolism chamber was constructed from galvanized sheet metal +(77.5 × 45.5 × 51.0 cm = 180 liters) and was painted black inside. +Within the chamber, the animal was held in a cage (71 × 39 × 33 cm) +constructed from turkey wire that also was painted black. This cage +prevented the raccoons from coming into contact with the walls of the +chamber, yet it was large enough to allow them to stand and freely +move about. The bottom of the cage was 11 cm above the chamber floor, +which was covered to a depth of one cm with mineral oil to trap urine +and feces. + +During measurements, the metabolism chamber was placed in a +controlled-temperature cabinet (modified Montgomery Ward model 8969 +freezer). Air temperature (T_{a}) in the metabolism chamber was +regulated with a Yellow Springs Instrument model 74 temperature +controller. T_{a} was controlled to ħ1.0°C at temperatures below +freezing, and to ħ0.5°C at temperatures above freezing. The chamber +air and wall temperatures were recorded continuously (Linseis model +LS-64 recorder) during each experiment, and, except during temperature +changes, they were always within 0.5°C of each other. + +Columns of Drierite(R) and Ascarite(R) removed water vapor and carbon +dioxide, respectively, from air entering and leaving the chamber. Dry +carbon-dioxide-free room air was pumped into the chamber (Gilman model +13152 pressure/vacuum pump) at a rate of 3.0 L/min (Gilmont model +K3203-20 flow meter). Downstream from the chemical absorbents, an +aliquot (0.1 L/min) of dry carbon-dioxide-free air was drawn off the +chamber exhaust line and analyzed for oxygen content (Applied +Electrochemistry model S-3A oxygen analyzer, model 22M analysis cell, +and model R-1 flow control). All gas values were corrected to standard +temperature and pressure for dry gas. Oxygen consumption was +calculated from the difference in oxygen content between inlet and +outlet air using Eq. 8 of Depocas and Hart (1957). + +Each raccoon was fasted for at least 12 hours before oxygen +consumption measurements began. At the start and end of each metabolic +trial the animal was weighed to the nearest 10 g (Doctors Infant +Scale, Detecto Scales, Inc., Brooklyn, N.Y., U.S.A.). The body mass +used in calculating minimum oxygen consumption and evaporative water +loss was estimated from timed extrapolations of the difference between +starting and ending weights, and the time at which these variables +were measured. + + +_Evaporative Water Loss_ + +During metabolic measurements at temperatures above freezing, +evaporative water loss was determined gravimetrically. Upstream from +the chemical columns, an aliquot of air (0.1 L/min) was drawn off the +exhaust line and diverted for a timed interval through a series of +preweighed (0.1 mg) U-tubes containing Drierite(R). The aliquot then +passed through a second series of U-tubes containing Ascarite(R) +before entering the oxygen analysis system. Evaporative water loss was +calculated using Eq. 1 + + [.E] = (m_{w}·[.V]_{e})/([.V]_{a}·t·m) Eq. 1 + +where [.E] is evaporative water loss (mg·g^{-1}·h^{-1}), m_{w} is mass +of water collected (mg), [.V]_{e} is rate of air flow into the chamber +(3.0 L/min), [.V]_{a} is rate of air flow through the U-tubes +(0.1 L/min), t is length of the timed interval (h), and m is the +estimated mass of the raccoon at the time of sampling (g). + + +_Body Temperature_ + +Veterinarians at the National Zoological Park surgically implanted +calibrated temperature-sensitive radio transmitters (Telonics, Inc., +Mesa, AZ, U.S.A.) into abdominal cavities of two female and two male +raccoons. Transmitter pulse periods were monitored with a digital +processor (Telonics TDP-2) coupled to a receiver (Telonics +TR-2-164/166). During some metabolic measurements, body temperatures +of these animals were recorded to the nearest 0.1°C at 30-minute +intervals. The daily cycle of body temperature of these raccoons also +was measured once a month. + + + +CALIBRATIONS + + +_Calorimeter_ + +At the conclusion of these experiments, the accuracy of our +calorimetry apparatus was tested by burning an ethanol lamp in the +metabolism chamber. During these tests a CO_{2} analyzer was +incorporated into the system (Beckman, LB-2). Results demonstrated +that we measured 84% of the oxygen consumed by the lamp as well as 84% +of the water and CO_{2} it produced; standard deviation = ħ2.6, ħ5.0, +and ħ3.6, respectively (n = 27). Average respiratory quotient (RQ) +calculated from these data was O.657 ħ0.008 (n = 27), which is 99.5% +of that predicted (0.66). McNab (1988b) reports that the accuracy of +open-flow indirect calorimetry systems, such as ours, depends on the +rate of air flow through the animal chamber. If flow rates are too +low, there is inadequate mixing of air within the chamber, and the +rate of oxygen consumption, as calculated from the difference in +oxygen content of air flowing into and out of the chamber (Depocas and +Hart, 1957), is underestimated. At some critical rate of air flow, +which is unique to each combination of chamber and animal, this +situation changes such that measured rates of oxygen consumption +become independent of any further increase in flow rate (McNab, +1988b). In recent tests of our system, where we burned the ethanol +lamp at a variety of chamber flow rates, the efficiency of +measurement increased linearly as flow rate increased, and the +critical rate of air flow was about 6.7 L/min. This appeared to +explain why a flow rate of 3.0 L/min underestimated oxygen consumption +of the ethanol lamp. + +Our earlier tests of the efficiency of our system indicated that +although we underestimated actual oxygen consumption of the ethanol +lamp, we did so with a fair degree of precision; probably because flow +rates were closely controlled. During our metabolic measurements, +chamber flow rates also were closely controlled at 3.0 L/min, and we +believe, therefore, that these measurements also were carried out with +a high degree of precision. Consequently, all measured values of +oxygen consumption and water production were considered to be 84% of +their actual value and were adjusted to 100% before being included in +this report. + + +_Body Temperature Transmitters_ + +The calibration of all temperature-sensitive radio transmitters +drifted over time. Transmitters were calibrated before they were +surgically implanted and again after they were removed from the +animals. Although the drift of each transmitter was unique, it was +also linear (S. Tomkiewicz, Telonics, Inc., pers. com.). All body +temperature measurements were corrected from timed extrapolations of +the difference between starting and ending calibrations. + + + +STATISTICAL METHODS + + +Values of oxygen consumption, evaporative water loss, and body +temperature were plotted as a function of chamber air temperature. +Linear regressions of oxygen consumption at temperatures below the +thermoneutral zone (T_{n}), and evaporative water loss at temperatures +above freezing, were determined with the SAS (1982) GLM procedure. +Lower critical temperature (T_{lc}) was determined graphically from +intersection of the line representing [.H]_{b} and the regression line +representing oxygen consumption below T_{n}. Slopes and intercepts of +regression lines, as well as other mean values, were compared with +_t_-tests (Statistical Analysis System, 1982; Ott, 1984:138-175). +Unless indicated otherwise, data are expressed as mean ħ standard +deviation (s.d.). + + + +ESTIMATING INTRINSIC RATE OF NATURAL INCREASE + + +We employed the method first described by Cole (1954) to calculate +r_{max}: + + 1 = e^{-r_{max}} + b·e^{-r_{max}(a)} - b·e^{r_{max}(n+1)} Eq. 2 + +where a is potential age of females first producing young, b is +potential annual birth rate of female young, and n is potential age of +females producing their final young. After life-history data were +substituted into Eq. 2, r_{max} was determined by trial and error +substitution (Hennemann, 1983). + +Because r_{max} represents the genetically fixed, physiologically +determined maximum possible rate of increase, data on earliest +possible age of female reproduction, highest possible birth rate of +female young, and longest possible female reproductive life span were +used for a, b, and n, respectively. Calculated values, therefore, +represent physiologically possible, not ecologically possible, +intrinsic rates of increase (Hennemann, 1983, 1984; Hayssen, 1984; +McNab, 1984b). Values of n were derived from longevity records for +captive animals, and as these were all large values of similar +duration (14-16 years), they had very little effect on r_{max}. All +species considered have one litter per year, and because their sex +ratios at birth are about 50:50, variation in b was due to differences +in litter size. Therefore, age of first reproduction and litter size +had the greatest effect on r_{max}. Intrinsic rate of increase scales +to body mass (Fenchel, 1974), and we removed this effect by comparing +each calculated r_{max} with the value expected (r_{maxe}) on the +basis of body mass (Hennemann, 1983). + + + +COMPARISON OF ADAPTIVE UNITS + + +Dimensionless numbers for each of the four variables used in +calculating composite scores were derived as follows. Ratios of +measured to predicted values were used for basal metabolism (H_{br}) +and minimum wet thermal conductance (C_{mwr}). Thermoregulatory +ability at low temperatures is closely related to the ratio +H_{br}/C_{mwr} (McNab, 1966). This ratio was used, therefore, to gauge +each species' cold tolerance. For D_{d} we used the ratio of food +categories actually used by a species to the total number of food +categories taken by all species tested (D_{dr}). The ratio of +calculated to expected intrinsic rates of natural increase was used to +derive r_{maxr}. Composite scores were calculated as + + Composite score = [(H_{br}/C_{mwr}) + D_{dr} + r_{maxr}]/3 Eq. 3 + +The correlation between number of climates these species occupy and +their composite scores was tested by linear regression. + + + + +$Results$ + + + +BODY MASS + + +According to monthly live-trapping records, the body mass of +free-ranging female raccoons increased from 3.6 ħ0.6 kg during summer +to 5.6 ħ0.8 kg in early winter, and the mass of free-ranging males +increased from 4.0 ħ0.5 to 6.7 ħ0.9 kg during the same interval. These +seasonal changes in body mass were due to fluctuations in the amount +of body fat and represent a mechanism for storing energy during fall +for use in winter. In summer, captive and trapped male and captive +female raccoons had the same body mass (4.73 ħ0.61, 4.41 ħ0.70, and +4.67 ħ0.88 kg, respectively, Table 2). Mass of captive females did +not change between seasons, whereas captive males were heavier in +winter than summer (p<0.005; Table 2). This seasonal change in mass of +our captive males was of a much smaller magnitude (0.6 kg) than that +observed for wild males (2.7 kg). During winter, captive males (5.34 +ħ1.39 kg) were heavier than captive females (4.49 ħ0.98 kg; p<0.005; +Table 2). Thus, our captive animals maintained a body mass throughout +the year that was intermediate to the range of values found for wild +raccoons in the same area. + + + TABLE 2.--Body mass in kg and basal metabolism + (mL O_{2}·kg^{-0.75}·h^{-1}) of _Procyon lotor_ + in summer and winter (s.d. = standard deviation + and n = number of observations). + + ----------------+----------------------------------------------------- + Season and sex | Body mass, ħs.d., (n) Basal metabolism, ħs.d., (n) + ----------------+----------------------------------------------------- + Summer | + Trapped male | 4.41 ħ0.70 (52) 780 ħ112 (20) + Captive male | 4.73 ħ0.61 (22) 680 ħ102 (8) + Captive female| 4.67 ħ0.88 (41) 618 ħ 92 (13) + Winter | + Captive male | 5.34 ħ1.39 (31) 704 ħ 81 (19) + Captive female| 4.49 ħ0.98 (42) 667 ħ139 (25) + ----------------+----------------------------------------------------- + + + +BASAL METABOLIC RATE + + +Within thermoneutrality, [.H]_{b} (mL O_{2}·g^{-1}·h^{-1}) was +0.54 ħ0.09 for trapped males in summer, 0.46 ħ0.07 for captive males +in summer, 0.42 ħ0.07 for captive females in summer, 0.47 ħ0.06 for +captive males in winter, and 0.46 ħ0.10 for captive females in winter +(Figures 2, 3). Ratios of these measured values to those predicted by +the Kleiber (1932, 1961:206) equation are 1.28, 1.12, 1.02, 1.17, and +1.09, respectively. To minimize the effect of body size (Mellen, 1963) +and to facilitate comparisons between sexes and seasons and between +captive and trapped animals, basal metabolism also was calculated as a +function of metabolic body size (mL O_{2}·kg^{-0.75}·h^{-1}; Table 2). +Based on this analysis, trapped summer males had a higher basal +metabolism than captive males (p<0.025) or females (p<0.005) in either +season (Table 2). There was no difference in basal metabolism between +captive males and females in either summer or winter, and there was no +seasonal difference in their basal metabolic rates (Table 2). + + + +MINIMUM THERMAL CONDUCTANCE + + +Minimum wet and dry thermal conductances were calculated using Eqs. 4 +and 5 + + C_{mw} = [.H]_{r} / (T_{b} - T_{a}) Eq. 4 + + C_{md} = ([.H]_{r} - [.E]_{eq}) / (T_{b} - T_{a}) Eq. 5 + +where C_{mw} is wet and C_{md} is dry conductance +(mL O_{2}·g^{-1}·h^{-1}·°C^{-1}); [.H]_{r} is the lowest resting +metabolic rate measured at each temperature (mL O_{2}·g^{-1}·h^{-1}); +[.E]_{eq} is oxygen equivalent for heat lost by evaporation +[[.E]_{eq} = mL O_{2}·g^{-1}·h^{-1} = [.E]·[lambda]/[gamma], where +[.E] is evaporative water loss (mg·g^{-1}·h^{-1}), [lambda] is heat of +vaporization for water (2.43 J/mg), and [gamma] is heat equivalent for +oxygen (20.097 J/mL)]; T_{b} is body temperature (°C); and T_{a} is +chamber air temperature (°C). Only data from animals equipped with +temperature-sensitive radio transmitters were used for these +calculations. + + + TABLE 3.--Minimum wet and dry thermal conductances + (mL O_{2}·g^{-1}·h^{-1}·°C^{-1}) of _Procyon lotor_ in summer + and winter. Means of values were calculated from equations 3 + and 4 (s.d. = standard deviation and n = number of observations). + + ----------------------+---------------------------------------- + | Thermal conductance + Season and sex |---------------------------------------- + | Wet ħs.d. (n) Dry ħs.d. (n) + ----------------------+---------------------------------------- + Summer | + Captive, both sexes | 0.0256 ħ0.0028 (18) 0.0246 ħ0.0019 (12) + Winter | + Captive, female | 0.0172 ħ0.0023 (10) 0.0161 ħ0.0027 (6) + ----------------------+---------------------------------------- + + + [Illustration: FIGURE 2.--Relationship between oxygen consumption + and chamber air temperature for raccoons in summer: captive + females, open circles; captive males, closed circles; trapped + males, open squares. Sloping lines represent regressions of oxygen + consumption on chamber air temperature, and horizontal lines, basal + metabolism.] + + [Illustration: FIGURE 3.--Relationship between oxygen consumption + and chamber air temperature for raccoons in winter: captive + females, open circles; captive males, closed circles. Solid sloping + line represents regression of oxygen consumption on chamber air + temperature for males and females, and the horizontal line, basal + metabolism for males and females.] + +C_{mw} was calculated for each season from metabolic measurements made +at all air temperatures below T_{lc} (Table 3). Because evaporative +water loss was not measured at temperatures below freezing, C_{md} +was calculated only from metabolic determinations made at air +temperatures between T_{lc} and 0°C. There was no difference +between males and females in summer for either C_{mw} or C_{md} +(mL O_{2}·g^{-1}·h^{-1}·°C^{-1}). Data for each sex were combined +to give a summer average of 0.0256 ħ0.0028 for C_{mw}, and 0.0246 +ħ0.0019 for C_{md} (Table 3). These summer conductances were 49% higher +(p<0.005) than those calculated for winter females (0.0172 ħ0.0023, and +0.0161 ħ0.0027 for C_{mw} and C_{md}, respectively; Table 3). C_{mw} and +C_{md} were not different from each other in either summer or winter, +which indicated that in both seasons evaporative water loss contributed +very little to heat dissipation at temperatures below T_{n}. Comparisons +of thermal conductances calculated on the basis of metabolic body size +(Mellen, 1963) gave the same results. + + + +EVAPORATIVE WATER LOSS + + +Evaporative water loss increased as chamber temperature increased in +both summer and winter (Figures 4, 5). In summer, the pattern of +increase was different for females and males. Polynomial regressions +for trapped and captive males produced equations that describe a +concave relationship between T_{a} and evaporative water loss, whereas +the equation for females describes a sigmoid curve (Table 4; Figure 4). +For females, water loss increased rapidly at temperatures above 25°C +(Figure 4). The intercepts and coefficients of the X, X², and X³ terms +of the polynomial regression equations (Table 4) were compared +(_t_-tests) to determine if they differed from each other. The +coefficients in the equation for trapped males differed from those for +captive females in the X² (p<0.05) and X³ (p<0.025) terms. The +intercept and coefficients of the equation for captive males, however, +were not different from those for either captive females or trapped +males. Although this lack of difference is understandable in the case +of trapped males, where the shape of the two curves is similar +(concave), it is not so clear for the sigmoid curve of captive females +(Figure 4). Perhaps the lack of difference in this case is simply due +to the small number of observations available for captive males (n = 10; +Table 4). Nonetheless, in summer at 35°C, both captive and trapped +males relied less on evaporative cooling than did captive females +(Figure 4). + +In winter, males and females had similar rates of evaporative water +loss across the full range of temperatures tested (Figure 5). +Therefore, data for both sexes were combined. The intercept and +coefficients of this equation (Table 4) did not differ from those for +summer females, but they did differ from those in the regression for +trapped males in the X² (p<0.05) and X³ (p<0.025) terms. As was +the case for females in summer, rates of water loss for winter animals +increased most rapidly at temperatures above 25°C (Figure 5). + + [Illustration: FIGURE 4.--Relationship between evaporative water + loss and chamber air temperature for raccoons in summer: captive + females, open circles; captive males, closed circles; trapped + males, open squares. Lines represent polynomial regressions of + evaporative water loss on chamber air temperature.] + + [Illustration: FIGURE 5.--Relationship between evaporative water + loss and chamber air temperature for raccoons in winter: captive + females, open circles; captive males, closed circles. Lines + represent polynomial regressions of evaporative water loss on + chamber air temperature.] + + + TABLE 4.--Polynomial regression equations describing evaporative + water loss (mg·g^{-1}·h^{-1}) of _Procyon lotor_ in summer and + winter (X = chamber temperature (°C), Y = evaporative water loss, + n = number of observations, R² = coefficient of determination, and + SEE = standard error of estimate). + + --------------+-------------------------------------------------------- + Season and sex| Equation (n) R² + --------------+-------------------------------------------------------- + Summer | + Trapped male |Y = 0.1899 + 0.0114·X + 0.0011·X² - 0.00002·X³ (32) 0.86 + SEE | 0.0885 0.0223 0.0015 0.00003 + Captive male |Y = 0.2174 + 0.0192·X + 0.0009·X² - 0.00003·X³ (10) 0.73 + SEE | 0.3983 0.0834 0.0048 0.00008 + Captive | + female |Y = 0.0127 + 0.0943·X - 0.0060·X² + 0.00013·X³ (31) 0.64 + SEE | 0.2218 0.0547 0.0036 0.00006 + Winter | + Captive, | + both sexes |Y = 0.1550 + 0.0426·X - 0.0025·X² + 0.00006·X³ (57) 0.80 + SEE | 0.0734 0.0192 0.0013 0.00002 + --------------+-------------------------------------------------------- + + + +THERMOREGULATION AT LOW TEMPERATURES + + +_Body Temperature_ + +Body temperatures in Figure 6 are those recorded during metabolic +measurements from animals equipped with surgically implanted, +temperature-sensitive radio transmitters. Each point was recorded +during the lowest level of oxygen consumption at each T_{a}. In both +summer and winter, T_{b}'s were lowest during metabolic measurements +at T_{a}'s around T_{lc}. At T_{a}'s below T_{lc}, T_{b}'s increased +(Figure 6), which is an unusual response. Under similar conditions, +other procyonids either maintain a nearly constant T_{b} or allow it +to fall slightly (Müller and Kulzer, 1977; Chevillard-Hugot et al., +1980; Müller and Rost, 1983; Chevalier, 1985). For our raccoons, +confinement in the metabolism chamber at low temperatures must have +stimulated a greater than necessary increase in metabolic rate such +that heat production exceeded heat loss, which caused T_{b} to become +elevated. + + [Illustration: FIGURE 6.--Relationship between body temperature and + chamber air temperature in summer (panel A), and winter (panel B): + captive females, open circles and solid lines; captive males, solid + circles and dashed lines. Solid vertical lines represent lower + critical temperatures.] + + + TABLE 5.--Regression equations describing oxygen consumption + (mL O_{2}·g^{-1}·h^{-1}) of _Procyon lotor_ at temperatures below + their lower critical temperature (I = x-intercept (°C), n = number + of observations, R² = coefficient of determination, SEE = standard + error of estimate for the y-intercept (a) and slope (b), X = chamber + temperature (°C), and Y = oxygen consumption). + + ----------------+------------------------------------------------------ + Season | SEE + and sex | ----------- + | Equation (n) R² a b I + ----------------+------------------------------------------------------ + Summer | + Trapped male | Y = 1.09 - 0.0281·X (30) 0.64 0.0353 0.0040 38.8 + Captive male | Y = 0.97 - 0.0258·X (12) 0.91 0.0235 0.0025 37.6 + Captive female| Y = 1.04 - 0.0251·X (29) 0.78 0.0288 0.0026 41.1 + Winter | + Captive, | + both sexes | Y = 0.68 - 0.0193·X (36) 0.68 0.0157 0.0023 35.2 + ----------------+------------------------------------------------------ + + +_Summer_ + +During summer, T_{lc} for male raccoons was 20°C, whereas for females +it was 25°C (Figure 2). Regression equations calculated to describe +oxygen consumption at T_{a}'s below T_{lc} are presented in Table 5. +For three groups of summer animals, slopes of regressions are +identical. This indicates that minimum conductances of these three +groups were equivalent. Intercepts of these equations are different, +which suggests a difference in metabolic cost of thermoregulation +between these groups (Figure 2); captive males had a lower intercept +than either trapped males (p<0.005) or captive females (p<0.05), but +there was no difference in intercepts of captive females and trapped +males. These regression equations, therefore, also were derived using +values of oxygen consumption expressed in terms of metabolic body mass +(Mellen, 1963). Relationships between intercepts of these equations +are different than those for regressions in Table 5. Intercept for +females was intermediate to, and not different from, those of the two +groups of males. However, captive males still had a lower intercept +than trapped males (p<0.025). Thus, in summer, thermoregulatory +metabolism was less expensive for captive than for trapped males, and +in spite of a 5°C difference in their T_{lc}'s (Figure 2), captive +males and females had similar thermoregulatory costs. + +Regression lines for three groups of animals in summer extrapolate to +zero metabolism at values equivalent to, or greater than, normal +T_{b}; 38.8°C for trapped males, 37.6°C for captive males, and 41.1°C +for captive females (Table 5). Thus, all three groups had minimized +thermal conductance at T_{a}'s below T_{lc} (Scholander et al., 1950b; +McNab, 1980b). Minimum wet thermal conductance calculated for raccoons +in summer with Eq. 4 (Table 3) is numerically similar to these "slope" +values (Table 5), and it was, therefore, considered to be the best +estimate of C_{mw} for _Procyon lotor_ during that season +(0.0256 mL O_{2}·g^{-1}·h^{-1}·°C^{-1}). + + +_Winter_ + +During winter T_{lc} for both sexes decreased to 11°C (Figure 3). +Regression equations of thermoregulatory metabolism for males and +females in winter are not different from each other in either slope or +intercept. These data, therefore, were combined into a single equation +(Table 5). Slope and intercept of this equation are both lower +(p<0.005 and p<0.05, respectively) than those for summer animals +(Table 5). Identical results were obtained from comparisons using +regressions derived from oxygen consumption expressed in terms of +metabolic body mass (Mellen, 1963). Thermoregulatory costs at any +temperature below 20°C were lower for winter than summer animals +(Figures 2, 3). + + + TABLE 6.--Regression equations describing oxygen consumption + (mL O_{2}·g^{-1}·h^{-1}) of _Procyon lotor_ at temperatures below + their lower critical temperature in winter (A = females with radio + transmitters, B = females without radio transmitters, C = males, + I = x-intercept (°C), n = number of observations, R² = coefficient + of determination, X = chamber temperature (°C), and Y = oxygen + consumption). + + -----+-------------------------------------- + Group| Equation (n) R² I + -----+-------------------------------------- + A | Y = 0.63 - 0.0158·X (10) 0.66 40.1 + B | Y = 0.72 - 0.0226·X (11) 0.71 32.1 + C | Y = 0.69 - 0.0200·X (15) 0.79 34.7 + -----+-------------------------------------- + + + [Illustration: FIGURE 7.--Relationship between body temperature and + time of day at various months of the year: captive females, open + circles; captive males, closed circles. Vertical cross-hatched + areas represent civil twilight.] + +The regression line for _Procyon lotor_ in winter (Table 5) +extrapolates to zero metabolism at 35.2°C, which is below normal T_{b} +(Figures 6, 7). This suggests that not all raccoons measured in winter +minimized thermoregulatory metabolism or conductances at T_{a}'s below +T_{lc} (Scholander et al., 1950b; McNab, 1980b). To assess this +possibility, data for these animals were divided into three groups: +(A) females with radio transmitters, (B) females without radio +transmitters, and (C) males (Table 6). Regression equations of +metabolism below T_{lc} were derived for each group, and based on +extrapolated T_{b}'s at zero metabolism, only the two females with +implanted radio transmitters (group A) minimized thermoregulatory +metabolism and conductance. Had animals in groups B and C also +minimized their thermal conductances, while retaining their measured +metabolic rates, their rates of heat production would have been +disproportionately higher than their rates of heat loss. Equation 4 +predicts that under these conditions their body temperatures would +have been elevated to 42.0°C and 40.4°C, respectively. Thus, in order +to avoid such a large increase in body temperature, animals in groups +B and C increased their thermal conductances in preference to lowering +their metabolic rates. The regression equation of thermoregulatory +metabolism for all winter animals (Table 5), therefore, overestimates +minimum metabolic cost of temperature regulation below T_{lc}, and its +slope underestimates C_{mw}. Consequently, the best estimate of C_{mw} +for _Procyon lotor_ in winter is the value calculated for group A +animals with Eq. 4 (0.0172 mL O_{2}·g^{-1}·h^{-1}·°C^{-1}; Table 3), +and the minimum cost of thermoregulatory metabolism at any T_{a} below +T_{lc} is best estimated by substituting this value into Eq. 4 and +solving for [.H]_{r}. + + + +THERMOREGULATION AT HIGH TEMPERATURES + + +_Body Temperature_ + +In both summer and winter, T_{b}'s increased during metabolic +measurements at T_{a}'s above T_{lc} (Figure 6). This response also +was seen during metabolic measurements conducted on other procyonids +(Müller and Kulzer, 1977; Chevillard-Hugot et al., 1980; Müller and +Rost, 1983; Chevalier, 1985). + + +_Summer_ + +During summer our data suggested that the upper critical temperature +(T_{uc}) was higher than 35°C. The lowest rates of oxygen consumption +at T_{a} = 35°C occurred after 1.5 to 2.5 hours of exposure to that +temperature. Prolonged exposure to this temperature in summer did not +make animals restless, and their rate of oxygen consumption was very +stable throughout each measurement. Body temperature responses at +T_{a} = 35°C were recorded from two males and two females that had +implanted radio transmitters. With the exception of one male, T_{b}'s +were maintained near 38°C (Figure 6). The one exception (a male) +maintained its T_{b} at 39.3°C. At T_{a} = 35°C, summer males had +rates of evaporative water loss that were lower than those of summer +females (Figure 4). At this temperature, males dissipated 35% ħ6% and +females 56% ħ18% of their metabolic heat via evaporative water loss. +Thus, at T_{a} = 35°C, males must have utilized modes of heat transfer +other than evaporative cooling (convective and conductive heat +transfer) to a greater extent than females. + + +_Winter_ + +Body temperature, evaporative water loss, and metabolic data indicated +that, in winter, T_{uc} was very close to 35°C. In winter, the lowest +level of oxygen consumption was recorded during the first hour after +the chamber had reached T_{a} = 35°C. Unlike summer, animals became +restless after the first hour at 35°C, at which point their oxygen +consumption increased and showed a high degree of variability. Body +temperature responses at 35°C were recorded from both females that had +implanted radio transmitters. In one case, T_{b} rose from 37.9°C at +the end of the first hour to 40.5°C by the end of the second hour, and +as it did not show signs of leveling off, we terminated the +experiment. We exposed that same animal to T_{a} = 35°C one other time +during winter. In that instance, its T_{b} rose to 40.0°C during the +first 30 minutes and was maintained at that level for three hours with +no apparent distress. The other female elevated its T_{b} from 37.3°C +to 39.0°C during the second hour at T_{a} = 35°C and maintained its +T_{b} at that level for two hours. Thus, during winter, prolonged +exposure to T_{a} = 35°C stimulated more of an increase in T_{b} than +it did in summer. During winter, both males and females increased +evaporative water loss at T_{a} = 35°C (Figure 5) but only to the +extent that they dissipated 35% ħ10% of their metabolic heat +production. Thus, even in winter, convective and conductive heat +transfers were still the most important modes of heat loss at this +temperature. + + + +DAILY CYCLE OF BODY TEMPERATURE + + +The daily cycle of raccoon T_{b}'s during summer and winter are +presented in Figure 7. In general, T_{b}'s showed a marked circadian +cycle in phase with photoperiod. T_{b}'s rose above 38°C for several +hours each night but remained below 38°C during daytime. During +summer, with the exception of one female whose record was not typical +(Figure 7), T_{b}'s rose above 38°C shortly after sunset, whereas in +winter T_{b}'s did not rise above 38°C until several hours after +sunset. Once T_{b} was elevated it usually remained so until just +before or after sunrise (Figure 7). During summer, T_{b} was above +38°C for 85% or more of the time between sunset and sunrise (87% for +the female with the typical body temperature pattern, and 85% and 98% +for males), whereas in winter it was elevated for only 47%-78% of the +time between sunset and sunrise (47% and 61% for females, and 67% and +78% for males). During night, T_{b} would oscillate between 38°C and +about 39°C, such that two peak values occurred. These peak values +presumably corresponded to two periods of heightened nighttime +activity. During summer, one of these peaks occurred before and the +other after 24:00 hours, whereas in winter both peaks occurred after +24:00 hours. With the exception of one female in winter (Figure 7), +the lowest T_{b} of the day for both sexes was near 37°C, and this +typically occurred during daytime (Figure 7). + + + + +$Discussion$ + + + +BASAL METABOLIC RATE + + +_Background_ + +Basal metabolism represents the minimum energy required by a mammal to +maintain endothermy and basic homeostasis (Lusk, 1917:141; Kleiber, +1932, 1961:251; Benedict, 1938:191-215; Brody, 1945:59; Robbins, +1983:105-111). Mammals with lower than predicted [.H]_{b} maintain +endothermy and enjoy its attendant advantages at a discount, whereas +others, with rates that are higher than predicted, pay a premium +(Calder, 1987). Such variation in [.H]_{b} appears to be tied to +ecological circumstances rather than taxonomic affinities (Vogel, +1980; McNab, 1986a, 1988a, 1989), and depending on environmental +conditions, each rate provides an individual with various advantages +and limitations. During the course of evolution, therefore, each +species' [.H]_{b} evolves to provide it with the best match between +its energy requirements for continuous endothermy, its food supply, +and the thermal characteristics of its environment. + + +_Captive versus Wild Raccoons_ + +Male raccoons trapped in summer had higher [.H]_{b}'s than our captive +animals in any season (Table 2). The higher rate of metabolism of +these trapped males could have been due to the stress of captivity or +to the fact that "wild" animals actually may have higher metabolic +rates than those that have adjusted to captivity. If the latter is +true, then our data for captive animals underestimated the actual +energy cost of maintenance metabolism for _Procyon lotor_ in the wild. +At present, we have no way of determining which of these alternatives +is true. + + +_Seasonal Metabolism of Raccoons_ + +In some temperate-zone mammals, [.H]_{b} is elevated in winter, which +presumably increases their "cold-hardiness." Conversely, lower summer +metabolism is considered to be a mechanism that reduces the potential +for heat stress. Such seasonal variation in [.H]_{b} has been found in +several species: collard peccary, _Tayassu tajacu_ (Zervanos, 1975); +antelope jackrabbit, _Lepus alleni_ (Hinds, 1977); desert cottontail, +_Sylvilagus audubonii_ (Hinds, 1973); and, perhaps, cold-acclimatized +rat, _Rattus norvegicus_ (Hart and Heroux, 1963). Unlike these +species, our captive raccoons showed no seasonal variation in [.H]_{b} +(Table 2). Instead, raccoons achieved "cold-hardiness" in winter and +reduced their potential for heat stress in summer with a large +seasonal change in thermal conductance (Table 3). + + + TABLE 7.--Metabolic characteristics of several procyonid species. + + ---------------------+------------------------------------------------ + |Body Basal[a] Minimum[b] + Species |mass metabolism conductance T_{b}[c] + |(g) ------------ ------------- ------------- + | Meas H_{br} Meas C_{mwr} [alpha] [rho] + ---------------------+------------------------------------------------ + _Bassariscus astutus_| 865 0.43 0.68 0.0288[e] 0.85 37.6 23 + _Procyon cancrivorus_|1160 0.40 0.69 0.0368[e] 1.25 + _Potos flavus_ |2030 0.36 0.51 + _Potos flavus_ |2400 0.32 0.65 38.1 36.0 + _Potos flavus_ |2600 0.34 0.71 0.0200[f] 1.02 + _Nasua nasua_ |3850 0.26 0.60 0.0200[f] 1.24 38.3 36.4 + _Nasua nasua_ |4847 0.33 0.79 0.0238[e] 1.65 39.1 37.9 + _Nasua narica_ |5554 0.25 0.62 0.0208[e] 1.55 38.9 37.4 + _Nasua narica_ |4150 0.42 1.20 0.0341[e] 2.20 + | 0.0224[g] 1.45 + _Procyon lotor_ | + Summer | + Trapped male |4400 0.54 1.28 + Captive male |4790 0.46 1.07 0.0256[f] 1.77 38.4 37.5 + Captive female |4670 0.42 1.02 0.0256[f] 1.79 38.2 37.6 + Winter | + Captive male |5340 0.47 1.17 38.6 37.6 + Captive female |4490 0.46 1.10 0.0172[f] 1.15 38.3 37.3 + ---------------------+------------------------------------------------ + + ---------------------+----------------------------------------------- + | + Species | T_{n}[d] + |--------------- + | T_{lc} T_{uc} References + ---------------------+----------------------------------------------- + _Bassariscus astutus_| 35.5 Chevalier (1985) + _Procyon cancrivorus_| 26 Scholander et al. (1950b, c) + _Potos flavus_ | McNab (1978a) + _Potos flavus_ | 23 30 Müller and Kulzer (1977) + _Potos flavus_ | 23 33 Müller and Rost (1983) + _Nasua nasua_ | 25 33 Chevillard-Hugot et al. (1980) + _Nasua nasua_ | 30 35 Mugaas et al. (in prep.) + _Nasua narica_ | 25 35 + _Nasua narica_ | Scholander et al. (1950b, c) + | + _Procyon lotor_ | This study + Summer | + Trapped male | 20 + Captive male | 20 + Captive female | 25 + Winter | + Captive male | 11 + Captive female | 11 + ---------------------+----------------------------------------------- + + [a] Meas is measured basal metabolism (mL O_{2}·g^{-1}·h^{-1}). H_{br} + is the ratio of measured to predicted basal metabolism where the + predicted value is calculated from [.H]_{b} = 3.42·m^{-.25} + (Kleiber, 1932, 1961:206) and m is body mass in grams. + + [b] Meas is measured minimum thermal conductance + (mL O_{2}·g^{-1}·h^{-1}·°C^{-1}). C_{mwr} is the ratio of measured + to predicted minimum thermal conductance where the predicted value + is calculated from C_{m} = 1.0·m^{-0.5} (McNab and Morrison, 1963; + Herreid and Kessel, 1967), and m is body mass in grams. + + [c] T_{b} is body temperature during the active ([alpha]) and rest + ([rho]) phases of the daily cycle (°C). + + [d] T_{n} is the thermoneutral zone as defined by the lower (T_{lc}) + and upper (T_{uc}) critical temperatures (°C). + + [e] Conductance calculated as the slope of the line describing oxygen + consumption at temperatures below the lower critical temperature. + + [f] Conductance calculated from C_{mw} = [.H]_{r}/(T_{b} - T_{a}), + where [.H]_{r} is resting metabolic rate at temperatures below + T_{lc}, and other symbols are as described elsewhere. + + [g] Inactive-phase thermal conductance: estimated from Scholander et + al. (1950b), assuming that active-phase thermal conductance is 52% + higher than values determined during the inactive phase (Aschoff, + 1981). + + +_Comparison of Procyon lotor with Other Procyonids_ + +_Procyon lotor_ has a much higher mass-specific [.H]_{b} than other +procyonids (Table 7). To quantify the magnitude of this difference, +we compared the measured value for _Procyon lotor_ with one calculated +for it from a mass-specific least-squares regression equation (Eq. 6; +R² = 0.78) derived from data for those procyonids with lower than +predicted [.H]_{b}: _Potos flavus_, _Procyon cancrivorus_, +_Nasua nasua_, _Nasua narica_, and _Bassariscus astutus_ (Table 7). + + [.H]_{b} = 2.39·m^{-0.25} Eq. 6 + +[.H]_{b} in Eq. 6 is basal metabolism (mL O_{2}·g^{-1}·h^{-1}) and m +is body mass (g). Measured values of [.H]_{b} for _Procyon lotor_ were +1.45 to 1.86 times greater than those predicted for it by Eq. 6 +(Table 8). + + + TABLE 8.--Basal metabolism (mL O_{2}·g^{-1}·h^{-1}) of _Procyon + lotor_ as predicted by Eq. 6 ([.H]_{b} = 2.39·m^{-0.25}). Body + masses, used to calculate predicted values, and measured values + were taken from Table 7. + + ----------------+-------------------------------- + Season and sex | Predicted Measured/Predicted + ----------------+-------------------------------- + Summer | + Trapped male | 0.29 1.86 + Captive male | 0.29 1.59 + Captive female| 0.29 1.45 + Winter | + Captive male | 0.28 1.68 + Captive female| 0.29 1.59 + ----------------+-------------------------------- + + +_Influence of Diet on Basal Metabolism_ + +BACKGROUND.--With respect to [.H]_{b}, McNab (1986a:1) maintains that +"the influence of climate is confounded with the influence of food +habits," and that departures from the Kleiber (1961) "norm" are best +correlated with diet. Although this does appear to be the case for +diet specialists, the analysis is not so clear-cut for omnivorous +species (McNab, 1986a). His analysis also indicates that an animal's +"behavior" (i.e., whether it is terrestrial, arboreal, subterranean, +aquatic, etc.), secondarily modifies the influence of food habits on +[.H]_{b}. For example, terrestrial frugivores have [.H]_{b}'s that are +very near predicted values, whereas arboreal frugivores have rates +that are much lower than predicted (McNab, 1986a). + + + TABLE 9.--Food habits of some Procyonids. References for foods were + as follows: _Potos flavus_, _Procyon cancrivorus_, and _Nasua nasua_ + taken from Bisbal (1986); _Nasua narica_ taken from Kaufmann + (1962:182-198); _Bassariscus astutus_ taken from Martin et al. + (1951), Taylor (1954), Wood (1954), Toweill and Teer (1977), and + Trapp (1978); _Procyon lotor_ taken from Hamilton (1936), Stuewer + (1943:218-220), Stains (1956:39-51), and Greenwood (1981). Symbols + represent either qualitative (#) or quantitative (+,|) assessments + of feeding habits: # indicates that the animal was observed eating + the food; + and | represent volume and frequency, respectively, of + food utilization. No attempt was made to account for seasonal + variation in the use of these foods. + + + <20% by volume when found. | 1%-19% frequency of occurrence. + ++ >20% by volume when found. || 20%-50% frequency of occurrence. + ||| >50% frequency of occurrence. + + -----------+-------------------------------------------------------------- + |_Potos_ _Procyon_ _Nasua_ _Nasua_ _Bassariscus_ _Procyon_ + Food |_flavus_ _cancrivorus_ _nasua_ _narica_ _astutus_ _lotor_ + -----------+-------------------------------------------------------------- + Mammalia | + | # ++ ||| ++ || + Aves | ++ | + || + Birds' eggs| ||| + Reptilia | + | + ||| # + | + | + Amphibia | + | # + | + Pices | ++ || ++ || + Insecta |++ | + ||| ++ ||| # + || ++ || + Arachnida | ++ ||| # + | + | + Chilopoda | ++ ||| + Diplopoda | # + | + Crustacea | ++ ||| # ++ ||| + Mollusca | + || # + || + Annelida | # + | + Nuts | ++ || + Grains | ++ || + Buds | + | + Fruit |++ ||| ++ # || ++ ||| + Leaves | + | + Grass | + | + -----------+-------------------------------------------------------------- + + +FOOD HABITS OF PROCYONIDS.--Food habits of six procyonids for which +metabolic data are available are presented in Table 9. All six species +clearly have mixed diets. Compared to other species, _Procyon lotor_ +is highly catholic in its diet, taking food from almost twice as +many categories as _Nasua narica_, three times as many as _Procyon +cancrivorus_, _Nasua nasua_, and _Bassariscus astutus_, and nine times +as many as _Potos flavus_. + +For those species for which food habit data are quantified, we used +Eisenberg's (1981:247-251) substrate/feeding matrix method, where +"substrate" is analogous to McNab's (1986a) "behavior," to construct +the following feeding categories that are based on the major food +groups utilized by each species (Table 9). + + 1. _Potos flavus:_ (1) arboreal/frugivore, insectivore. + + 2. _Procyon cancrivorus:_ (1) semiaquatic/crustacivore, + molluscivore, insectivore, piscivore, carnivore. + + 3. _Nasua nasua:_ (1) terrestrial/insectivore, arachnidivore, + carnivore, frugivore. + + 4. _Bassariscus astutus:_ (1) terrestrial/carnivore, insectivore, + frugivore. + + 5. _Procyon lotor:_ (1) terrestrial/carnivore, granivore, + frugivore, insectivore; and (2) semiaquatic/crustacivore, + molluscivore, insectivore, piscivore, carnivore. + +FOOD HABITS AND BASAL METABOLISM.--The most important foods in the +diet of _Procyon lotor_ are vertebrates, nuts, seeds, and fruits +(Table 9). These are the same foods that are eaten by those dietary +specialists that have [.H]_{b}'s equivalent to, or higher than, values +predicted for them by the Kleiber equation (McNab, 1986a). The most +important foods in the diets of _Potos flavus_, _Procyon cancrivorus_, +and _Nasua nasua_ are invertebrates and fruit (Table 9), and these +foods are eaten by dietary specialists that have lower than predicted +[.H]_{b}'s (McNab, 1986a). Major foods in the diet of _Bassariscus +astutus_ are terrestrial vertebrates, insects, and fruit (Table 9). +Dietary specialists that eat terrestrial vertebrates have higher than +predicted [.H]_{b}'s, whereas those that feed on insects have [.H]_{b}'s +that are lower than predicted (McNab, 1986a). Year-round utilization of +vertebrates by _Bassariscus astutus_ suggests that it also should have +a metabolic rate that is equivalent to or higher than predicted, rather +than lower (McNab, 1986a). However, perhaps year-round inclusion of +insects in its diet (Martin et al., 1951; Taylor, 1954; Wood, 1954; +Toweill and Teer, 1977; Trapp, 1978), plus water- and energy-conserving +advantages of a low metabolic rate, each exert a stronger selective +influence on [.H]_{b} than do vertebrates in its diet. + +SUMMARY.--The basal metabolic rate of these procyonids does appear to +be influenced by diet. But, it is apparent from this family's +evolutionary history and tropical origins that climate also has had a +profound influence on its member's metabolism. The history of the +family and the data presented here (Table 7) suggest that lower than +predicted [.H]_{b} is a feature that evolved very early as the primary +metabolic adjustment to a tropical climate. From this perspective, it +could be argued that climate would have been the major selective force +determining [.H]_{b}, whereas food habits would have had a secondary +influence. + + +_Basal Metabolism and Intrinsic Rate of Natural Increase_ + +BACKGROUND.--McNab (1980a) suggested that if food is not restricted +during an animal's reproductive period, the factor that will limit +growth and reproduction will be the rate at which energy can be used +in growth and development. Under these conditions, an increase in +[.H]_{b} would actually increase r_{max} because it would provide a +higher rate of biosynthesis, a faster growth rate, and a shorter +generation time. Hennemann (1983) tested McNab's (1980a) premise and +found a significant correlation between r_{max} and metabolic rate, +independent of body size, for 44 mammal species. A low correlation +coefficient for this relationship, however, indicated to him +(Hennemann, 1983) that factors such as (1) food supply, (2) thermal +characteristics of the environment, and (3) brain size also contribute +toward shaping a species' reproductive potential, particularly when +these factors strongly influence rates of biosynthesis or growth or +for some reason alter generation time. Results of our estimates of +r_{max} for procyonids are presented in Table 10. + +_Procyon lotor._--This species had the highest [.H]_{b} and D_{d}, and +also had the highest r_{max} (1.34; Table 10). Such a high r_{max} may +infer that this trait evolved under conditions where food and +temperature were not limiting to reproduction. Under these conditions +selection could have favored those reproductive characteristics +sensitive to a higher [.H]_{b} (biosynthesis, growth, and generation +time; McNab, 1980a). _Procyon lotor_'s high reproductive potential is +due to its early age of first female reproduction and its large litter +size, characteristics that may reflect metabolically driven increases +in both biosynthesis and growth. + +_Bassariscus astutus._--This species has a low [.H]_{b} but an r_{max} +that was 124% of expected (Table 10). This suggests that r_{max} +evolved under conditions where food and temperature were not +limiting to reproduction. Reduced litter size should restrict this +species' reproductive potential and may be a reflection of its low +[.H]_{b}. The factor that is responsible for increasing its +reproductive potential, however, is its early age of first female +reproduction. _Bassariscus astutus_ is the smallest of these +procyonids, and even though it has a low [.H]_{b}, its small mass +may contribute to its ability to reach adult size and sexual maturity +in its first year. The high quality of its diet (a high proportion of +small vertebrates; Table 9) also may be a factor that is permissive to +early female reproduction. Thus, small body size and diet may be +factors that have allowed this species to evolve a higher than expected +reproductive potential in spite of its low [.H]_{b}. + +_Nasua narica._--This species is one of the largest procyonids +(Table 7), and it possesses characteristics that should limit its +reproductive potential: lower than predicted [.H]_{b} (Table 7), a +relatively low-quality diet (Kaufmann, 1962:182-198; Table 9), and +delayed time of first reproduction (Table 10). In spite of this, +_Nasua narica_ has a higher than expected r_{max} (111% of predicted; +Table 10). The life history feature that enhances _Nasua narica_'s +reproductive potential, and increases r_{max} beyond expected, is its +large litter size. In this species females live in bands. Each year +just before their young are born these bands break up, and each female +seeks out a den for herself and her litter. Once the young are able to +leave the den (approximately five weeks), bands reform. In this +situation, females not only care for their own young but also for those +of other females in the band (Kaufmann, 1962:157-159, 1982, 1987; +Russell, 1983). This social structure may contribute to this species' +ability to produce large litters and in this way increase its +reproductive potential. + + + TABLE 10.--Intrinsic rate of natural increase (r_{max}) of several + procyonids. (a = potential age of females producing first young; + b = potential annual birth rate of female young (= average litter + size/2; average litter size was calculated from the published range + of litter sizes for each species); n = potential age of females + producing their final young; r_{maxe} = intrinsic rate of natural + increase expected from body mass (Hennemann, 1983); r_{maxr} = ratio + of calculated to expected intrinsic rate of natural increase + (r_{max}/r_{maxe}).) + + ---------------------+------------------------------------------------ + | + Species |Body mass a b n r r [a] r [b] + | (g) max maxe maxr + ---------------------+------------------------------------------------ + _Procyon lotor_ | 4940 0.83 2.25 16 1.34 0.53 2.52 + | + | + | + _Bassariscus astutus_| 900 0.83 1.50 14 1.02 0.82 1.24 + | + | + | + _Nasua narica_ | 3900 2.50 2.25 14 0.62 0.56 1.11 + | + _Nasua nasua_ | 3850 + _Procyon cancrivorus_| 1160 0.83 1.50 15 1.02[c] 0.77 1.32 + | 1.75 0.65[c] 0.84 + _Potos flavus_ | 2490 1.75 0.50 12 0.30 0.63 0.48 + | + _Bassaricyon gabbii_ | 1600 1.75 0.50 15 0.32 0.71 0.45 + | + ---------------------+------------------------------------------------ + + ---------------------+------------------------------------------------ + | + Species | References + ---------------------+------------------------------------------------ + _Procyon lotor_ | Dunn and Chapman (1983); Eisenberg (1981:489); + | Kaufmann (1987); Lotze and Anderson (1979); + | Nowak and Paradiso (1983:981); Sanderson + | (1987); Stains (1956:28-31); This study + _Bassariscus astutus_| Kaufmann (1982, 1987); Nowak and Paradiso + | (1983:979, 980); Poglayen-Neuwall and + | Poglayen-Neuwall (1980); Poglayen-Neuwall + | and Toweill (1988); Russell (1983) + _Nasua narica_ | Kaufmann (1982, 1987); Nowak and Paradiso + | (1983:983); Sanderson (1983) + _Nasua nasua_ | Chevillard-Hugot et al. (1980) + _Procyon cancrivorus_| Crandall (1964:312); Poglayen-Neuwall (1987) + | + _Potos flavus_ | Ford and Hoffmann (1988); Nowak and Paradiso + | (1983:984) + _Bassaricyon gabbii_ | Eisenberg (1981:489); Nowak and Paradiso + | (1983:985) + ---------------------+------------------------------------------------ + + [a] r_{maxe} = 4.9·m^{0.2622}, where m is body mass in grams. + + [b] Regression of r_{max} on body mass (m). Assume r_{max} = 1.02 for + _Procyon cancrivorus_: r_{max} = 0.00005·m + 0.623; R = 0.19; + R² = 0.03; Regression of r_{maxr} (Table 10) on H_{br} (Table 7); + assume _Nasua nasua_ has the same r_{maxr} as _Nasua narica_: + r_{maxr} = 3.35·H_{br} - 1.11; R = 0.93; R² = 0.86. + + [c] Estimate based on females reproducing in their first (a = 0.83) or + second (a = 1.75) year. + + +_Nasua nasua._--Unfortunately, there is not enough reproductive data +to allow calculation of r_{max} for _Nasua nasua_ (Table 10), therefore, +it is not possible to compare the reproductive potential of this South +American coati with its North American relative, _Nasua narica_. Given +its low [.H]_{b} and relatively low-quality diet of fruit and +terrestrial invertebrates (Table 9), however, r_{max} of _Nasua nasua_ +may be very similar to that of _Nasua narica_. + +_Procyon cancrivorus._--The age of first female reproduction for +_Procyon cancrivorus_ has not been reported. However, if one assumes +females can reproduce in their first year, r_{max} for _Procyon +cancrivorus_ would be 1.02 (132% of expected; Table 10). If, on the +other hand, first female reproduction is delayed until the second year, +r_{max} would be 0.65 (84% of predicted; Table 10). _Procyon +cancrivorus_ has a low [.H]_{b}, reduced litter size, and small body +mass. Its low [.H]_{b} may limit litter size, but as with _Bassariscus +astutus_, the quality of its diet (a high percentage of small +vertebrates; Table 9) and its small body size may make it possible for +females to reproduce in their first year and thus increase the species' +reproductive potential. This reasoning would argue that _Procyon +cancrivorus_ probably enjoys higher, rather than lower, than expected +r_{max}. + +_Potos flavus._--In addition to a low [.H]_{b}, this species possesses +other characteristics that limit its reproductive potential: +low-quality diet, delayed reproduction, and birth of a single young +each year. Because there does not appear to be any other feature of +its life history that can counteract the influence of these factors, +r_{max} in _Potos flavus_ has evolved to be only 48% of expected +(0.30; Table 10). Its close relative, the olingo, _Bassaricyon gabbii_, +appears to share the same condition (Table 10). + +SUMMARY.--This brief survey illustrates that, with the exception of +_Potos flavus_, procyonids tend to have values of r_{max} that are +higher than those predicted for them on the basis of mass (Table 10). +Regression analysis indicates that, within the family, body mass +accounts for only a small amount (3%) of the variation in r_{max}, +whereas the positive slope of the correlation between r_{maxr} and +H_{br} (R = 0.93) suggests that low metabolism has a limiting effect +on r_{max} (see Table 10, footnote f). The implication here is that +low [.H]_{b} would be associated with a lower rate of biosynthesis, +a slower growth rate, and a longer generation time. Procyonids with +low [.H]_{b} but higher than expected r_{max} must possess other +traits that serve to offset the effects of low metabolism. Our survey +indicates that the following features compensate for low [.H]_{b} and +help increase r_{max}: (1) a high-quality diet may make biosynthesis +and growth more efficient, thus optimizing the time element associated +with each of these processes; (2) larger litter sizes and cooperation +in care of the young may increase survivorship in spite of a slower +growth rate; and (3) an early age of first reproduction, a long +reproductive life span, and moderate-size litters (two to four young) +may in the long run add as many individuals to the population as a +shortened generation time. Our survey also suggests that, at the other +extreme, factors such as a low-quality diet, reduced litter size, +absence of cooperative care of the young, delayed age of first +reproduction, and shortened reproductive life span all serve to +decrease r_{max}. Thus, it is obvious that diet, litter size, social +structure, reproductive strategy, and reproductive life span can operate +synergistically with [.H]_{b} to magnify its influence on r_{max} (as +with _Procyon lotor_ and _Potos flavus_), or they can function in +opposition to [.H]_{b} to change the direction of its influence on +r_{max} (as with _Bassariscus astutus_, _Procyon cancrivorus_, _Nasua +narica_, and perhaps _Nasua nasua_). + + +_Basal Metabolism and Climatic Distribution_ + +_Procyon lotor._--The evolution of a higher [.H]_{b} (Tables 7, 8) may +have been the physiological cornerstone that enabled _Procyon lotor_ +to break out of the mold being exploited by other procyonids and to +generalize its use of habitats and climates. Once this basic +physiological change was in place, selection for appropriate +alterations in thermal conductance, capacity for evaporative cooling, +diversity of diet, and energy storage would have provided this species +with the suite of adaptations needed to extend its distribution into +other habitats and climates. Support for this concept follows from the +fact that high levels of [.H]_{b} are associated with (1) cold-hardiness +in mammals that live in cold-temperate and arctic climates (Scholander +et al., 1950c; Irving et al., 1955; Irving, 1972:115, 116; Shield, 1972; +Vogel, 1980; Golightly and Ohmart, 1983); (2) the ability to utilize a +wide variety of food resources and to occupy a large number of different +environments and habitats (McNab, 1980a); and (3) a high intrinsic rate +of natural increase (McNab, 1980a; Hennemann, 1983; Lillegraven et al., +1987; Nicoll and Thompson, 1987; Thompson, 1987). + +OTHER PROCYONIDS.--Other procyonids (_Potos flavus_, _Procyon +cancrivorus_, _Nasua narica_, and _Nasua nasua_) have lower than +predicted [.H]_{b}'s (Table 7), a characteristic that is considered to +be an energy-saving adaptation for those that live in relatively stable +tropical and subtropical habitats (Müller and Kulzer, 1977; +Chevillard-Hugot et al., 1980; Müller and Rost, 1983). However, +_Bassariscus astutus_ is found in tropical, subtropical, and temperate +climates. This species is found from tropical Mexico to temperate +regions of the western United States (Kaufmann, 1982, 1987; Nowak and +Paradiso, 1983:979). In the northern part of its distribution, +_Bassariscus astutus_ lives in habitats that are unstable (arid +regions), that are low in productivity, and that characteristically +have marked seasonal changes in temperature. Its lower than predicted +[.H]_{b} could be an important water-conserving adaptation at times when +temperatures are high (McNab and Morrison, 1963; McNab, 1966; MacMillen +and Lee, 1970; Noll-Banholzer, 1979) and an important energy-conserving +mechanism when cold weather may limit food availability and hunting time +(Scholander et al., 1950c; Wang et al., 1973). As will be seen later, +_Bassariscus astutus_ is unique among procyonids with lower than +predicted [.H]_{b}'s in that it also has a lower than predicted +C_{mw} (Table 7). This allows it to use less energy than expected for +thermoregulation at low temperatures. Another species with a similar set +of adaptations (lower than predicted [.H]_{b} and C_{mw}) is the arctic +hare, _Lepus arcticus_ (Wang et al., 1973), which lives in one of +the coldest and least-productive regions on earth. Wang et al. (1973) +suggest that this combination of adaptations allows _Lepus arcticus_ +to better match its energy requirements to the low productivity of its +environment. A similar relationship may hold for _Bassariscus astutus_, +particularly in colder arid portions of its distribution, and may be the +reason that it, but not other procyonids with low [.H]_{b}'s, has been +able to inhabit temperate climates. + + + +MINIMUM THERMAL CONDUCTANCE + + +_Background_ + +Thermal conductance is a measure of the ease with which heat is +passively transferred to or from a body through its tissues and pelt. +Within T_{n}, a mammal is able to vary its thermal conductance over a +wide range of values by changing heat transfer characteristics of both +of these layers. Minimum thermal conductance occurs when total heat +transfer through these layers is reduced to its lowest possible rate. +This minimum value, which is the reciprocal of maximum resistance, +occurs, theoretically, but not always practically (see McNab, 1988b), +at the animal's T_{lc} and is best estimated under standard conditions +in a metabolism chamber (McNab, 1980b; Aschoff, 1981). Minimum thermal +conductance scales to body mass (McNab and Morrison, 1963; Herreid and +Kessel, 1967; McNab, 1970, 1979b; Bradley and Deavers, 1980; Aschoff, +1981). Therefore, to make comparisons between species of various sizes, +we scaled out body mass by expressing C_{mw} as the ratio of measured +to predicted values (C_{mwr}; Table 7). These ratios were used to make +comparisons of heat-transfer characteristics between species that +occupy different habitats or climates. + + +_Effect of Molt on Thermal Conductance_ + +In summer, T_{lc}'s of male and female _Procyon lotor_ (Figure 2) were +very similar to those of other procyonids (22°C-26°C; Table 7). In +winter, T_{lc} of both sexes shifted downward to 11°C (Figure 3). This +seasonal shift in T_{lc} occurred as the result of a seasonal change +in minimum thermal conductance (Table 3). For many northern mammals, a +seasonal change in thermal conductance is partly mediated via cyclic +changes in the insulative quality of their pelt (Scholander et al., +1950a; Irving et al., 1955; Hart, 1956, 1957; Irving, 1972:165). + +_Procyon lotor_ begins to shed its heavy winter coat about the time +its young are born. Molt progresses through summer and by late August +the new coat is complete (Stuewer, 1942). During its summer molt, +_Procyon lotor_'s C_{mw} increased by about 49% over the value for +female raccoons in winter (Table 3). In summer, therefore, it had the +highest mass specific C_{mw} of those procyonids considered +(C_{mwr} = 1.77 and 1.79; Table 7). An increase in thermal conductance +facilitates passive heat loss for temperate and arctic species, and +this serves as an important thermoregulatory adaptation during warm +summer months (Scholander et al., 1950c; Irving et al., 1955; +Hart, 1956, 1957; Irving, 1972:165). This adaptation is particularly +important to those temperate- and arctic-zone species (including +raccoons) whose [.H]_{b}'s do not decrease during summer (Irving et al., +1955). From August on, the fur of _Procyon lotor_ becomes increasingly +longer and heavier, with peak, or prime, condition occurring in late +fall and early winter (Stuewer, 1942). Minimum conductance of our +captive raccoons was lowest in winter (C_{mwr} = 1.15) when their pelts +were in prime condition (Tables 3, 7). Because "primeness" of raccoon +pelts varies geographically, thicker pelts being associated with colder +climates (Goldman, 1950:21; Whitney and Underwood, 1952:24-41), the +degree of seasonal change in C_{mw} must also vary geographically. + +The only other procyonid for which a seasonal molt has been described +is _Bassariscus astutus_. Molt in this species extends from late summer +to late fall (Toweill and Toweill, 1978). How molt effects thermal +conductance in _Bassariscus astutus_ is not known because metabolic +data for this species (Table 7) apparently were collected only when +their pelts were in prime condition (Chevalier, 1985). + +Goldman (1950:20) reports that _Procyon cancrivorus_ does not have a +seasonal molt. Like other tropical procyonids, _Procyon cancrivorus_ +lives in an environment that has the following characteristics: high +even temperatures throughout the year (1°C-13°C difference in monthly +mean temperature), a greater range in temperature between day and +night than in mean monthly temperature throughout the year, uniform +lengths of day and night, seasonal variation in rainfall, and lowest +temperatures during the rainy season(s) (Kendeigh, 1961:340). In such +a stable environment there would be no advantage to a sharply defined +seasonal molt cycle that could place an animal in thermoregulatory +jeopardy by increasing its thermal conductance. This would be +particularly true for animals like tropical procyonids that have lower +than predicted [.H]_{b}'s but that maintain typical eutherian body +temperatures (Table 7). Consequently, molt in all tropical procyonids +may either be prolonged or continuous. This is a feature of their +biology that needs to be examined in more detail. + + +_Comparison of Thermal Conductances_ + +_Procyon lotor_ VERSUS TROPICAL PROCYONIDS.--C_{mwr} for _Procyon +lotor_ in winter was 1.15, which is similar to the values for _Potos +flavus_ and _Procyon cancrivorus_, 1.02 and 1.25, respectively +(Table 7). These two tropical species, therefore, have C_{mw}'s that +are similar on a mass specific basis to the value for _Procyon lotor_ +in winter. However, at their T_{lc}'s, the thermal gradient sustained +by these tropical animals is only about 11°C, whereas for _Procyon +lotor_ in winter it was 26.5°C. Examination of Eq. 4 with respect to +these thermal gradients suggests that tropical procyonids achieve such +low C_{mw}'s by virtue of their lower than predicted [.H]_{b}'s rather +than by having pelts that are exceptionally good insulators. In fact, +the insulation afforded by the pelts of these tropical procyonids is +about the same as that of the 50 g arctic lemming, _Dicrostonyx +groenlandicus rubricatus_, whose coat has an insulative value that is +about half that of the hare, _Lepus americanus_, red fox, _Vulpes fulva +alascensis_, and pine martin, _Martes americana_, animals comparable in +size to these procyonids (Scholander et al., 1950a). Therefore, pelts +of these tropical procyonids do not have the same insulative value as +the prime winter coat of _Procyon lotor_. + +_Nasua narica_ and _Nasua nasua_ have tropical and subtropical +distributions and they are the only procyonids that are diurnal +(Kaufmann, 1962:103-105, 1982, 1987). Because they are active during +the day they experience a more extreme thermal environment (higher +T_{a}'s and solar radiation) than their nocturnal cousins. Values of +C_{mwr} for _Nasua narica_ (1.45 and 1.55) and _Nasua nasua_ (1.24 and +1.65) are higher than those for _Procyon cancrivorus_ or _Potos flavus_ +(Table 7). Thus, these coatis have higher mass specific C_{mw}'s than +their nocturnal tropical cousins. A high C_{mw} reduces the cost of +thermoregulation in hot environments because it increases an animal's +ability to lose excess heat passively. The higher C_{mw}'s of these +coatis serve as an adaptation that contributes to the success of their +diurnal life style as well as their ability to expand their habitat use +to areas with less thermal stability, such as oak and pine woodlands +and deserts. + +_Bassariscus astutus._--This species has the lowest mass specific +C_{mw} of these procyonids (C_{mwr} = 0.85; Table 7), which indicates +that its pelt has a greater insulative value than the coats of _Potos +flavus_, _Procyon cancrivorus_, _Nasua nasua_, or _Nasua narica_. This, +coupled with a lower than predicted [.H]_{b}, allows _Bassariscus +astutus_ to maintain T_{b} with less energy expenditure than is +possible for any other procyonid of comparable size; and this +combination of adaptations provides _Bassariscus astutus_ with a +distinct energy advantage in environments that have low productivity +(Wang et al., 1973). The evolution of a pelt that provides better +insulation must be considered an important contributing factor for +the spread of this species into desert regions of the western United +States. + + + +THERMOREGULATION AND USE OF STORED FAT AT LOW TEMPERATURES + + +_Background_ + +THERMOREGULATION.--At temperatures below a mammal's T_{n}, heat loss +exceeds [.H]_{b}. To maintain T_{b} under these conditions, metabolic +rate must be increased (Eq. 4). _Procyon lotor_ in summer during its +annual molt (Table 5; Figure 2), _Bassariscus astutus_ (Chevalier, +1985), _Nasua nasua_ (Chevillard-Hugot et al., 1980; Mugaas et al., +in prep.), _Nasua narica_ (Scholander et al., 1950b; Mugaas et al., +in prep.), and _Potos flavus_ (Müller and Kulzer, 1977; Müller and Rost, +1983) all are able to elevate their metabolic rates by 130% above basal +when they are exposed to T_{a} = 0°C. _Procyon cancrivorus_ responds to +0°C with an increase in metabolic rate of 257% above basal (Scholander +et al., 1950b). All animals listed have about the same T_{lc} and T_{b}, +so the temperature differential producing this response is about the +same for each species. Metabolic ability to defend body temperature +against low ambient temperatures, therefore, is well developed in these +procyonids. Such large increases in metabolic rate are energetically +expensive, and if these animals were routinely exposed to T_{a} = 0°C, +it would be difficult for them to acquire enough food each day to +maintain endothermy. Raccoons in winter pelage, however, need only +elevate their metabolic rate by 47% above basal to maintain endothermy +at T_{a} = 0°C (Table 5; Figure 3). Each year at the completion of its +molt, the raccoon's highly insulative pelt is renewed. This lowers +their T_{lc} by 9°C to 15°C below that measured for them in summer +(Figure 3) and decreases their cost of thermoregulation at low +temperatures. The increased insulative capacity of their pelt is one +of the primary adaptations that has allowed _Procyon lotor_ to extend +its distribution into cold climates. + +STORED FAT.--Cyclic fattening is an integral and important part of a +raccoon's annual cycle (Mugaas and Seidensticker, ms); however, it has +not been reported for other procyonids. During winter in parts of the +United States and Canada, raccoons are confined to their dens for +variable periods of time (days to months) depending on the severity of +the weather (Stuewer, 1943:223-225; Whitney and Underwood, 1952:108-116; +Sharp and Sharp, 1956; Mech et al., 1968; Schneider et al., 1971). +During this confinement, they do not hibernate but rather enter a state +of "dormancy" and become inactive. While dormant they remain endothermic +(T_{b} > 35°C; Thorkelson, 1972:87-90) and derive most of their energy +requirement from fat reserves accumulated during fall. The rate at which +fat stores are consumed during winter dormancy depends on the +thermoregulatory requirement imposed on them by local weather +conditions, the insulative quality of their pelt, and any advantage +they may gain by seeking shelter in a den. + + +_Thermal Model of the Raccoon and Its Den_ + +Heat transfer between an animal and its environment is a function of +the interaction of its body temperature and thermal conductance with +various environmental variables (air temperature, wind speed, vapor +pressure, and thermal radiation). When a raccoon is outside its den, +its thermal conductance (C_{mw}) is the only barrier to heat transfer +with the external environment. However, when it enters a tree den, a +raccoon imposes two other thermal barriers between itself and the +external environment: (1) conductance of the air space between its fur +and the den's walls (C_{a}) and (2) conductance of the den's walls +(C_{d}; Thorkelson, 1972:59-63; Thorkelson and Maxwell, 1974). +Thorkelson and Maxwell (1974) modeled heat transfer of a simulated +raccoon (a water-filled aluminum cylinder equipped with a heater and +covered with a raccoon pelt) in a closed tree den. In their system, +65% of resistance to heat flux was attributable to the pelt, whereas +the remainder (35%) was due to C_{a} and C_{d}. Because resistance is +the inverse of conductance, and resistances for the raccoon and its +den are arranged in series, we can estimate total conductance (C_{t}) +of this system with Eq. 7. + + 1/C_{t} = 1/C_{mw} + 1/C_{a} + 1/C_{d} Eq. 7 + +Minimum thermal conductance C_{mw} for raccoons in winter was +0.0172 mL O_{2}·g^{-1}·h^{-1}·°C^{-1} (Table 3). Based on Thorkelson +and Maxwell's (1974) model we let 1/C_{mw} = 0.65(1/C_{t}) = +1/0.0172 mL O_{2}·g^{-1}·h^{-1}·°C^{-1}, and 1/C_{a} + 1/C_{d} = +0.35(1/C_{t}). Substituting these values into Eq. 7 and solving +for C_{t} yields 0.0112 mL O_{2}·g^{-1}·h^{-1}·°C^{-1}, a value +that is 35% lower than that of the animal alone. Substituting +this value and the value for basal metabolism of winter raccoons +(0.47 mL O_{2}·g^{-1}·h^{-1}; Table 7) into Eq. 4 and solving for +(T_{b} - T_{a}) yields a new temperature differential of 42°C. +Therefore, by using tree dens, raccoons in north central Virginia, +with T_{b} = 37°C (Figure 7), could effectively reduce their T_{lc} +from 11°C to -5°C and markedly reduce their metabolic cost of +thermoregulation. + + +_Metabolic Advantage of the Den_ + +Given prevailing winter temperatures in north central Virginia (see +"Materials and Methods"), adult raccoons in that area should be able +to sustain endothermy most of the time they are in their dens by simply +maintaining [.H]_{b}. Depending on the mass of their stored fat, they +could remain in their dens for several weeks without eating (Mugaas and +Seidensticker, ms). The thermal advantage of a den could be further +enhanced during colder temperatures if two or more raccoons occupied it +at the same time and huddled together, and/or if these animals could +reduce C_{mw} even more by lowering T_{b} and cooling their extremities. +Although we do not have any data to verify the second mechanism, there +are many accounts in natural history literature that document raccoons +occupying dens together (Lotze and Anderson, 1979). This habit could be +particularly important for the young of the year and may be one reason +why they often continue to den with their mothers during winter (Lotze +and Anderson, 1979; Seidensticker et al., 1988). Raccoons that live in +colder climates, such as Minnesota, undoubtedly obtain the same +advantage from a den as Virginia animals, but because of their greater +body mass, longer fur, and potentially lower C_{mw}, T_{lc} of a +Minnesota raccoon in a den could be even lower than what we calculated +for Virginia raccoons. Therefore, when they are in their dens, raccoons +living in very cold climates also may be able to maintain homeothermy +with a basal level of metabolism. + + + +THERMOREGULATION AT HIGH TEMPERATURES + + +_Background_ + +In hot environments mammals depend on behavior to minimize their +thermal load (escape to shaded or cooler microclimates, use posture +and orientation to wind and sun, restrict activity, become nocturnal, +etc.) and on evaporative water loss to rid themselves of excess heat. +With regard to evaporative heat loss, Calder and King (1974:326) +arbitrarily subdivided the response to various T_{a}'s as follows: +"(1) cool temperatures at which water loss should be minimized, both +to reduce heat loss and as an adaptation to terrestriality; (2) an +intermediate temperature range wherein evaporation is gradually +increased as dry heat losses are proportionately reduced with smaller +thermal gradients; and (3) warm to hot temperatures at which +evaporation must be actively increased to dispose of metabolic and +exogenous heat loads." Some mammals are able to thermoregulate very +well at high ambient temperatures via panting or sweating, whereas +others have a very limited capacity. Hence, there is no general +approach to calculating evaporative water loss under these conditions +(Campbell, 1977:85). However, the ratio of evaporative heat lost to +metabolic heat produced can be used to quantify a species' capacity +for evaporative cooling and to make comparisons between species. + + +_Comparison of Procyonid Responses to Heat Stress_ + +_Potos flavus._--This species lives in Neotropical forests of Central +and South America. It is nocturnal, arboreal in habit, and appears to +be the most heat-sensitive of these procyonids. Its T_{uc} is at 30°C +to 33°C (Table 7; Müller and Kulzer, 1977; Müller and Rost, 1983). It +begins to pant at about 30°C, but its efforts at evaporative cooling +are very ineffective. At 33°C _Potos flavus_ can dissipate 33% of its +metabolic heat via evaporative water loss, but at 35°C the efficiency +of this mechanism falls to 20% (Müller and Rost, 1983). Consequently, +when exposed to T_{a}'s above 33°C, any kind of excitement causes its +T_{b} to rise rapidly in an uncontrolled manner (Müller and Kulzer, +1977; Müller and Rost, 1983). These animals rely on their nocturnal +and arboreal habits to keep them out of situations that could lead to +hyperthermia (Müller and Kulzer, 1977; Müller and Rost, 1983). + +_Nasua nasua_ and _Nasua narica_.--_Nasua nasua_ is abundant in +tropical and subtropical South America, whereas _Nasua narica_ occupies +the same climates in North America from southern Arizona and New Mexico +south through Panama and on into Colombia and Ecuador (Hall and Kelson, +1959:892; Ewer, 1973:391, 392; Poglayen-Neuwall, 1975). Both coatis +are diurnal and forage primarily on the ground (Kaufmann, 1962:185-188, +1987; Poglayen-Neuwall, 1975; Nowak and Paradiso, 1983:982), +consequently they are exposed to a more severe thermal environment +while active (higher T_{a}'s and solar radiation) than are nocturnal +procyonids. Both coatis are more heat-tolerant than _Potos flavus_; +their T_{uc}'s are higher (33°C-35°C; Table 7), they can tolerate +T_{a}'s of 35°C without raising their T_{b}'s (Chevillard-Hugot et al., +1980; Mugaas et al., in prep.), and they have a greater capacity for +evaporative cooling than _Potos flavus_ (Mugaas et al., in prep.). The +greater heat tolerance of these coatis is compatible with their diurnal +habits and widespread distribution in a variety of forest habitats in +both tropical and subtropical areas of the western hemisphere. + +_Bassariscus astutus._--In addition to living in Neotropical forests of +Mexico, _Bassariscus astutus_ also flourishes in hot arid climates, and +it has extended its range much farther north than _Nasua narica_ (Hall +and Kelson, 1959:881,892; Poglayen-Neuwall, 1975; Kaufmann, 1982). Its +T_{uc} is higher (35.5°C; Table 7) than that of _Potos flavus_, but it +is comparable to those of _Nasua nasua_ and _Nasua narica_. Its capacity +for evaporative cooling is well developed; at 40°C _Bassariscus astutus_ +is able to dissipate 100% of its resting metabolic heat via evaporative +water loss, and at 45°C it is able to dissipate 172% (Chevalier, 1985). +In spite of its great capacity for evaporative cooling, this species is +nocturnal, a habit that, along with its low [.H]_{b}, should allow it +to keep thermoregulatory water requirements to a minimum. + +_Procyon lotor._--Our data suggested that T_{uc} for _Procyon lotor_ in +winter was comparable to that for _Bassariscus astutus_ (35°C), and +that in summer it was even higher. When exposed to temperatures near +the upper end of its T_{n}, _Procyon lotor_ increased the gradient for +passive heat loss with a controlled rise in T_{b} (Figure 6). In summer +its capacity for passive heat loss was enhanced by the molt of its +heavy winter fur. _Procyon lotor_'s capacity for evaporative cooling +also appeared to be well developed, although our animals were not +heated to the point that evaporative cooling was fully expressed +(Figures 4, 5). However, _Procyon lotor_ is nocturnal, and this may +allow it to eliminate, or at least reduce, the need for evaporative +cooling, even in hot climates. Thus, _Procyon lotor_ appears to be +well equipped physiologically and behaviorally to cope with thermal +demands of hot environments in its distribution. + +_Procyon cancrivorus._--Unfortunately, data for the crab-eating +raccoon are not complete enough at high temperatures to include it in +this survey. + +SUMMARY.--This comparison demonstrates that capacity for evaporative +cooling, tolerance of an elevated T_{b} to enhance passive heat loss, +and behavioral avoidance of thermal stress are the primary methods used +by procyonids to thermoregulate at high temperatures. _Procyon lotor_ +and _Bassariscus astutus_, whose distributions extend into temperate +regions, have developed these abilities to a greater extent than other +procyonids. _Potos flavus_, whose distribution is confined to +lowland tropical forests, has the least ability in this regard. _Nasua +nasua_ and _Nasua narica_ appear to have thermoregulatory abilities +that are intermediate to those of _Bassariscus astutus_ and _Potos +flavus_. This suggests that ancestral procyonids may have had poor to +modest ability to thermoregulate at high temperatures, a condition that +would have limited their ability to leave the thermal stability +afforded by tropical forests. Dispersal into temperate climates, +therefore, required not only increased cold tolerance but also +selective enhancement of those mechanisms used in thermoregulation at +high temperatures. + + + TABLE 11.--Distribution by climate of selected procyonid species. + + -----------------------+--------------------------------------------- + | Mild[a] Cold[b] + Species | Tropics Subtropics temperate temperate + -----------------------+--------------------------------------------- + _Procyon lotor_ | + + + + + _Bassariscus astutus_ | + + + + _Nasua nasua_ | + + + _Nasua narica_ | + + + _Procyon cancrivorus_ | + + + _Potos flavus_ | + + -----------------------+--------------------------------------------- + + [a] Extends from the subtropics north to the northern limit of + _Bassariscus astutus_' distribution (Hall and Kelson, 1959:881), + which approximates the 10°C isotherm for average annual + temperature in the United States (Kincer, 1941). + + [b] Extends northward from the 10°C isotherm for average annual + temperature in the United States. + + + +COMPOSITE SCORES OF ADAPTIVE UNITS AND GEOGRAPHIC DISTRIBUTION + + +In Table 11, procyonid species are arranged in descending order with +respect to the number of major climates that are included in their +geographic distributions (Hall and Kelson, 1959:878-897; +Poglayen-Neuwall, 1975; Kortlucke and Ramirez-Pulido, 1982; Nowak and +Paradiso, 1983:977-985). Composite scores ranged from a high of 1.47 +for _Procyon lotor_ to a low of 0.39 for _Potos flavus_, whereas +_Nasua nasua_, _Nasua narica_, _Procyon cancrivorus_, and _Bassariscus +astutus_ had intermediate values ranging from 0.64 to 0.79 (Table 12). +Figure 8 demonstrates that there is a direct relationship between the +number of climates these species occupy and their composite scores. +Regression analysis (Y = 2.68·X + 0.24; where Y is number of climates, +and X is composite score) demonstrates a high degree of correlation +between these variables (R = 0.94) and indicates that 89% of the +variance in distribution can be explained by composite scores. The +various combinations of adaptations expressed by these species do, +therefore, play a role in delimiting their climatic (latitudinal) +distributions. + +_Procyon lotor's_ normalized scores were higher in all categories than +those of other procyonids. _Procyon lotor_, therefore, possesses those +traits that have allowed it to become the premier climate generalist +of the procyonid family. As an adaptive unit, these traits provide +_Procyon lotor_ with the physiological and behavioral flexibility +required to take full advantage of a wide range of climates and +habitats, and its distribution verifies that it has done so. Even so, +it is probably not fair to assume that this species represents a +perfect physiological match with climate over its entire distribution. +_Procyon lotor_ is, in many respects, still a forest-dwelling species, +and its ability to expand its distribution into other habitats such as +prairie and desert may well be due, in part, to its use of behavior to +take advantage of favorable microclimates in otherwise hostile +environments (Bartholomew, 1958, 1987). This feature of _Procyon +lotor's_ biology needs to be further examined. + + + TABLE 12.--Normalized and composite scores for selected procyonids. + (H_{br} = ratio of measured to predicted basal metabolism (Table 7), + C_{mwr} = ratio of measured to predicted minimum thermal conductance + (Table 7), D_{dr} = ratio of food categories actually utilized by + each species to total food categories eaten by all six species + (calculated from Table 9), r_{maxr} = ratio of calculated to + expected r_{max} (Table 10).) + + ----------------------+---------------------------------------------- + | Normalized scores + Species |---------------------------- Composite[a] + |H_{br}/C_{mwr} D_{dr} r_{maxr} score + ----------------------+---------------------------------------------- + _Procyon lotor_ | 0.95 0.95 2.52 1.47 + _Bassariscus astutus_ | 0.80 0.33 1.24 0.79 + _Nasua nasua_ | 0.48 0.33 1.11[b] 0.64 + _Nasua nasua_ | 0.48 0.33 1.11[b] 0.64 + _Nasua narica_ | 0.40 0.53 1.11 0.68 + _Procyon cancrivorus_ | 0.55 0.33 1.32 0.73 + _Potos flavus_ | 0.60 0.11 0.48 0.39 + ----------------------+---------------------------------------------- + + [a] Composite score = [(H_{br}/C_{mwr}) + D_{dr} + r_{maxr}]/3. + + [b] Value calculated for _Nasua narica_ (Table 10) and used with the + assumption that it must be similar to the value for _Nasua nasua_. + + +All five species with low [.H]_{b}'s have composite scores less than +1.0 (Table 12; Figure 8). Four of these five, _Nasua nasua_, _Nasua +narica_, _Procyon cancrivorus_, and _Potos flavus_, have +H_{br}/C_{mwr} ratios that are 0.6 or less, which indicates they are +the least cold-tolerant procyonids (McNab, 1966). These four species +also are confined to either tropic, or tropic and subtropic climates +(Table 11). This suggests that these species share a common +thermoregulatory adaptation that represents a specialization to these +climates. Attendant with this adaptation, however, is a high cost of +thermoregulation at temperatures below their T_{lc}, and this must be +an important factor in limiting their distributions to tropic and +subtropic climates. Differences in their distributions within these +climates, therefore, must hinge more on differences in their D_{dr} +and r_{maxr} values than on differences in their H_{br}/C_{mwr} +ratios. This is supported by the fact that _Potos flavus_, which has +the lowest D_{dr} and r_{maxr} values, is confined to a single +climate, whereas _Nasua nasua_, _Nasua narica_, and _Procyon +cancrivorus_ each possess larger D_{dr} and r_{maxr} values and are +found in two climates. Thus, _Potos flavus_, with its highly +specialized diet and low reproductive potential, is the most +ecologically specialized of these procyonids, and its distribution is +limited to the single climate that can provide its requirements. +_Nasua nasua_, _Nasua narica_, and _Procyon cancrivorus_ are less +specialized and thus show more ecological flexibility in their +distributions. + + [Illustration: FIGURE 8.--Relationship between number of climates + in which a species is found and its composite score. Symbols for + _Nasua nasua_ overlap at coordinates (0.64, 2). Solid line + represents linear regression of climates (Y) on composite scores + (X): Y = 2.68·X + 0.24; R = 0.94.] + +_Bassariscus astutus_, the other species with low [.H]_{b}, is found +in three climates, which indicates that it has greater ecological +flexibility than _Nasua nasua_, _Nasua narica_, or _Procyon +cancrivorus_. D_{dr} and r_{maxr} are comparable for these four +species (Table 12). This suggests that the greater ecological +flexibility of _Bassariscus astutus_ is derived largely from its +greater cold tolerance. _Bassariscus astutus_ has a more insulative +pelt than these other procyonids (C_{mwr} = 0.85; Table 7), so its +H_{br}/C_{mwr} ratio is higher (0.80; Table 12). This, and its greater +capacity for evaporative cooling (Chevalier, 1985), allows +_Bassariscus astutus_ to take advantage of a wider range of thermal +environments than these other species. However, even with its higher +H_{br}/C_{mwr} ratio, the composite score for _Bassariscus astutus_ +is not much different than those for _Nasua nasua_, _Nasua narica_, +and _Procyon cancrivorus_ (Table 12). Consequently, _Bassariscus +astutus_ is found in more climates than would be predicted for it on +the basis of its composite score (Figure 8). This suggests that either +the H_{br}/C_{mwr} ratio carries greater weight in determining +distribution than is reflected in this analysis, or as has been +described for some other species (Bartholomew, 1958, 1987), +_Bassariscus astutus_ may extend its distribution farther than +expected via use of its behavior. In either case, for procyonids with +low [.H]_{b}, _Bassariscus astutus_ represents the pinnacle of +adaptation for climate generalization. + + + +EVOLUTION OF METABOLIC ADAPTATIONS + + +_Evolution of Low Basal Metabolic Rate_ + +A radiation of frugivorous and omnivorous Procyoninae (Table 1) +occurred in the middle and late Miocene of North America. It included +origins of such terrestrial genera as _Cyonasua_, _Nasua_, and +_Procyon_ (Webb, 1985b). The earliest procyonid genus to find its way +to South America was _Cyonasua_, an omnivorous carnivore that +presumably split, along with its sister genus _Arctonasua_, from a +common North American ancestor (Baskin, 1982; Webb, 1985b). +_Cyonasua_, about the size of present-day raccoons, was adapted to a +wide range of habitats and was probably comparable to modern raccoons +with respect to the breadth of its feeding habits (Webb, 1985b; +Marshall, 1988). Because North American _Arctonasua_ was about the +same size as _Cyonasua_ (Webb, 1985b) and shared a number of +characters with it (Baskin, 1982), we speculate that it also may have +had similar habits and occupied similar climates and habitats. +_Bassariscus_, another member of Procyoninae, had an even earlier +origin in tropical North America (Webb, 1985b). The origin of the +small arboreal forms _Potos_ and _Bassaricyon_ (subfamily Potosinae) +is obscure but is thought to have occurred in the rainforests of +Central America (Webb, 1985b). What were the metabolic capabilities of +these early procyonids? We do not know, but for several million years, +from middle to late Miocene, procyonids lived in tropical and +subtropical forests of Central and North America (Webb, 1985b; +Marshall, 1988). Then, in the Pleistocene, several modern forms +crossed the Panamanian land bridge into similar habitats and climates +in South America; but none of them appear to have spread far enough +northward to have crossed the Bering land bridge. + +Several million years exposure to a tropical environment, with its +continuous high temperatures and modest range of thermal extremes, +would have favored selection of metabolic and thermoregulatory traits +that would minimize energy requirements: a lower than predicted basal +metabolic rate, a prolonged or continuous molt resulting in very +little annual change in minimum thermal conductance, and a modest +capacity for evaporative cooling. In addition, we would expect +selection to have favored a diverse diet, good reproductive potential, +and behavioral flexibility to utilize a variety of habitats within +these climates. Our analysis has shown that such characteristics are +the norm for extant members of this family living in tropical and +subtropical climates, and we speculate that these traits also were +common to early procyonids and served to restrict them to these +climates. Our speculation is supported by the fact that their known +fossil history from the Miocene is confined to geographic areas that +had tropical and subtropical climates. + +Later on, during Pleistocene glaciations, tropical and subtropical +forests shrank, savannas expanded, and temperate climate was pushed +toward equatorial regions. The opposite occurred during interglacial +periods (Raven and Axelrod, 1975; Webb, 1977, 1978; Marshall, 1988). +Consequently, mid-latitudes experienced alternating periods of +temperate and tropical, or at least subtropical, climate change. +Selection of characteristics that would have adapted a species with +low [.H]_{b} to temperate as well as tropic or subtropic climates +could have occurred in mid-latitudes at the temperate edge of these +tropical advances and retreats. Our analysis indicates that, for this +purpose, selection would have favored lower than predicted thermal +conductance, seasonal molt, increased capacity for evaporative +cooling, increased tolerance of elevated T_{b}, increased flexibility +of thermoregulatory behavior, food habits that provided for year-round +access to a high-quality diet in all three climates, and a higher than +predicted r_{max}. + +_Bassariscus astutus_ is the only species with low [.H]_{b} that has +all these characteristics, and it is the only one of them that has +added temperate climate to its distribution (Table 11). This suggests +that _Bassariscus astutus_ is a species that evolved away from the +norm for procyonids with low [.H]_{b}, toward characteristics that +allowed it to become more of a climate generalist. _Potos flavus_, +with its dietary specialization, low tolerance to high temperatures, +and arboreal mode of existence, has become a highly specialized +species totally dependent on tropical forests for its survival. As +such, it also represents a species that has evolved away from the +procyonid norm and portrays the extreme in climate specialization. +Olingos, _Bassaricyon gabbii_ (Table 1), may be similar to _Potos +flavus_ in this respect (see also Table 10). This suggests that of the +extant procyonids, _Nasua nasua_, _Nasua narica_, and _Procyon +cancrivorus_ have retained metabolic and behavioral characteristics +that are closest to those of their Miocene ancestors. + + +_Evolution of High Basal Metabolic Rate_ + +Between the time that _Cyonasua_ appeared and the Panamanian land +bridge was established in the upper Pliocene (4 to 5 million years +ago), northern climates continued their gradual cooling. This, along +with ongoing elevation of the continents and continuous modification +of their mountain ranges, served to shrink the tropical forest and +create pockets of climatic instability within it and on its edges +(Darlington, 1963:578-596; Marshall, 1988). In areas of instability, +selection would have favored traits that provided for a broader range +of thermal tolerance: higher [.H]_{b}, improved insulative quality of +pelt, a more sharply defined molt cycle, improved capacity for +evaporative cooling, greater D_{d}, and higher r_{max}. Consequently, +by the upper Pliocene, two metabolically distinct groups of procyonids +could have been established: those species with low [.H]_{b} living in +climatically stable forests and those with higher [.H]_{b} living in +unstable tropical, subtropical, and perhaps temperate climates. + +_Procyon lotor_ is the only extant procyonid with high [.H]_{b}. +_Procyon cancrivorus_ is its congeneric counterpart in Central and +South America (Table 1), and the two species are sympatric in Panama +and Costa Rica. However, in terms of its metabolism, thermal +conductance, molt, diversity of diet, r_{max}, and climatic +distribution, _Procyon cancrivorus_ shares more in common with other +procyonids than it does with _Procyon lotor_ (Tables 7, 11, 12; Figure +8). This suggests that metabolically _Procyon lotor_ portrays a +divergent line of this genus that arose as the result of a series of +mutations that gave rise to different metabolic characteristics. This +view is in keeping with a recent phylogenetic analysis of this family +that shows the genus _Procyon_ to be highly derived (Decker and +Wozencraft, 1991). Consequently, it would be instructive and would add +to our knowledge of the evolution of climatic adaptation to know more +about the genetic relatedness of these two species as well as their +historical relationship. + +Genus _Procyon_ appears in the fossil record (Hemphillian and Blancan +ages; Baskin, 1982) prior to Pleistocene glaciations. During the +Pleistocene, there were four different glacial advances and retreats +in a relatively short time period (the first appearing little more +than a million years ago; Darlington, 1963:578-596; Webb, 1985a; +Marshall, 1988). Glacial retreats created pulses of time during which +subtropic and temperate climates advanced toward the poles into areas +with large seasonal differences in light/dark cycles, whereas glacial +advances pushed these climates southward into areas having smaller +seasonal differences in light/dark cycles (Raven and Axelrod, 1975; +Webb, 1977, 1978; Marshall, 1988). Those members of the genus +_Procyon_ caught in these wide latitudinal fluctuations would have +experienced conditions favorable to continued selection for +characteristics conducive to physiologic adaptation to a wide range of +climatic conditions. _Procyon lotor_ is the only member of its genus +to have survived this selective process, and as we have seen, it does +possess traits that adapt it to a wide range of climatic conditions. +Primary among these is its higher [.H]_{b}, which provides it with +advantages not shared with other procyonids (see earlier discussion). +Three other adaptations also have had a profound influence on _Procyon +lotor_'s ability to generalize its use of climate: (1) the increased +insulative quality of its pelt coupled with its sharply defined molt +cycle, which allows for a large annual change in thermal conductance; +(2) its annual cycle of fat storage; and (3) a diverse high-quality +diet. The first two of these adaptations required evolution of +neuroendocrine pathways capable of responding to time-dependent +environmental cues such as changing day length, changing temperature, +etc. Such conditions would have been available as selective stimuli in +high-latitude forests and savannas of interglacial periods. _Procyon +lotor_'s elevated basal metabolic rate would have increased its +overall energy requirement, and it makes good intuitive sense, +therefore, that evolution during the Pleistocene also would have +favored selection of a diverse diet containing many items of high +nutritive value. + + + +SUMMARY + + +Our analysis has illustrated that within Procyonidae there are two +distinct modes of metabolic adaptation to climate. One is typified by +those species with low [.H]_{b}'s (_Bassariscus astutus_, _Nasua +nasua_, _Nasua narica_, _Procyon cancrivorus_, and _Potos flavus_), +and the other by _Procyon lotor_ with its higher [.H]_{b}. Those with +low [.H]_{b}'s have more restricted geographic distributions, and, +with the exception of _Bassariscus astutus_, they are all confined to +tropical and subtropical areas. The fossil history of this family +indicates that it had its origins in tropical forests of North and +Central America. This indicates that those procyonids whose +distributions are still primarily restricted to tropical forests share +many of the metabolic adaptations characteristic of their ancestors. +We speculate, therefore, that ancestral procyonids had a lower than +predicted [.H]_{b}, a pelt with modest to poor insulative quality, +good thermogenic ability but poor heat tolerance, modest to poor +capacity for evaporative cooling, no well-defined molt cycle, no +cyclic period of fattening, nocturnal habits, and a modestly diverse +diet of high-enough quality to provide for an average reproductive +potential. Although this pedigree contributed to the success of this +family in tropical and subtropical forests, it limited the ability of +its members to expand their distributions into cooler, less stable +climates. Viewed in this perspective, _Procyon lotor_'s high basal +metabolic rate, extraordinarily diverse diet, well-defined cyclic +changes in fat content and thermal conductance, high level of heat +tolerance, high capacity for evaporative cooling, and high +reproductive potential all stand out in sharp contrast to the +condition described for other procyonids. This suggests that the North +American raccoon represents culmination of a divergent evolutionary +event that has given this species the ability to break out of the old +procyonid mold and carry the family into new habitats and climates. + + + + +APPENDIX: LIST OF SYMBOLS + + + a potential age of females first producing young + + b potential annual birth rate of female young + + C_{a} conductance of air + + C_{d} conductance of den walls + + C_{m} minimum thermal conductance + + C_{md} minimum dry thermal conductance + + C_{mw} minimum wet thermal conductance + + C_{mwr} ratio of measured to predicted minimum wet thermal + conductance + + C_{t} total conductance + + D_{d} diversity of diet + + D_{dr} ratio of food categories actually used by a species to + the total number of food categories taken by all species + tested + + [.E] evaporative water loss + + E_{c} ratio of evaporative heat lost to metabolic heat produced + + [.E]_{eq} oxygen equivalent for heat lost by evaporation + + [.H]_{b} basal metabolic rate + + [.H]_{r} lowest resting metabolic rate at each temperature + + H_{br} ratio of measured to predicted basal metabolic rate + + m mass of animal + + m_{w} mass of water + + n potential age of females producing their final young + + r_{max} intrinsic rate of natural increase + + r_{maxe} expected intrinsic rate of natural increase + + r_{maxr} ratio of calculated to expected intrinsic rate of natural + increase + + RQ respiratory quotient + + T_{a} chamber air temperature + + T_{b} body temperature + + T_{lc} lower critical temperature + + T_{n} thermoneutral zone + + T_{uc} upper critical temperature + + t time + + [.V]_{a} rate of air flow through U-tubes + + [.V]_{e} rate of air flow into metabolism chamber + + [alpha] active phase of the daily cycle + + [gamma] heat equivalent of oxygen + + [lambda] heat of vaporization of water + + [rho] rest phase of the daily cycle + + + + +LITERATURE CITED + + + Aschoff, Jürgen + + 1981. 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Mugaas and John Seidensticker and Kathleen P. Mahlke-Johnson + +This eBook is for the use of anyone anywhere at no cost and with +almost no restrictions whatsoever. You may copy it, give it away or +re-use it under the terms of the Project Gutenberg License included +with this eBook or online at www.gutenberg.org + + +Title: Metabolic Adaptation to Climate and Distribution of the Raccoon Procyon Lotor and Other Procyonidae + +Author: John N. Mugaas + John Seidensticker + Kathleen P. Mahlke-Johnson + +Release Date: May 5, 2011 [EBook #36036] + +Language: English + +Character set encoding: ISO-8859-1 + +*** START OF THIS PROJECT GUTENBERG EBOOK THE RACCOON PROCYON LOTOR *** + + + + +Produced by Colin Bell, Tom Cosmas, Joseph Cooper and the +Online Distributed Proofreading Team at http://www.pgdp.net + + + + + + +</pre> + + + + + + +<p><span class="pagenum"><a name="Cover" id="Cover">[Cover]</a></span></p> +<br> +<div class="center"> + <img src="images/cover.png" width="448" height="570" title="cover" alt="cover"> +</div> +<br> +<br> +<br> + +<p><span class="pagenum"><a name="Page_i" id="Page_i">[Pg i]</a></span></p> + +<div class="caption3 gesperrt">SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY · NUMBER 542</div> + +<div class="caption1">Metabolic Adaptation to Climate<br> +and Distribution of the Raccoon<br> +<i>Procyon lotor</i> and Other Procyonidae</div> + +<div class="caption2"><i>John N. Mugaas, John Seidensticker,<br> +and Kathleen P. Mahlke-Johnson</i></div> + +<div class="center"> +<img src="images/smithsonian_logo.png" width="92" height="87" title="" alt=""> +<br> + +SMITHSONIAN INSTITUTION PRESS<br> +Washington, D.C.<br> +1993 +</div> + + +<br> +<br> +<p><span class="pagenum"><a name="Page_ii" id="Page_ii">[Pg ii]</a></span></p> + +<div class="caption2">ABSTRACT</div> + + +<p class="noidt">Mugaas, J. N., J. Seidensticker, and K. Mahlke-Johnson. Metabolic Adaptation to Climate and +Distribution of the Raccoon <i>Procyon lotor</i> and Other Procyonidae. <i>Smithsonian Contributions +to Zoology</i>, number 542, 34 pages, 8 figures, 12 tables, 1993.—Although the family +Procyonidae is largely a Neotropical group, the North American raccoon, <i>Procyon lotor</i>, is more +versatile in its use of climate, and it is found in nearly every habitat from Panama to 60°N in +Canada. We hypothesized that most contemporary procyonids have remained in tropic and +subtropic climates because they have retained the metabolic characteristics of their +warm-adapted ancestors, whereas <i>Procyon lotor</i> evolved a different set of adaptations that have +enabled it to generalize its use of habitats and climates. To test this hypothesis we compared +<i>Procyon lotor</i> with several other procyonids (<i>Bassariscus astutus</i>, <i>Nasua nasua</i>, <i>Nasua narica</i>, +<i>Procyon cancrivorus</i>, and <i>Potos flavus</i>) with respect to (1) basal metabolic rate (Ḣ<sub>b</sub>), (2) +minimum wet thermal conductance (C<sub>mw</sub>), (3) diversity of diet (D<sub>d</sub>), (4) intrinsic rate of natural +increase (r<sub>max</sub>), and, where possible, (5) capacity for evaporative cooling (E<sub>c</sub>). We measured +basal and thermoregulatory metabolism, evaporative water loss, and body temperature of both +sexes of <i>Procyon lotor</i> from north central Virginia, in summer and winter. Metabolic data for +other procyonids were from literature, as were dietary and reproductive data for all species.</p> + +<p>Procyon lotor differed from other procyonids in all five variables. (1) <i>Procyon lotor</i>'s mass +specific Ḣ<sub>b</sub> (0.46 mL O<sub>2</sub>·g<sup>-1</sup>·h<sup>-1</sup>) was 1.45 to 1.86 times greater than values for other procyonids. +(2) Because of its annual molt, <i>Procyon lotor</i>'s C<sub>mw</sub> was about 49% higher in summer than +winter, 0.0256 and 0.0172 mL O<sub>2</sub>·g<sup>-1</sup>·h<sup>-1</sup>·°C<sup>-1</sup>, respectively. The ratio of measured to predicted +C<sub>mw</sub> for <i>Procyon lotor</i> in winter (1.15) was similar to values calculated for <i>Potos flavus</i> (1.02) +and <i>Procyon cancrivorus</i> (1.25). Values for other procyonids were higher than this, but less than +the value for <i>Procyon lotor</i> (1.76) in summer. On a mass specific basis, <i>Bassariscus astutus</i> had +the lowest C<sub>mw</sub> with a ratio of 0.85. (3) <i>Procyon lotor</i> utilized three times as many food +categories as <i>Procyon cancrivorus</i>, <i>Nasua nasua</i>, and <i>Bassariscus astutus</i>; about two times as +many as <i>Nasua narica</i>; and nine times as many as <i>Potos flavus</i>. (4) Intrinsic rate of natural +increase correlated positively with Ḣ<sub>b</sub>. <i>Procyon lotor</i> had the highest r<sub>max</sub> (2.52 of expected) and +<i>Potos flavus</i> the lowest (0.48 of expected). The other procyonids examined also had low Ḣ<sub>b</sub>, but +their r<sub>max</sub>'s were higher than predicted (1.11-1.32 of expected). Early age of first female +reproduction, fairly large litter size, long life span, high-quality diet, and, in one case, female +social organization all compensated for low Ḣ<sub>b</sub> and elevated r<sub>max</sub>. (5) Although data on the +capacity for evaporative cooling were incomplete, this variable appeared to be best developed in +<i>Procyon lotor</i> and <i>Bassariscus astutus</i>, the two species that have been most successful at +including temperate climates in their distributions.</p> + +<p>These five variables are functionally interrelated, and have co-evolved in each species to form +a unique adaptive unit that regulates body temperature and energy balance throughout each +annual cycle. The first four variables were converted into normalized dimensionless numbers, +which were used to derive a composite score that represented each species' adaptive unit. +<i>Procyon lotor</i> had the highest composite score (1.47) and <i>Potos flavus</i> the lowest (0.39). Scores +for the other procyonids were intermediate to these extremes (0.64-0.79). There was a positive +correlation between the number of climates a species occupies and the magnitude of its +composite score. Linear regression of this relationship indicated that 89% of the variance in +climatic distribution was attributed to the composite scores. Differences in metabolic adaptation, +therefore, have played a role in delimiting climatic distribution of these species.</p> + +<p>It was clear that <i>Procyon lotor</i> differed from the other procyonids with respect to +thermoregulatory ability, diet, and reproductive potential. These differences have enabled it to +become a highly successful climate generalist, and its evolution of an Ḣ<sub>b</sub> that is higher than the +procyonid norm appears to be the cornerstone of its success.</p> +<br> + +<p class="noidt"> +<span class="smcap">Official publication date</span> is handstamped in a limited number of initial copies and is +recorded in the Institution's annual report, <i>Smithsonian Year</i>. <span class="smcap">Series cover design:</span> The coral +<i>Montastrea cavernosa</i> (Linnaeus).</p> + +<hr> +<p class="noidt"><small> +Library of Congress Cataloging-in-Publication Data<br> + +Mugaas, John N.<br> + +Metabolic adaptation to climate and distribution of the raccoon Procyon lotor and other Procyonidae / John N. Mugaas, +John Seidensticker, and Kathleen P. Mahlke-Johnson.<br> + +p. cm.—(Smithsonian contributions to zoology; no. 542)<br> + +Includes bibliographical references (p. )<br> + +1. Raccoons-Metabolism-Climatic factors. 2. Procyonidae-Metabolism-Climatic factors. 3. Raccoons-Geographical +distribution. 4. Procyonidae-Geographical distribution. I. Seidensticker, John. II. Mahlke-Johnson, +Kathleen. III. Title. IV. Series.<br> + +QL1.S54 no. 542 [QL737.C26] 591 s-dc20 [599.74´443´04542] 93-3119</small><br> +<br> + +<img src="images/perm_paper.png" width="22" height="21" title="permanent paper" alt="permanent paper"> The paper used in this publication meets the minimum requirements of the American +National Standard for Permanence of Paper for Printed Library Materials z39.48—1984.</p> + + +<br> +<br> +<p><span class="pagenum"><a name="Page_iii" id="Page_iii">[Pg iii]</a></span></p> +<a name="toc"></a> +<div class="caption2">Contents</div> + +<table width="100%" summary="ToC"> +<tr><td> </td><td class="text_rt"><i>Page</i></td></tr> +<tr><td><a href="#Introduction">Introduction</a></td><td class="text_rt">1</td></tr> +<tr><td> <a href="#Defining_the_Problem">Defining the Problem</a></td><td class="text_rt">1</td></tr> +<tr><td> <a href="#Procyonid_Origins">Procyonid Origins</a></td><td class="text_rt">1</td></tr> +<tr><td> <a href="#Typical_Procyonids">Typical Procyonids</a></td><td class="text_rt">2</td></tr> +<tr><td> <a href="#The_Atypical_Procyonid">The Atypical Procyonid</a></td><td class="text_rt">3</td></tr> +<tr><td> <a href="#The_Hypothesis">The Hypothesis</a></td><td class="text_rt">4</td></tr> +<tr><td> <a href="#Hypothesis_Testing">Hypothesis Testing</a></td><td class="text_rt">4</td></tr> +<tr><td> <a href="#Adaptive_Significance">Adaptive Significance of the Variables</a></td><td class="text_rt">4</td></tr> +<tr><td> <a href="#Basal_Metabolic_Rate_1">Basal Metabolic Rate and Intrinsic Rate of Natural Increase</a></td><td class="text_rt">4</td></tr> +<tr><td> <a href="#Minimum_Thermal_Conductance_1">Minimum Thermal Conductance</a></td><td class="text_rt">4</td></tr> +<tr><td> <a href="#Capacity_for_Evaporative_Cooling">Capacity for Evaporative Cooling</a></td><td class="text_rt">5</td></tr> +<tr><td> <a href="#Diet">Diet</a></td><td class="text_rt">5</td></tr> +<tr><td> <a href="#Experimental_Design_and_Summary">Experimental Design and Summary</a></td><td class="text_rt">5</td></tr> +<tr><td> <a href="#Acknowledgments">Acknowledgments</a></td><td class="text_rt">5</td></tr> +<tr><td><a href="#Materials_and_Methods">Materials and Methods</a></td><td class="text_rt">6</td></tr> +<tr><td> <a href="#Live-trapping">Live-trapping</a></td><td class="text_rt">6</td></tr> +<tr><td> <a href="#Metabolic_Studies">Metabolic Studies</a></td><td class="text_rt">6</td></tr> +<tr><td> <a href="#Basal_and_Thermoregulatory_Metabolism">Basal and Thermoregulatory Metabolism</a></td><td class="text_rt">6</td></tr> +<tr><td> <a href="#Evaporative_Water_Loss_1">Evaporative Water Loss</a></td><td class="text_rt">7</td></tr> +<tr><td> <a href="#Body_Temperature">Body Temperature</a></td><td class="text_rt">7</td></tr> +<tr><td> <a href="#Calibrations">Calibrations</a></td><td class="text_rt">7</td></tr> +<tr><td> <a href="#Calorimeter">Calorimeter</a></td><td class="text_rt">7</td></tr> +<tr><td> <a href="#Body_Temperature_Transmitters">Body Temperature Transmitters</a></td><td class="text_rt">8</td></tr> +<tr><td> <a href="#Statistical_Methods">Statistical Methods</a></td><td class="text_rt">8</td></tr> +<tr><td> <a href="#Estimating_Intrinsic_Rate">Estimating Intrinsic Rate of Natural Increase</a></td><td class="text_rt">8</td></tr> +<tr><td> <a href="#Comparison_of_Adaptive_Units">Comparison of Adaptive Units</a></td><td class="text_rt">8</td></tr> +<tr><td><a href="#Results">Results</a></td><td class="text_rt">8</td></tr> +<tr><td> <a href="#Body_Mass">Body Mass</a></td><td class="text_rt">8</td></tr> +<tr><td> <a href="#Basal_Metabolic_Rate_2">Basal Metabolic Rate</a></td><td class="text_rt">9</td></tr> +<tr><td> <a href="#Minimum_Thermal_Conductance_2">Minimum Thermal Conductance</a></td><td class="text_rt">9</td></tr> +<tr><td> <a href="#Evaporative_Water_Loss_2">Evaporative Water Loss</a></td><td class="text_rt">11</td></tr> +<tr><td> <a href="#Thermoregulation_at_Low_Temperatures">Thermoregulation at Low Temperatures</a></td><td class="text_rt">12</td></tr> +<tr><td> <a href="#Body_Temperature_1">Body Temperature</a></td><td class="text_rt">12</td></tr> +<tr><td> <a href="#Summer_1">Summer</a></td><td class="text_rt">14</td></tr> +<tr><td> <a href="#Winter_1">Winter</a></td><td class="text_rt">14</td></tr> +<tr><td> <a href="#Thermoregulation_at_High_Temperatures">Thermoregulation at High Temperatures</a></td><td class="text_rt">16</td></tr> +<tr><td> <a href="#Body_Temperature_2">Body Temperature</a></td><td class="text_rt">16</td></tr> +<tr><td> <a href="#Summer_2">Summer</a></td><td class="text_rt">16</td></tr> +<tr><td> <a href="#Winter_2">Winter</a></td><td class="text_rt">16</td></tr> +<tr><td> <a href="#Daily_Cycle_of_Body_Temperature">Daily Cycle of Body Temperature</a></td><td class="text_rt">16</td></tr> +<tr><td><a href="#Discussion">Discussion</a></td><td class="text_rt">16</td></tr> +<tr><td> <a href="#Basal_Metabolic_Rate_3">Basal Metabolic Rate</a></td><td class="text_rt">16</td></tr> +<tr><td> <a href="#Background_1">Background</a></td><td class="text_rt">16</td></tr> +<tr><td> <a href="#Captive_versus_Wild_Raccoons">Captive versus Wild Raccoons</a></td><td class="text_rt">17</td></tr> +<tr><td> <a href="#Seasonal_Metabolism_of_Raccoons">Seasonal Metabolism of Raccoons</a></td><td class="text_rt">17</td></tr> +<tr><td> <a href="#Comparison">Comparison of <i>Procyon lotor</i> with Other Procyonids</a></td><td class="text_rt">17</td></tr> +<tr><td> <a href="#Influence_of_Diet">Influence of Diet on Basal Metabolism</a><span class="pagenum"><a name="Page_iv" id="Page_iv">[Pg iv]</a></span></td><td class="text_rt">18</td></tr> +<tr><td> <a href="#Background_2">Background</a></td><td class="text_rt">18</td></tr> +<tr><td> <a href="#Food_Habits_of_Procyonids">Food Habits of Procyonids</a></td><td class="text_rt">18</td></tr> +<tr><td> <a href="#Food_Habits_and_Basal_Metabolism">Food Habits and Basal Metabolism</a></td><td class="text_rt">19</td></tr> +<tr><td> <a href="#Summary_1">Summary</a></td><td class="text_rt">19</td></tr> +<tr><td> <a href="#Basal_Metabolism_1">Basal Metabolism and Intrinsic Rate of Natural Increase</a></td><td class="text_rt">19</td></tr> +<tr><td> <a href="#Background_3">Background</a></td><td class="text_rt">19</td></tr> +<tr><td> <a href="#Procyon_lotor_1"><i>Procyon lotor</i></a></td><td class="text_rt">19</td></tr> +<tr><td> <a href="#Bassariscus_astutus_1"><i>Bassariscus astutus</i></a></td><td class="text_rt">19</td></tr> +<tr><td> <a href="#Nasua_narica_1"><i>Nasua narica</i></a></td><td class="text_rt">19</td></tr> +<tr><td> <a href="#Nasua_nasua_1"><i>Nasua nasua</i></a></td><td class="text_rt">20</td></tr> +<tr><td> <a href="#Procyon_cancrivorus_1"><i>Procyon cancrivorus</i></a></td><td class="text_rt">20</td></tr> +<tr><td> <a href="#Potos_flavus_1"><i>Potos flavus</i></a></td><td class="text_rt">20</td></tr> +<tr><td> <a href="#Summary_2">Summary</a></td><td class="text_rt">20</td></tr> +<tr><td> <a href="#Basal_Metabolism_2">Basal Metabolism and Climatic Distribution</a></td><td class="text_rt">21</td></tr> +<tr><td> <a href="#Procyon_lotor_2"><i>Procyon lotor</i></a></td><td class="text_rt">21</td></tr> +<tr><td> <a href="#Other_Procyonids">Other Procyonids</a></td><td class="text_rt">21</td></tr> +<tr><td> <a href="#Minimum_Thermal_Conductance_3">Minimum Thermal Conductance</a></td><td class="text_rt">21</td></tr> +<tr><td> <a href="#Background_4">Background</a></td><td class="text_rt">21</td></tr> +<tr><td> <a href="#Effect_of_Molt_on_Thermal_Conductance">Effect of Molt on Thermal Conductance</a></td><td class="text_rt">21</td></tr> +<tr><td> <a href="#Comparison_of_Thermal_Conductances">Comparison of Thermal Conductances</a></td><td class="text_rt">22</td></tr> +<tr><td> <a href="#Procyon_lotor_versus_Tropical_Procyonids"><i>Procyon lotor</i> versus Tropical Procyonids</a></td><td class="text_rt">22</td></tr> +<tr><td> <a href="#Bassariscus_astutus_2"><i>Bassariscus astutus</i></a></td><td class="text_rt">22</td></tr> +<tr><td> <a href="#Thermoregulation_1">Thermoregulation and Use of Stored Fat at Low Temperatures</a></td><td class="text_rt">22</td></tr> +<tr><td> <a href="#Background_5">Background</a></td><td class="text_rt">22</td></tr> +<tr><td> <a href="#Thermoregulation_2">Thermoregulation</a></td><td class="text_rt">22</td></tr> +<tr><td> <a href="#Stored_Fat">Stored Fat</a></td><td class="text_rt">23</td></tr> +<tr><td> <a href="#Thermal_Model">Thermal Model of the Raccoon and Its Den</a></td><td class="text_rt">23</td></tr> +<tr><td> <a href="#Metabolic_Advantage_of_the_Den">Metabolic Advantage of the Den</a></td><td class="text_rt">23</td></tr> +<tr><td> <a href="#Thermoregulation_3">Thermoregulation at High Temperatures</a></td><td class="text_rt">24</td></tr> +<tr><td> <a href="#Background_6">Background</a></td><td class="text_rt">24</td></tr> +<tr><td> <a href="#Comparison_of_Procyonid_Responses">Comparison of Procyonid Responses to Heat Stress</a></td><td class="text_rt">24</td></tr> +<tr><td> <a href="#Potos_flavus_2"><i>Potos flavus</i></a></td><td class="text_rt">24</td></tr> +<tr><td> <a href="#Nasua_nasua_2"><i>Nasua nasua and Nasua narica</i></a></td><td class="text_rt">24</td></tr> +<tr><td> <a href="#Bassariscus_astutus_3"><i>Bassariscus astutus</i></a></td><td class="text_rt">24</td></tr> +<tr><td> <a href="#Procyon_lotor_3"><i>Procyon lotor</i></a></td><td class="text_rt">24</td></tr> +<tr><td> <a href="#Procyon_cancrivorus_2"><i>Procyon cancrivorus</i></a></td><td class="text_rt">24</td></tr> +<tr><td> <a href="#Summary_3">Summary</a></td><td class="text_rt">24</td></tr> +<tr><td> <a href="#Composite_Scores">Composite Scores of Adaptive Units and Geographic Distribution</a></td><td class="text_rt">25</td></tr> +<tr><td> <a href="#Evolution_of_Metabolic_Adaptations">Evolution of Metabolic Adaptations</a></td><td class="text_rt">26</td></tr> +<tr><td> <a href="#Evolution_of_Low_Basal_Metabolic_Rate">Evolution of Low Basal Metabolic Rate</a></td><td class="text_rt">26</td></tr> +<tr><td> <a href="#Evolution_of_High_Basal_Metabolic_Rate">Evolution of High Basal Metabolic Rate</a></td><td class="text_rt">27</td></tr> +<tr><td> <a href="#Summary_4">Summary</a></td><td class="text_rt">28</td></tr> +<tr><td><a href="#Appendix_List_of_Symbols">Appendix: List of Symbols</a></td><td class="text_rt">29</td></tr> +<tr><td><a href="#Literature_Cited">Literature Cited</a></td><td class="text_rt">30</td></tr> +</table> + + +<br> +<br> +<p><span class="pagenum"><a name="Page_1" id="Page_1">[Pg 1]</a></span></p> + +<div class="caption2">Metabolic Adaptation to Climate<br> +and Distribution of the Raccoon<br> +<i>Procyon lotor</i> and Other Procyonidae</div> + +<div class="caption4">John N. Mugaas, John Seidensticker,<br> +and Kathleen P. Mahlke-Johnson</div> + +<div class="blockquot"> +<i>John N. Mugaas, Department of Physiology, Division of Functional +Biology, West Virginia School of Osteopathic Medicine, Lewisburg, +West Virginia 24901. John Seidensticker and Kathleen P. Mahlke-Johnson, +National Zoological Park, Smithsonian Institution, Washington, +D.C. 20008.</i> +</div> + + +<br> +<a name="Introduction"></a> +<a name="Defining_the_Problem"></a> +<a name="Procyonid_Origins"></a> +<span class="pagenum"><a href="#toc">[↑ TOC]</a></span> +<div class="caption2">Introduction</div> +<div class="caption3"><span class="smcap">Defining the Problem</span></div> +<div class="caption4">Procyonid Origins</div> + +<p>The major carnivore radiations took place about 40 million +years before present (MYBP) in the late Eocene and early +Oligocene (Ewer, 1973:363; Wayne et al., 1989). Between 30 +and 40 MYBP, a progenitor split into the ursid and procyonid +lineages, which evolved into present-day bears, pandas, and +raccoons (Wayne et al., 1989). The taxonomic relatedness of +pandas to bears and raccoons has been tested extensively and a +number of authors have summarized current thinking on the +problem (Martin, 1989; Wayne et al., 1989; Wozencraft, +1989a, 1989b; Decker and Wozencraft, 1991). Davis +(1964:322-327) and others (Leone and Wiens, 1956; Todd and +Pressman, 1968; Sarich, 1976; O'Brien et al., 1985) place the +giant panda, <i>Ailuropoda melanoleuca</i>, with the ursids. The +taxonomic status of the red panda, <i>Ailurus fulgens</i>, appears to +be less certain. Some current investigations align the red panda +with bears (Segall, 1943; Todd and Pressman, 1968; Hunt, +1974; Ginsburg, 1982; Wozencraft, 1984:56-110; 1989a), +whereas others place them intermediate to procyonids and +bears (Wurster and Benirschke, 1968; Sarich, 1976; O'Brien et +al., 1985), or in close relationship to the giant panda (Tagle et +al., 1986).</p> + +<p>The procyonid radiation took place in North America and +produced forms that were mostly arboreal and omnivorous +(Eisenberg, 1981:122; Martin, 1989). The center of this +diversification occurred in Middle America (Baskin, 1982; +Webb, 1985b) during the Miocene (Darlington, 1963:367; +Webb, 1985b). Fossil procyonids from the late Miocene are +represented in Florida, California, Texas, Nebraska, Kansas, +and South Dakota (Baskin, 1982; Martin, 1989) and include +such genera as <i>Bassariscus</i>, <i>Arctonasua</i>, <i>Cyonasua</i>, <i>Paranasua</i>, +<i>Nasua</i>, and <i>Procyon</i> (Baskin, 1982; Webb, 1985b). +During the Miocene procyonids underwent a modest radiation +within tropical and subtropical climates of North America's +central and middle latitudes. <i>Cyonasua</i>, which has close +affinities to <i>Arctonasua</i> (Baskin, 1982), appears in tropical +South America in the late Miocene and immigrated there either +by rafting across the Bolivar Trough or by island-hopping +through the Antilles archipelagoes (Marshall et al., 1982; +Marshall, 1988). Thus, procyonids were found on both +continents prior to formation of the Panamanian land bridge +(Darlington, 1963:367, 395; Marshall et al., 1982; Marshall, +1988). Origins of <i>Bassaricyon</i> and <i>Potos</i> are obscure but +probably occurred in tropical rainforests of Middle America +(Baskin, 1982; Webb, 1985b). A subsequent Pleistocene +dispersal carried several modern genera (<a href="#Table_1">Table 1</a>) across the +Panamanian land bridge into South America (Webb, 1985b). +<i>Bassariscus</i> and <i>Bassaricyon</i> represent the most primitive +genera in Procyoninae and Potosinae subfamilies, respectively +(<a href="#Table_1">Table 1</a>; Wozencraft, 1989a; Decker and Wozencraft, 1991).</p> + +<p>In the early Tertiary, mid-latitudes of North America were +much warmer than they are now, but not fully tropical, and +temperate deciduous forests, associated with strongly seasonal +climates, occurred only in the far north (Barghoorn, 1953; +Colbert, 1953; Darlington, 1963:589, 590). Major climatic +deteriorations, with their attendant cooling of northern continents, +occurred during the Eo-Oligocene transition, in the +middle Miocene, at the end of the Miocene, and at about 3 +MYBP (late Pliocene). This last deterioration corresponds with +closure of the Panamanian isthmus (Berggren, 1982; Webb, +<span class="pagenum"><a name="Page_2" id="Page_2">[Pg 2]</a></span> +1985a). Climatic deterioration went on at an accelerating rate +during the late Tertiary, with glacial conditions developing at +the poles by the mid-Pliocene (Barghoorn, 1953). Therefore, +throughout the Tertiary, as continents cooled, northern climate +zones moved toward the tropics (Barghoorn, 1953; Colbert, +1953; Darlington, 1963:589, 590, 594, 595; Webb, 1985a).</p> + +<a name="Table_1"></a> +<div class="tab_cap"> +<span class="smcap">Table 1.</span>—Classification of recent Procyonidae after Wozencraft (1989a) and Decker and Wozencraft (1991). Information in parenthesis indicates general geographic distribution (modified from Kortlucke and Ramirez-Pulido (1982) and Poglayen-Neuwall (1975)): S.A. = South America; C.A. = Central America; M. = Mexico; U.S. = United States; C. = Canada. Lower case letters preceding geographic areas signify north (n), south (s), and west (w). +</div> + +<div class="blockquot"><p class="noidt"> +Order <span class="smcap">Carnivora</span> Bowdich, 1821<br> +<span style="margin-left: 1em;">Suborder <span class="smcap">Caniformia</span> Kretzoi, 1945</span><br> +<span style="margin-left: 2em;">Family <span class="smcap">Procyonidae</span> Gray, 1825</span><br> +<span style="margin-left: 3em;">Subfamily <span class="smcap">Potosinae</span> Trouessart, 1904</span><br> +<span style="margin-left: 4em;">Genus <i>Potos</i> E. Geoffroy and G. Cuvier, 1795</span><br> +<span class="m_left5"><i>P. flavus</i> (S.A., C.A., M.)</span><br> +<span style="margin-left: 4em;">Genus <i>Bassaricyon</i> Allen, 1876</span><br> +<span class="m_left5"><i>B. alleni</i><a name="FNanchor_A_2" id="FNanchor_A_2"></a><a href="#Footnote_A_2" class="fnanchor">[A]</a> (S.A.)</span><br> +<span class="m_left5"><i>B. beddardi</i><a href="#Footnote_A_2" class="fnanchor">[A]</a> (S.A.)</span><br> +<span class="m_left5"><i>B. gabbii</i><a href="#Footnote_A_2" class="fnanchor">[A]</a> (nS.A., C.A.)</span><br> +<span class="m_left5"><i>B. lasius</i><a href="#Footnote_A_2" class="fnanchor">[A]</a> (C.A.)</span><br> +<span class="m_left5"><i>B. pauli</i><a href="#Footnote_A_2" class="fnanchor">[A]</a> (C.A.)</span><br> +<span style="margin-left: 3em;">Subfamily <span class="smcap">Procyoninae</span> Gray, 1825</span><br> +<span style="margin-left: 4em;">Genus <i>Bassariscus</i> Coues, 1887</span><br> +<span class="m_left5"><i>B. astutus</i> (M., wU.S.)</span><br> +<span class="m_left5"><i>B. sumichrasti</i> (C.A., M.)</span><br> +<span style="margin-left: 4em;">Genus <i>Nasua</i> Storr, 1780</span><br> +<span class="m_left5"><i>N. narica</i><a name="FNanchor_B_3" id="FNanchor_B_3"></a><a href="#Footnote_B_3" class="fnanchor">[B]</a> (nS.A., C.A., M., swU.S.)</span><br> +<span class="m_left5"><i>N. nasua</i><a href="#Footnote_B_3" class="fnanchor">[B]</a> (S.A., sC.A.)</span><br> +<span style="margin-left: 4em;">Genus <i>Nasuella</i> Hollister, 1915</span><br> +<span class="m_left5"><i>N. olivacea</i> (S.A.)</span><br> +<span style="margin-left: 4em;">Genus <i>Procyon</i> Storr, 1780</span><br> +<span class="m_left5"><i>P. cancrivorus</i> (S.A., sC.A.)</span><br> +<span class="m_left5"><i>P. gloveralleni</i><a name="FNanchor_C_4" id="FNanchor_C_4"></a><a href="#Footnote_C_4" class="fnanchor">[C]</a> (Barbados)</span><br> +<span class="m_left5"><i>P. insularis</i><a href="#Footnote_C_4" class="fnanchor">[C]</a> (Maria Madre Is., Maria Magdalene Is.)</span><br> +<span class="m_left5"><i>P. lotor</i><a href="#Footnote_C_4" class="fnanchor">[C]</a> (C.A., M., U.S., sC.)</span><br> +<span class="m_left5"><i>P. maynardi</i><a href="#Footnote_C_4" class="fnanchor">[C]</a> (Bahamas, New Providence Is.)</span><br> +<span class="m_left5"><i>P. minor</i><a href="#Footnote_C_4" class="fnanchor">[C]</a> (Guadeloupe Is.)</span><br> +<span class="m_left5"><i>P. pygmaeus</i><a href="#Footnote_C_4" class="fnanchor">[C]</a> (M., Quintana Roo, Cozumel Is.)</span><br> +</p> +</div> + +<div class="footnote"> +<a name="Footnote_A_2" id="Footnote_A_2"></a><a href="#FNanchor_A_2"><span class="label">[A]</span></a> The several named forms of <i>Bassaricyon</i> are a single species, <i>Bassaricyon gabbii</i> (Wozencraft, 1989a).<br> +<br> +<a name="Footnote_B_3" id="Footnote_B_3"></a><a href="#FNanchor_B_3"><span class="label">[B]</span></a> These are considered conspecific in some current taxonomies (Kortlucke and Ramirez-Pulido, 1982); however, the scheme followed here maintains them as separate species (Decker, 1991).<br> +<br> +<a name="Footnote_C_4" id="Footnote_C_4"></a><a href="#FNanchor_C_4"><span class="label">[C]</span></a> Several named forms of <i>Procyon</i> are a single species, <i>Procyon lotor</i> (Wozencraft, 1989a).<br> +<br> +</div> + + +<p>During the late Miocene, late Pliocene, and Pleistocene, the +Bering land bridge between North America and Asia formed +periodically, offering an avenue for dispersal between northern +continents (Darlington, 1963:366; Webb, 1985a). However, by +the late Tertiary, northern continents had cooled to the extent +that climate, with its attendant sharply defined vegetative +zones, became the major factor limiting dispersal by this route +(Darlington, 1963:366; Webb, 1985a). Those Holarctic mammals +that did cross the Bering land bridge in the late Tertiary +were "cold-adapted" species associated with relatively cool, +but not alpine, climates (Darlington, 1963:366; Ewer, +1973:369). Among carnivores this included some canids, +ursids, mustelids, and felids (Darlington, 1963:393-395, 397; +Webb, 1985a). Procyonids, however, did not cross the Bering +land bridge into Asia, and Ewer (1973:369) ascribes this to +their being an "essentially tropical group." Miocene radiation +of procyonids occurred at a time when two of the four major +climatic deteriorations (middle and late Miocene) were taking +place (Webb, 1985a, 1985b). These deteriorations had the +effect of cooling the middle latitudes to the extent that +temperate forest forms began to appear in mid-latitude floras, +along with a rapid influx of herbaceous plants (Barghoorn, +1953). The procyonid radiation did not penetrate beyond these +climatically changing middle latitudes, which implies that +these animals were "warm-adapted," and were, therefore, +physiologically excluded from reaching the Bering land bridge. +Today, three of the six genera and over half of the 18 species +that comprise Procyonidae (<a href="#Table_1">Table 1</a>; Wozencraft, 1989b) +remain confined to tropical regions of North and South +America (Hall and Kelson, 1959:878-897; Poglayen-Neuwall, +1975; Kortlucke and Ramirez-Pulido, 1982; Nowak and +Paradiso, 1983:977-985).</p> + + + +<a name="Typical_Procyonids"></a> +<span class="pagenum"><a href="#toc">[↑ TOC]</a></span> +<div class="caption4">Typical Procyonids</div> + +<p>McNab (1988a) contends that basal metabolism is a highly +plastic character in evolution, and he has amply shown that +ecologically uniform species are more apt to share common +metabolic rates than taxonomically allied species from drastically +different environments (McNab, 1984a, 1986a, 1986b, +1988a). Procyonids represent a taxonomically allied group that +shared a common ecological situation for millions of years; +consequently, members of this family might be expected to +show some uniformity in their Ḣ<sub>b</sub>. Basal and thermoregulatory +metabolism of several procyonids have been measured: +kinkajou, <i>Potos flavus</i> (Müller and Kulzer, 1977; McNab, +1978a; Müller and Rost, 1983), coatis, <i>Nasua nasua</i> (Chevillard-Hugot +et al., 1980; Mugaas et al., in prep.), and <i>Nasua +narica</i> (Scholander et al., 1950c; Mugaas et al., in prep.), +ringtail, <i>Bassariscus astutus</i> (Chevalier, 1985), and crab-eating +raccoon, <i>Procyon cancrivorus</i> (Scholander et al., 1950c). In +general, these species have Ḣ<sub>b</sub>'s that are 40%-80% of the +values predicted for them by the Kleiber (1961:206) equation. +Lower than predicted Ḣ<sub>b</sub> is viewed as an energy-saving +adaptation for procyonids living in relatively stable tropical +climates (Müller and Kulzer, 1977; Chevillard-Hugot et al., +1980; Müller and Rost, 1983). This implies that lower than +predicted Ḣ<sub>b</sub> is a general procyonid condition and that it +represents a characteristic that evolved in response to the +family's long association with tropical and subtropical forest +environments.<span class="pagenum"><a name="Page_3" id="Page_3">[Pg 3]</a></span></p> + + +<a name="The_Atypical_Procyonid"></a> +<span class="pagenum"><a href="#toc">[↑ TOC]</a></span> +<div class="caption4">The Atypical Procyonid</div> + +<p>Although most procyonids are found in only tropical to +subtropical climates, the North American raccoon, <i>Procyon +lotor</i>, (<a href="#Fig_1">Figure 1</a>) has a much broader distribution that extends +from tropical Panama (8°N) to southern Canada. In Alberta, +Canada, its range reaches the edge of the Hudsonian Life Zone +at 60°N (for distribution maps see Hall and Kelson, 1959:878-897, +and Poglayen-Neuwall, 1975). Range extensions and an +increase in numbers have been noted in Canada and in parts of +the United States since the 19th century (Lotze and Anderson, +1979; Kaufmann, 1982; Nowak and Paradiso, 1983:977-985). +Thus, <i>Procyon lotor</i> is more complex ecologically than other +procyonids, particularly when one takes into account its highly +generalized food habits (Hamilton, 1936; Stuewer, 1943; +Stains, 1956:39-51; Greenwood, 1981) and the wide range of +habitat types (forest, prairie, desert, mountain, coastal marsh, +freshwater marsh) and climates (tropical to north temperate) in +which it is successful (Whitney and Underwood, 1952:1; Hall +and Kelson, 1959:885; Lotze and Anderson, 1979; Kaufmann, +1982). On this basis it is clear that <i>Procyon lotor</i> has deviated +from the typical procyonid portrait and has become the +consummate generalist of the Procyonidae.</p> + +<a name="Fig_1"></a> +<div class="center"> + <img src="images/fig_1.png" width="459" height="593" title="North American raccoon" alt="North American raccoon"><br><br> + <div class="center"><b><span class="smcap">Figure 1.</span>—North American raccoon, <i>Procyon lotor</i>.</b></div> +</div> + + +<a name="The_Hypothesis"></a> +<span class="pagenum"><a href="#toc">[↑ TOC]</a></span> +<div class="caption4">The Hypothesis</div> + +<p><span class="pagenum"><a name="Page_4" id="Page_4">[Pg 4]</a></span>Our +general hypothesis was that whereas most contemporary +procyonids have retained the metabolic characteristics of their +warm-adapted ancestors, <i>Procyon lotor</i> possesses a different +set of adaptations, which either evolved as characteristics +unique to this species or were acquired from its ancestral stock. +In either case, its unique adaptations have given <i>Procyon lotor</i> +the physiological flexibility to generalize its use of habitats and +climates and expand its geographic distribution to a much +greater extent than other procyonids.</p> + +<a name="Hypothesis_Testing"></a> +<span class="pagenum"><a href="#toc">[↑ TOC]</a></span> +<div class="caption4">Hypothesis Testing</div> + +<p>We tested our hypothesis by comparing <i>Procyon lotor</i> with +several other procyonids (<i>Bassariscus astutus</i>, <i>Nasua nasua</i>, +<i>Nasua narica</i>, <i>Procyon cancrivorus</i>, and <i>Potos flavus</i>) on the +basis of their (1) basal metabolic rate (Ḣ<sub>b</sub>), (2) minimum wet +thermal conductance (C<sub>mw</sub>), (3) diversity of diet (D<sub>d</sub>), (4) +intrinsic rate of natural increase (r<sub>max</sub>), and, when data were +available, (5) capacity for evaporative cooling (E<sub>c</sub>). In a genetic +sense each one of these variables is a complex adaptive +characteristic, expression of which is determined by the +interaction of several genes (Prosser, 1986:110-165). Experience +has shown that a given species will express each one of +these variables in a specific manner that is relevant to its mass, +physiology, behavior, and environmental circumstance. Thus, +different expressions of these variables may represent specific +climatic adaptations (Prosser, 1986:16) that have been selected-for +by evolutionary process. Because these variables are +interrelated with respect to regulation of body temperature and +energy balance, they have co-evolved in each species to form +an adaptive unit. For each species, measured and calculated +values for the first four variables were converted into +dimensionless numbers and used to derive a composite score +that represented its adaptive unit. Climatic distributions of +these species were then compared relative to their composite +scores.</p> + +<a name="Adaptive_Significance"></a> +<a name="Basal_Metabolic_Rate_1"></a> +<span class="pagenum"><a href="#toc">[↑ TOC]</a></span> +<div class="caption3">Adaptive Significance of the Variables</div> +<div class="caption4">Basal Metabolic Rate and Intrinsic Rate of Natural Increase</div> + +<p>Basal metabolic rate represents the minimum energy +required by an animal to maintain basic homeostasis (Lusk, +1917:141; Kleiber, 1932, 1961:251; Benedict, 1938; Brody, +1945:59; Robbins, 1983:105-111). For mammals, Ḣ<sub>b</sub> appears +to be determined by complex interactions between their body +size (Kleiber, 1932, 1961:206; Benedict, 1938; Brody, +1945:368-374; Hemmingsen, 1960:15-36; McNab, 1983b; +Calder, 1987), the climate in which they live (Scholander et al., +1950c; McNab and Morrison, 1963; Hulbert and Dawson, +1974; Shkolnik and Schmidt-Nielsen, 1976; McNab, 1979a; +Vogel, 1980), their food habits (McNab, 1978a, 1978b, 1980a, +1983a, 1984a, 1986a, 1986b, 1988a, 1989), and their circadian +period (Aschoff and Pohl, 1970; Prothero, 1984). Some species +have higher mass-specific Ḣ<sub>b</sub> than others, and this variation +appears to be tied to ecological circumstances rather than +taxonomic affinities (McNab, 1988a, 1989). Basal metabolic +rate is important ecologically because it serves as a measure of +a species' minimum "obligatory" energy requirement, and +under many circumstances, it represents the largest energy +demand associated with a daily energy budget (King, 1974:38-55; +McNab, 1980a; Mugaas and King, 1981:37-40). Recently +it also has been implicated as a permissive factor with respect +to r<sub>max</sub> of mammals (Hennemann, 1983; Lillegraven et al., +1987; Nicoll and Thompson, 1987; Thompson, 1987) via its +direct effect on their rates of development and fecundity +(McNab, 1980a, 1983a, 1986b; Hennemann, 1983; Schmitz +and Lavigne, 1984; Glazier, 1985a, 1985b). The implication of +this latter point is that those species with higher Ḣ<sub>b</sub>'s also have +faster rates of development and greater fecundity and hence +enjoy the competitive advantage of a higher r<sub>max</sub>. Basal +metabolism is, therefore, "a highly plastic character in the +course of evolution" (McNab, 1988a:25) that has a profound +influence on each species' life history.</p> + +<a name="Minimum_Thermal_Conductance_1"></a> +<span class="pagenum"><a href="#toc">[↑ TOC]</a></span> +<div class="caption4">Minimum Thermal Conductance</div> + +<p>Whole-body resistance to passive heat transfer is equal to +tissue resistance plus coat resistance. Within limits, these +resistances can be altered; tissue resistance can be varied by +changes in blood flow, whereas coat resistance can be changed +by piloerection, molt, and behavior. When whole-body +resistance is maximized (maximum tissue and coat resistances), +passive heat transfer is minimized. The inverse of resistance is +conductance; therefore, maximum whole-body resistance is the +inverse of minimum thermal conductance (C<sub>m</sub>). Minimum +thermal conductance is readily derived from metabolic chamber +data, and it is commonly used to describe an animal's +capacity to minimize passive heat transfer. Minimum thermal +conductance interacts with Ḣ<sub>b</sub> and body mass to set the +maximum temperature differential a mammal can maintain +without increasing its basal level of heat production. The low +temperature in this differential is the lower critical temperature +(T<sub>lc</sub>).</p> + +<p>Mass-specific C<sub>m</sub> for mammals is negatively correlated with +body mass (McNab and Morrison, 1963; Herreid and Kessel, +1967; McNab, 1970, 1979b; Bradley and Deavers, 1980; +Aschoff, 1981), and for any given mass its magnitude is 52% +higher during the active, rather than the inactive, phase of the +daily cycle (Aschoff, 1981). However, some mammals have +C<sub>m</sub>'s that are higher or lower than would be predicted for them +on the basis of body mass and circadian phase. Seasonal +<span class="pagenum"><a name="Page_5" id="Page_5">[Pg 5]</a></span> +variation in C<sub>m</sub> (higher values during summer than winter) has +been reported for many northern mammals that experience +large annual variations in air temperature (Scholander et al., +1950a; Irving et al., 1955; Hart, 1956, 1957; Irving, 1972:165). +Some tropical mammals with very thin fur coats, and others +with nearly hairless bodies, have high C<sub>m</sub>'s (McNab, 1984a), as +do burrowing mammals (McNab, 1966, 1979b, 1984a) and the +kit fox, <i>Vulpes macrotis</i> (Golightly and Ohmart, 1983). Some +small mammals with low basal metabolic rates tend to have +lower than predicted C<sub>m</sub>'s: small marsupials (McNab, 1978a), +heteromyid rodents (McNab, 1979a), several ant eaters +(McNab, 1984a), the arctic hare, <i>Lepus arcticus</i> (Wang et al., +1973), the ringtail, <i>Bassariscus astutus</i> (Chevalier, 1985), and +the fennec, <i>Fennecus zerda</i> (Noll-Banholzer, 1979). Thus, in +spite of its mass dependence, C<sub>m</sub> also has been modified during +the course of evolution by selective factors in the environment +and by the animal's own metabolic characteristics.</p> + +<a name="Capacity_for_Evaporative_Cooling"></a> +<span class="pagenum"><a href="#toc">[↑ TOC]</a></span> +<div class="caption4">Capacity for Evaporative Cooling</div> + +<p>Latent heat loss occurs as a result of evaporation from the +respiratory tract and through the skin, and except under +conditions of heat stress, it "is a liability in thermal and osmotic +homeostasis" (Calder and King, 1974:302). E<sub>c</sub>, defined as the +ratio of evaporative heat lost to metabolic heat produced, can be +used to quantify thermoregulatory effectiveness of evaporative +cooling and to make comparisons of heat tolerance between +species. Thermoregulatory effectiveness of latent heat loss is +not just a function of the rate of evaporative water loss but also +of the rate of metabolic heat production (Lasiewski and +Seymour, 1972). For example, a low metabolic rate minimizes +endogenous heat load and thus conserves water, whereas the +opposite is true of high metabolic rates (Lasiewski and +Seymour, 1972). Some mammals that live in arid regions have +evolved low metabolic rates and thus capitalize on this +relationship to reduce their thermoregulatory water requirement +(McNab and Morrison, 1963; McNab, 1966; MacMillen +and Lee, 1970; Noll-Banholzer, 1979). What is evident, +therefore, is that an animal's capacity for increasing latent heat +loss must evolve together with its Ḣ<sub>b</sub> and C<sub>m</sub> in response to +specific environmental demands.</p> + +<a name="Diet"></a> +<span class="pagenum"><a href="#toc">[↑ TOC]</a></span> +<div class="caption4">Diet</div> + +<p>McNab (1986a, 1988a, 1989) demonstrated that, for mammals, +departures of Ḣ<sub>b</sub> from the Kleiber (1961:206) "norm" are +highly correlated with diet and independent of phylogenetic +relationships. McNab's analysis indicates that for mammals +that feed on invertebrates, those species with body mass less +than 100 g have Ḣ<sub>b</sub>'s that are equal to or greater than values +predicted by the Kleiber equation, whereas those with body +mass greater than 100 g have metabolic rates that are lower than +predicted. Grazers, vertebrate eaters, nut eaters, and terrestrial +frugivores also have Ḣ<sub>b</sub>'s that are equal to or greater than +predicted, whereas insectivorous bats, arboreal folivores, +arboreal frugivores, and terrestrial folivores all have rates that +are lower than predicted. McNab (1986a) found animals with +mixed diets harder to categorize, but in general he predicted +that their Ḣ<sub>b</sub>'s would be related to (1) a food item that is +constantly available throughout the year, (2) a food item that is +most available during the worst conditions of the year, or (3) a +mix of foods available during the worst time of the year. +Although these correlations do not establish cause and effect +between food habits and Ḣ<sub>b</sub>, McNab's analysis does make it +clear that the relationship between these variables has very real +consequences for an animal's physiology, ecology, and +evolution.</p> + +<a name="Experimental_Design_and_Summary"></a> +<span class="pagenum"><a href="#toc">[↑ TOC]</a></span> +<div class="caption3">Experimental Design and Summary</div> + +<p>In this investigation we measured basal and thermoregulatory +metabolism, evaporative water loss, and body temperature +of raccoons from north central Virginia. Measurements were +conducted on both sexes in summer and winter to determine +how season and sex influenced these variables. We then +compared the data for this widely distributed generalist with +data from literature for its ecologically more restricted +relatives. Dietary data for all species were taken from literature, +as were reproductive data for calculation of r<sub>max</sub>.</p> + +<p>Our analysis demonstrated clear differences between <i>Procyon +lotor</i> and other procyonids with respect to Ḣ<sub>b</sub>, C<sub>mw</sub>, D<sub>d</sub>, +and r<sub>max</sub>. The composite score calculated from these variables +for <i>Procyon lotor</i> was much higher than those derived for other +species, and there was a positive correlation between the +number of climates a species occupies and the magnitude of its +composite score. Data on evaporative water loss, although not +complete for all species, suggested that tropical and subtropical +procyonids have less capacity for evaporative cooling than +<i>Procyon lotor</i> or <i>Bassariscus astutus</i>. It was clear, therefore, +that with respect to its thermal physiology, <i>Procyon lotor</i> +differed markedly from other procyonids, and we contend that +these differences have allowed this species to become a highly +successful climate generalist and to expand its distribution into +many different habitats and climates. Our analysis also +suggested that the cornerstone of <i>Procyon lotor</i>'s success as a +climate generalist is its Ḣ<sub>b</sub>, which is higher than the procyonid +norm.</p> + +<a name="Acknowledgments"></a> +<span class="pagenum"><a href="#toc">[↑ TOC]</a></span> +<div class="caption3">Acknowledgments</div> + +<p>The authors would like to thank John Eisenberg and Devra +Kleiman for their support and encouragement throughout the +study. This investigation was supported by research grants +from the West Virginia School of Osteopathic Medicine +(WVSOM), and Friends of the National Zoo (FONZ). Logistic +support was provided by the National Zoological Park's +Conservation and Research Center (CRC), and the departments +of Mammalogy and Zoological Research. Our ability to +<span class="pagenum"><a name="Page_6" id="Page_6">[Pg 6]</a></span> +conduct physiological research at CRC was made possible by +the thoughtful support and encouragement provided by Chris +Wemmer. His excellent staff at CRC, especially Jack Williams, +Junior Allison, and Red McDaniel, were very helpful in +providing hospitality and logistical support to the senior author +and his family during their various visits to the Center. The +assistance of several people at the National Zoo also is +gratefully acknowledged: Mitch Bush and Lyndsay Phillips not +only provided veterinary support throughout the investigation, +but also performed surgical procedures required to implant +temperature-sensitive radio transmitters in several raccoons; +Olav Oftedal made his laboratory available to us at various +times and loaned us equipment to use at CRC; Miles Roberts +and his staff provided care for our captive raccoons in the +Department of Zoological Research during various parts of the +investigation. Greg Sanders and Ken Halama, supported by +FONZ assistantships, cared for our captive raccoons at CRC, +provided assistance in the laboratory whenever needed, and +were an invaluable source of aid. Their friendship and help is +gratefully acknowledged. Ellen Broudy and Andy Meyer, +supported by WVSOM and a student work study grant, +respectively, provided assistance in the laboratory. David +Brown, John Eisenberg, Mary Etta Hight, Brian McNab, Steve +Thompson, and W. Chris Wozencraft critically reviewed +various phases of the manuscript and provided many helpful +suggestions. We deeply appreciate the work of Jean B. +McConville, whose beneficial editorial suggestions helped us +improve several early versions of the manuscript. We also +gratefully acknowledge Diane M. Tyler, our editor at the +Smithsonian Institution Press, whose expertise helped us mold +the manuscript into its final form. Jill Mellon and Sriyanie +Miththalapa, supported by FONZ traineeships, assisted in +measuring the daily cycle of body temperature in raccoons. The +Virginia Commission of Game and Inland Fisheries gave us +permission to use wild-caught raccoons in this project.</p> + + + +<br> +<a name="Materials_and_Methods"></a> +<a name="Live-trapping"></a> +<span class="pagenum"><a href="#toc">[↑ TOC]</a></span> +<div class="caption2">Materials and Methods</div> +<div class="caption3">Live-trapping</div> + +<p>Raccoons were caught from May 1980 through December +1984 on a trapping grid of 30 to 35 stations (one or two "live +traps" per station) that covered about one-third of the National +Zoological Park's Conservation and Research Center (CRC) +near Front Royal, Virginia (Seidensticker et al., 1988; Hallett et +al., 1991). Animals were trapped during 10 consecutive days +each month, and in this five-year interval 407 raccoons were +captured and marked with tattoos and ear tags. All captured +animals were individualized with respect to age, reproductive +status, physical condition, parasite load, and mass and body +dimensions. These data characterized the structure and dynamics +of the raccoon population at CRC and provided information +on the annual cycle of fattening for raccoons in north central +Virginia.</p> + +<p>Animals used for metabolic measurements were captured at +CRC about 1.5 km south of the trapping grid and thus were +genetically representative of the area. Six males were captured +and measured during the summer of 1983. These animals were +kept isolated for a week before being measured and were +released later that summer at the site of their capture. The other +seven animals used in our study were from the collection of the +National Zoological Park and all of them had their origins at CRC.</p> + +<a name="Metabolic_Studies"></a> +<a name="Basal_and_Thermoregulatory_Metabolism"></a> +<span class="pagenum"><a href="#toc">[↑ TOC]</a></span> +<div class="caption3">Metabolic Studies</div> +<div class="caption4">Basal and Thermoregulatory Metabolism</div> + +<p>Metabolic measurements, conducted at CRC, were carried +out on eight males during July and August 1983, on four +females and three males from November 1983 through March +1984, and on four females during June and July 1984.</p> + +<p>Raccoons were housed throughout the study such that they +were constantly exposed to a natural cycle of temperature and +photoperiod. Weather records for the Front Royal area indicate +that average temperatures are around -0.5°C in January and +23.3°C in July (Crockett, 1972). Light:dark (L:D) periods for +the latitude of CRC (48°55'N; United States Department of the +Interior Geological Survey, 1972), calculated from duration of +daylight tables (List, 1971:506-512), were 14.9:9.1 and +9.4:14.6 hours L:D for summer and winter solstices, +respectively, and 12.2:11.8 hours L:D for vernal and autumnal +equinoxes.</p> + +<p>Our animals were fed a measured amount of food daily, and +they usually ate most of what was provided. Occasionally these +animals would eat very little or none of their ration, and on +some days they would eat all that was given to them. We fed +them either feline diet (ground horse meat) or canned mackerel +(Star-kist<span class="sup2">®</span><a name="FNanchor_A_5" id="FNanchor_A_5"></a><a href="#Footnote_A_5" class="fnanchor">[1]</a>) along with high-protein dog chow (Purina<span class="sup2">®</span>). +When available, fresh fruit also was added to their diet. Water +was always provided ad libitum.</p> + +<div class="footnote"> +<a name="Footnote_A_5" id="Footnote_A_5"></a><a href="#FNanchor_A_5"><span class="label">[1]</span></a> <i>The use of product brand names in this publication is not intended as an endorsement of the products by the Smithsonian Institution.</i> +</div> + +<p>Measurements were conducted during the raccoons' daily +inactive period (sunrise to sunset) in both summer and winter. +Oxygen consumption was measured in a flow-through metabolism +chamber at 5°C intervals from -10°C to 35°C. Animals +were held at each temperature until the lowest rate of oxygen +consumption had been obtained and maintained for at least 15 +minutes. During each determination, oxygen consumption was +monitored for 30 minutes to one hour beyond a suspected +minimum value to see if an even lower reading could be +obtained. Raccoons attained minimum levels of oxygen +consumption more quickly at warm (>10°C) than at cold +<span class="pagenum"><a name="Page_7" id="Page_7">[Pg 7]</a></span> +temperatures. Depending on the temperature, therefore, each +measurement took from two to five hours to complete. On days +when two measurements could be completed, the second trial +was always at a temperature 10°C warmer than the first.</p> + +<p>The metabolism chamber was constructed from galvanized +sheet metal (77.5 × 45.5 × 51.0 cm = 180 liters) and was painted +black inside. Within the chamber, the animal was held in a cage +(71 × 39 × 33 cm) constructed from turkey wire that also was +painted black. This cage prevented the raccoons from coming +into contact with the walls of the chamber, yet it was large +enough to allow them to stand and freely move about. The +bottom of the cage was 11 cm above the chamber floor, which +was covered to a depth of one cm with mineral oil to trap urine +and feces.</p> + +<p>During measurements, the metabolism chamber was placed +in a controlled-temperature cabinet (modified Montgomery +Ward model 8969 freezer). Air temperature (T<sub>a</sub>) in the +metabolism chamber was regulated with a Yellow Springs +Instrument model 74 temperature controller. T<sub>a</sub> was controlled +to ± 1.0°C at temperatures below freezing, and to ± 0.5°C at +temperatures above freezing. The chamber air and wall +temperatures were recorded continuously (Linseis model +LS-64 recorder) during each experiment, and, except during +temperature changes, they were always within 0.5°C of each +other.</p> + +<p>Columns of Drierite<span class="sup2">®</span> and Ascarite<span class="sup2">®</span> removed water vapor +and carbon dioxide, respectively, from air entering and leaving +the chamber. Dry carbon-dioxide-free room air was pumped +into the chamber (Gilman model 13152 pressure/vacuum +pump) at a rate of 3.0 L/min (Gilmont model K3203-20 flow +meter). Downstream from the chemical absorbents, an aliquot +(0.1 L/min) of dry carbon-dioxide-free air was drawn off the +chamber exhaust line and analyzed for oxygen content +(Applied Electrochemistry model S-3A oxygen analyzer, +model 22M analysis cell, and model R-1 flow control). All gas +values were corrected to standard temperature and pressure for +dry gas. Oxygen consumption was calculated from the +difference in oxygen content between inlet and outlet air using +Eq. 8 of Depocas and Hart (1957).</p> + +<p>Each raccoon was fasted for at least 12 hours before oxygen +consumption measurements began. At the start and end of each +metabolic trial the animal was weighed to the nearest 10 g +(Doctors Infant Scale, Detecto Scales, Inc., Brooklyn, N.Y., +U.S.A.). The body mass used in calculating minimum oxygen +consumption and evaporative water loss was estimated from +timed extrapolations of the difference between starting and +ending weights, and the time at which these variables were +measured.</p> + + +<a name="Evaporative_Water_Loss_1"></a> +<span class="pagenum"><a href="#toc">[↑ TOC]</a></span> +<div class="caption4">Evaporative Water Loss</div> + +<p>During metabolic measurements at temperatures above +freezing, evaporative water loss was determined gravimetrically. +Upstream from the chemical columns, an aliquot of air +(0.1 L/min) was drawn off the exhaust line and diverted for a +timed interval through a series of preweighed (0.1 mg) <img src="images/u_tube.png" width="14" height="14" border="0" alt="U-tube" title="U-tube">-tubes +containing Drierite<span class="sup2">®</span>. The aliquot then passed through a second +series of <img src="images/u_tube.png" width="14" height="14" border="0" alt="U-tube" title="U-tube">-tubes containing Ascarite<span class="sup2">®</span> before entering the +oxygen analysis system. Evaporative water loss was calculated +using <a href="#Eq_1">Eq. 1</a></p> + +<a name="Eq_1"></a> +<table width="100%" summary="Eq. 1"> +<tr><td class="center">Ė = (m<sub>w</sub>·<span class="dot">.</span><span class="V">V</span><sub>e</sub>)/(<span class="dot">.</span><span class="V">V</span><sub>a</sub>·t·m)</td><td class="text_rt">Eq. 1</td></tr> +</table> + +<p class="noidt"> +where Ė is evaporative water loss (mg·g<sup>-1</sup>·h<sup>-1</sup>), m<sub>w</sub> is mass of +water collected (mg), <span class="dot">.</span><span class="V">V</span><sub>e</sub> is rate of air flow into the chamber (3.0 +L/min), <span class="dot">.</span><span class="V">V</span><sub>a</sub> is the rate of air flow through the <img src="images/u_tube.png" width="14" height="14" border="0" alt="U-tube" title="U-tube">-tubes (0.1 L/min), +t is length of the timed interval (h), and m is the estimated mass +of the raccoon at the time of sampling (g).</p> + +<a name="Body_Temperature"></a> +<span class="pagenum"><a href="#toc">[↑ TOC]</a></span> +<div class="caption4">Body Temperature</div> + +<p>Veterinarians at the National Zoological Park surgically +implanted calibrated temperature-sensitive radio transmitters +(Telonics, Inc., Mesa, AZ, U.S.A.) into abdominal cavities of +two female and two male raccoons. Transmitter pulse periods +were monitored with a digital processor (Telonics TDP-2) +coupled to a receiver (Telonics TR-2-164/166). During some +metabolic measurements, body temperatures of these animals +were recorded to the nearest 0.1°C at 30-minute intervals. The +daily cycle of body temperature of these raccoons also was +measured once a month.</p> + +<a name="Calibrations"></a> +<a name="Calorimeter"></a> +<span class="pagenum"><a href="#toc">[↑ TOC]</a></span> +<div class="caption3">Calibrations</div> +<div class="caption4">Calorimeter</div> + +<p>At the conclusion of these experiments, the accuracy of our +calorimetry apparatus was tested by burning an ethanol lamp in +the metabolism chamber. During these tests a CO<sub>2</sub> analyzer was +incorporated into the system (Beckman, LB-2). Results +demonstrated that we measured 84% of the oxygen consumed +by the lamp as well as 84% of the water and CO<sub>2</sub> it produced; +standard deviation = ± 2.6, ± 5.0, and ± 3.6, respectively (n = +27). Average respiratory quotient (RQ) calculated from these +data was O.657 ± 0.008 (n = 27), which is 99.5% of that +predicted (0.66). McNab (1988b) reports that the accuracy of +open-flow indirect calorimetry systems, such as ours, depends +on the rate of air flow through the animal chamber. If flow rates +are too low, there is inadequate mixing of air within the +chamber, and the rate of oxygen consumption, as calculated +from the difference in oxygen content of air flowing into and +out of the chamber (Depocas and Hart, 1957), is underestimated. +At some critical rate of air flow, which is unique to each +combination of chamber and animal, this situation changes +such that measured rates of oxygen consumption become +independent of any further increase in flow rate (McNab, +1988b). In recent tests of our system, where we burned the +ethanol lamp at a variety of chamber flow rates, the efficiency +<span class="pagenum"><a name="Page_8" id="Page_8">[Pg 8]</a></span> +of measurement increased linearly as flow rate increased, and +the critical rate of air flow was about 6.7 L/min. This appeared +to explain why a flow rate of 3.0 L/min underestimated oxygen +consumption of the ethanol lamp.</p> + +<p>Our earlier tests of the efficiency of our system indicated that +although we underestimated actual oxygen consumption of the +ethanol lamp, we did so with a fair degree of precision; +probably because flow rates were closely controlled. During +our metabolic measurements, chamber flow rates also were +closely controlled at 3.0 L/min, and we believe, therefore, that +these measurements also were carried out with a high degree of +precision. Consequently, all measured values of oxygen +consumption and water production were considered to be 84% +of their actual value and were adjusted to 100% before being +included in this report.</p> + +<a name="Body_Temperature_Transmitters"></a> +<span class="pagenum"><a href="#toc">[↑ TOC]</a></span> +<div class="caption4">Body Temperature Transmitters</div> + +<p>The calibration of all temperature-sensitive radio transmitters +drifted over time. Transmitters were calibrated before they +were surgically implanted and again after they were removed +from the animals. Although the drift of each transmitter was +unique, it was also linear (S. Tomkiewicz, Telonics, Inc., pers. +com.). All body temperature measurements were corrected +from timed extrapolations of the difference between starting +and ending calibrations.</p> + +<a name="Statistical_Methods"></a> +<span class="pagenum"><a href="#toc">[↑ TOC]</a></span> +<div class="caption3">Statistical Methods</div> + +<p>Values of oxygen consumption, evaporative water loss, and +body temperature were plotted as a function of chamber air +temperature. Linear regressions of oxygen consumption at +temperatures below the thermoneutral zone (T<sub>n</sub>), and evaporative +water loss at temperatures above freezing, were determined +with the SAS (1982) GLM procedure. Lower critical temperature +(T<sub>lc</sub>) was determined graphically from intersection of the +line representing Ḣ<sub>b</sub> and the regression line representing +oxygen consumption below T<sub>n</sub>. Slopes and intercepts of +regression lines, as well as other mean values, were compared +with <i>t</i>-tests (Statistical Analysis System, 1982; Ott, 1984:138-175). +Unless indicated otherwise, data are expressed as mean +± standard deviation (s.d.).</p> + +<a name="Estimating_Intrinsic_Rate"></a> +<span class="pagenum"><a href="#toc">[↑ TOC]</a></span> +<div class="caption3">Estimating Intrinsic Rate of Natural Increase</div> + +<p>We employed the method first described by Cole (1954) to +calculate r<sub>max</sub>:</p> + +<a name="Eq_2"></a> +<table width="100%" summary="Eq. 2"> +<tr><td class="center">1 = e<sup>-r<sub>max</sub></sup> + b·e<sup>-r<sub>max</sub>(a)</sup> - b·e<sup>-r<sub>max</sub>(n+1)</sup></td><td class="text_rt">Eq. 2</td></tr> +</table> + +<p class="noidt">where a is potential age of females first producing young, b is +potential annual birth rate of female young, and n is potential +age of females producing their final young. After life-history +data were substituted into <a href="#Eq_2">Eq. 2</a>, r<sub>max</sub> was determined by trial +and error substitution (Hennemann, 1983).</p> + +<p>Because r<sub>max</sub> represents the genetically fixed, physiologically +determined maximum possible rate of increase, data on +earliest possible age of female reproduction, highest possible +birth rate of female young, and longest possible female +reproductive life span were used for a, b, and n, respectively. +Calculated values, therefore, represent physiologically possible, +not ecologically possible, intrinsic rates of increase +(Hennemann, 1983, 1984; Hayssen, 1984; McNab, 1984b). +Values of n were derived from longevity records for captive +animals, and as these were all large values of similar duration +(14-16 years), they had very little effect on r<sub>max</sub>. All species +considered have one litter per year, and because their sex ratios +at birth are about 50:50, variation in b was due to differences +in litter size. Therefore, age of first reproduction and litter size +had the greatest effect on r<sub>max</sub>. Intrinsic rate of increase scales +to body mass (Fenchel, 1974), and we removed this effect by +comparing each calculated r<sub>max</sub> with the value expected (r<sub>maxe</sub>) +on the basis of body mass (Hennemann, 1983).</p> + +<a name="Comparison_of_Adaptive_Units"></a> +<span class="pagenum"><a href="#toc">[↑ TOC]</a></span> +<div class="caption3">Comparison of Adaptive Units</div> + +<p>Dimensionless numbers for each of the four variables used in +calculating composite scores were derived as follows. Ratios of +measured to predicted values were used for basal metabolism +(H<sub>br</sub>) and minimum wet thermal conductance (C<sub>mwr</sub>). Thermoregulatory +ability at low temperatures is closely related to +the ratio H<sub>br</sub>/C<sub>mwr</sub> (McNab, 1966). This ratio was used, +therefore, to gauge each species' cold tolerance. For D<sub>d</sub> we used +the ratio of food categories actually used by a species to the +total number of food categories taken by all species tested (D<sub>dr</sub>). +The ratio of calculated to expected intrinsic rates of natural +increase was used to derive r<sub>maxr</sub>. Composite scores were +calculated as</p> + +<table width="100%" summary="Eq. 3"> +<tr><td class="center">Composite score = [(H<sub>br</sub>/C<sub>mwr</sub>) + D<sub>dr</sub> + r<sub>maxr</sub>]/3</td><td class="text_rt">Eq. 3</td></tr> +</table> + +<p>The correlation between number of climates these species +occupy and their composite scores was tested by linear +regression.</p> + + +<br> +<a name="Results"></a> +<a name="Body_Mass"></a> +<span class="pagenum"><a href="#toc">[↑ TOC]</a></span> +<div class="caption2">Results</div> +<div class="caption3">Body Mass</div> + +<p>According to monthly live-trapping records, the body mass +of free-ranging female raccoons increased from 3.6 ± 0.6 kg +during summer to 5.6 ± 0.8 kg in early winter, and the mass of +free-ranging males increased from 4.0 ± 0.5 to 6.7 ± 0.9 kg +during the same interval. These seasonal changes in body mass +were due to fluctuations in the amount of body fat and represent +a mechanism for storing energy during fall for use in winter. In +summer, captive and trapped male and captive female raccoons +had the same body mass (4.73 ± 0.61, 4.41 ± 0.70, and 4.67 +<span class="pagenum"><a name="Page_9" id="Page_9">[Pg 9]</a></span> +± 0.88 kg, respectively, <a href="#Table_2">Table 2</a>). Mass of captive females did +not change between seasons, whereas captive males were +heavier in winter than summer (p<0.005; <a href="#Table_2">Table 2</a>). This +seasonal change in mass of our captive males was of a much +smaller magnitude (0.6 kg) than that observed for wild males +(2.7 kg). During winter, captive males (5.34 ± 1.39 kg) were +heavier than captive females (4.49 ± 0.98 kg; p<0.005; <a href="#Table_2">Table +2</a>). Thus, our captive animals maintained a body mass +throughout the year that was intermediate to the range of values +found for wild raccoons in the same area.</p> + +<a name="Table_2"></a> +<div class="tab_cap"><span class="smcap">Table 2.</span>—Body mass in kg and basal metabolism <span class="nobreak">(mL O<sub>2</sub>·kg<sup>-0.75</sup>·h<sup>-1</sup>)</span> of <i>Procyon lotor</i> in summer and +winter (s.d. = standard deviation and n = number of observations).</div> +<br> +<table width="100%" class="center" cellpadding="4" summary="Body Mass and Basal Metabolism Comparison"> +<tr><th class="bt bb">Season and sex</th><th class="bt bl bb">Body mass, ± s.d.,</th><th class="bt bb">(n)</th><th class="bt bb">Basal metabolism, ± s.d.,</th><th class="bt bb">(n)</th></tr> +<tr><td class="text_lf">Summer</td><td colspan=4 class="bl"> </td></tr> +<tr><td class="text_lf"> Trapped male</td><td class="bl">4.41 ± 0.70</td><td>(52)</td><td>780 ± 112</td><td>(20)</td></tr> +<tr><td class="text_lf"> Captive male</td><td class="bl">4.73 ± 0.61</td><td>(22)</td><td>680 ± 102</td><td> (8)</td></tr> +<tr><td class="text_lf"> Captive female</td><td class="bl">4.67 ± 0.88</td><td>(41)</td><td>618 ± 92</td><td>(13)</td></tr> +<tr><td class="text_lf">Winter</td><td colspan=4 class="bl"> </td></tr> +<tr><td class="text_lf"> Captive male</td><td class="bl">5.34 ± 1.39</td><td>(31)</td><td>704 ± 81</td><td>(19)</td></tr> +<tr><td class="text_lf bb"> Captive female</td><td class="bl bb">4.49 ± 0.98</td><td class="bb">(42)</td><td class="bb">667 ± 139</td><td class="bb">(25)</td></tr> +</table> +<br> + +<a name="Basal_Metabolic_Rate_2"></a> +<span class="pagenum"><a href="#toc">[↑ TOC]</a></span> +<div class="caption3">Basal Metabolic Rate</div> + +<p>Within thermoneutrality, Ḣ<sub>b</sub> <span class="nobreak">(mL O<sub>2</sub>·g<sup>-1</sup>·h<sup>-1</sup>)</span> was 0.54 ± 0.09 +for trapped males in summer, 0.46 ± 0.07 for captive males in +summer, 0.42 ± 0.07 for captive females in summer, 0.47 ± 0.06 +for captive males in winter, and 0.46 ± 0.10 for captive females +in winter (<a href="#Fig_2">Figures 2</a>,<a href="#Fig_3"> 3</a>). Ratios of these measured values to +those predicted by the Kleiber (1932, 1961:206) equation are +1.28, 1.12, 1.02, 1.17, and 1.09, respectively. To minimize the +effect of body size (Mellen, 1963) and to facilitate comparisons +between sexes and seasons and between captive and trapped +animals, basal metabolism also was calculated as a function of +metabolic body size <span class="nobreak">(mL O<sub>2</sub>·kg<sup>-0.75</sup>·h<sup>-1</sup>;</span> <a href="#Table_2">Table 2</a>). Based on this +analysis, trapped summer males had a higher basal metabolism +than captive males (p<0.025) or females (p<0.005) in either +season (<a href="#Table_2">Table 2</a>). There was no difference in basal metabolism +between captive males and females in either summer or winter, +and there was no seasonal difference in their basal metabolic +rates (<a href="#Table_2">Table 2</a>).</p> + +<a name="Minimum_Thermal_Conductance_2"></a> +<p><span class="pagenum"><a href="#toc">[↑ TOC]</a></span></p> +<div class="caption3">Minimum Thermal Conductance</div> + +<p>Minimum wet and dry thermal conductances were calculated +using Eqs. 4 and 5</p> + +<a name="Eq_4"></a> +<table width="100%" summary="Eq. 4"> +<tr><td class="center">C<sub>mw</sub> = Ḣ<sub>r</sub> / (T<sub>b</sub> - T<sub>a</sub>)</td><td class="text_rt">Eq. 4</td></tr> +</table> + +<table width="100%" summary="Eq. 5"> +<tr><td class="center">C<sub>md</sub> = (Ḣ<sub>r</sub> - Ė<sub>eq</sub>) / (T<sub>b</sub> - T<sub>a</sub>)</td><td class="text_rt">Eq. 5</td></tr> +</table> + +<p class="noidt"> +where C<sub>mw</sub> is wet and C<sub>md</sub> is dry conductance <span class="nobreak">(mL +O<sub>2</sub>·g<sup>-1</sup>·h<sup>-1</sup>·°C<sup>-1</sup>);</span> Ḣ<sub>r</sub> is the lowest resting metabolic rate +measured at each temperature <span class="nobreak">(mL O<sub>2</sub>·g<sup>-1</sup>·h<sup>-1</sup>);</span> +Ė<sub>eq</sub> is oxygen equivalent for heat lost by evaporation +[Ė<sub>eq</sub> = mL O<sub>2</sub>·g<sup>-1</sup>·h<sup>-1</sup> = Ė·λ/γ, where Ė is evaporative water loss +(mg·g<sup>-1</sup>·h<sup>-1</sup>), λ is heat of vaporization for water (2.43 J/mg), and +γ is heat equivalent for oxygen (20.097 J/mL)]; T<sub>b</sub> is body +temperature (°C); and T<sub>a</sub> is chamber air temperature (°C). Only +data from animals equipped with temperature-sensitive radio +transmitters were used for these calculations.</p> + +<a name="Table_3"></a> +<div class="tab_cap"><span class="smcap">Table 3.</span>—Minimum wet and dry thermal conductances <span class="nobreak">(mL O<sub>2</sub>·g<sup>-1</sup>·h<sup>-1</sup>·°C<sup>-1</sup>)</span> of <i>Procyon lotor</i> in summer and +winter. Means of values were calculated from equations 3 and 4 (s.d. = standard deviation and n = number of +observations).</div> +<br> +<table width="100%" class="center" cellpadding="4" summary="Thermal Conductance"> +<tr><th rowspan=2 class="bt bb">Season and sex</th><th class="bt bl"> </th><th colspan=4 class="bt bb">Thermal conductance</th></tr> +<tr><th class="bl bb"> </th><th class="bb">Wet ± s.d.</th><th class="bb">(n)</th><th class="bb">Dry ± s.d.</th><th class="bb">(n)</th></tr> +<tr><td class="text_lf">Summer</td><td colspan=5 class="bl"> </td></tr> +<tr><td class="text_lf"> Captive, both sexes</td><td class="bl"> </td><td>0.0256 ± 0.0028</td><td>(18)</td><td>0.0246 ± 0.0019</td><td>(12)</td></tr> +<tr><td class="text_lf">Winter</td><td colspan=5 class="bl"> </td></tr> +<tr><td class="bb text_lf"> Captive, female</td><td class="bl bb"> </td><td class="bb">0.0172 ± 0.0023</td><td class="bb">(10)</td><td class="bb">0.0161 ± 0.0027</td><td class="bb"> (6)</td></tr> +</table> +<br> +<a name="Fig_2"></a> +<p><span class="pagenum"><a name="Page_10" id="Page_10">[Pg 10]</a></span></p> +<div class="center"> + <img src="images/fig_2.png" width="436" height="311" title="Relationship between oxygen consumption and chamber air temperature for raccoons in summer" alt="summer - oxygen consumpsion vs air temp"><br><br> + <div class="fig_cap"><span class="smcap">Figure 2.</span>—Relationship between oxygen consumption and chamber air temperature for raccoons in summer: captive females, open circles; captive males, closed circles; trapped males, open squares. Sloping lines represent regressions of oxygen consumption on chamber air temperature, and horizontal lines, basal metabolism.</div> +</div> + +<a name="Fig_3"></a> +<div class="center"> + <img src="images/fig_3.png" width="436" height="307" title="Relationship between oxygen consumption and chamber air temperature for raccoons in winter" alt="winter - oxygen consumpsion vs air temp"><br><br> + <div class="fig_cap"><span class="smcap">Figure 3.</span>—Relationship between oxygen consumption and chamber air temperature for raccoons in winter: captive females, open circles; captive males, closed circles. Solid sloping line represents regression of oxygen consumption on chamber air temperature for males and females, and the horizontal line, basal metabolism for males and females.</div> +</div> + +<p><span class="pagenum"><a name="Page_11" id="Page_11">[Pg 11]</a></span> +C<sub>mw</sub> was calculated for each season from metabolic +measurements made at all air temperatures below T<sub>lc</sub> (<a href="#Table_3">Table 3</a>). +Because evaporative water loss was not measured at temperatures +below freezing, C<sub>md</sub> was calculated only from metabolic +determinations made at air temperatures between T<sub>lc</sub> and 0°C. +There was no difference between males and females in summer +for either C<sub>mw</sub> or C<sub>md</sub> <span class="nobreak">(mL O<sub>2</sub>·g<sup>-1</sup>·h<sup>-1</sup>·°C<sup>-1</sup>).</span> Data for each sex +were combined to give a summer average of 0.0256 ± 0.0028 +for C<sub>mw</sub>, and 0.0246 ± 0.0019 for C<sub>md</sub> (<a href="#Table_3">Table 3</a>). These summer +conductances were 49% higher (p<0.005) than those calculated +for winter females (0.0172 ± 0.0023, and 0.0161 ± 0.0027 for +C<sub>mw</sub> and C<sub>md</sub>, respectively; <a href="#Table_3">Table 3</a>). C<sub>mw</sub> and C<sub>md</sub> were not +different from each other in either summer or winter, which +indicated that in both seasons evaporative water loss contributed +very little to heat dissipation at temperatures below T<sub>n</sub>. +Comparisons of thermal conductances calculated on the basis +of metabolic body size (Mellen, 1963) gave the same results.</p> + +<a name="Evaporative_Water_Loss_2"></a> +<span class="pagenum"><a href="#toc">[↑ TOC]</a></span> +<div class="caption3">Evaporative Water Loss</div> + +<p>Evaporative water loss increased as chamber temperature +increased in both summer and winter (<a href="#Fig_4">Figures 4</a>,<a href="#Fig_5"> 5</a>). In summer, +the pattern of increase was different for females and males. +Polynomial regressions for trapped and captive males produced +equations that describe a concave relationship between T<sub>a</sub> and +evaporative water loss, whereas the equation for females +describes a sigmoid curve (<a href="#Table_4">Table 4</a>; <a href="#Fig_4">Figure 4</a>). For females, +water loss increased rapidly at temperatures above 25°C +(<a href="#Fig_4">Figure 4</a>). The intercepts and coefficients of the X, X<sup>2</sup>, and X<sup>3</sup> +terms of the polynomial regression equations (<a href="#Table_4">Table 4</a>) were +compared (<i>t</i>-tests) to determine if they differed from each other. +The coefficients in the equation for trapped males differed from +those for captive females in the X<sup>2</sup> (p<0.05) and X<sup>3</sup> (p<0.025) +terms. The intercept and coefficients of the equation for captive +males, however, were not different from those for either captive +females or trapped males. Although this lack of difference is +understandable in the case of trapped males, where the shape of +the two curves is similar (concave), it is not so clear for the +sigmoid curve of captive females (<a href="#Fig_4">Figure 4</a>). Perhaps the lack +of difference in this case is simply due to the small number of +observations available for captive males (n = 10; <a href="#Table_4">Table 4</a>). +Nonetheless, in summer at 35°C, both captive and trapped +males relied less on evaporative cooling than did captive +females (<a href="#Fig_4">Figure 4</a>).</p> + +<p>In winter, males and females had similar rates of evaporative +water loss across the full range of temperatures tested (<a href="#Fig_5">Figure +5</a>). Therefore, data for both sexes were combined. The intercept +and coefficients of this equation (<a href="#Table_4">Table 4</a>) did not differ from +those for summer females, but they did differ from those in the +regression for trapped males in the X<sup>2</sup> (p<0.05) and X<sup>3</sup> +(p<0.025) terms. As was the case for females in summer, +rates of water loss for winter animals increased most rapidly at +temperatures above 25°C (<a href="#Fig_5">Figure 5</a>).</p> + +<br> +<a name="Fig_4"></a> +<div class="center"> + <img src="images/fig_4.png" width="429" height="378" title="Relationship between evaporative water loss and chamber air temperature for raccoons in summer" alt="summer - evap water loss vs temp"><br><br> + <div class="fig_cap"><span class="smcap">Figure 4.</span>—Relationship between evaporative water loss and chamber air temperature for raccoons in summer: +captive females, open circles; captive males, closed circles; trapped males, open squares. Lines represent +polynomial regressions of evaporative water loss on chamber air temperature.</div> +</div> + +<a name="Fig_5"></a> +<span class="pagenum"><a name="Page_12" id="Page_12">[Pg 12]</a></span> +<div class="center"> + <img src="images/fig_5.png" width="382" height="333" title="Relationship between evaporative water loss and chamber air temperature for raccoons in winter" alt="winter - evap water loss vs temp"><br><br> + <div class="fig_cap"><span class="smcap">Figure 5.</span>—Relationship between evaporative water loss and chamber air temperature for raccoons in winter: captive females, open circles; captive males, closed circles. Lines represent polynomial regressions of evaporative water loss on chamber air temperature.</div> +</div> + +<a name="Table_4"></a> +<div class="tab_cap"><span class="smcap">Table 4.</span>—Polynomial regression equations describing evaporative water loss (mg·g<sup>-1</sup>·h<sup>-1</sup>) of <i>Procyon lotor</i> in +summer and winter (X = chamber temperature (°C), Y = evaporative water loss, n = number of observations, R<sup>2</sup> + = coefficient of determination, and SEE = standard error of estimate).</div> +<br> +<table width="100%" class="center" summary="Polnomial Regression Equations"> +<tr><th class="bt bb">Season and sex</th><th colspan=8 class="bt bl bb">Equation</th><th class="bt bb">(n)</th><th class="bt bb">R<sup>2</sup></th></tr> +<tr><td class="text_lf">Summer</td><td colspan=10 class="bl"> </td></tr> +<tr><td class="text_lf"> Trapped male</td><td class="bl">Y = </td><td class="text_lf">0.1899</td><td class="text_lf">+</td><td class="text_lf">0.0114 X</td><td class="text_lf">+ </td><td class="text_lf">0.0011 X<sup>2</sup></td><td class="text_lf">-</td><td class="text_lf">0.00002 X<sup>3</sup></td><td>(32)</td><td>0.86</td></tr> +<tr><td class="text_lf"> SEE</td><td class="bl"> </td><td class="text_lf">0.0885</td><td class="text_lf"> </td><td class="text_lf">0.0223</td><td class="text_lf"> </td><td class="text_lf">0.0015</td><td class="text_lf"> </td><td class="text_lf">0.00003</td><td> </td><td> </td></tr> +<tr><td class="text_lf"> </td><td class="bl" colspan=10> </td></tr> +<tr><td class="text_lf"> Captive male</td><td class="bl">Y = </td><td class="text_lf">0.2174</td><td class="text_lf">+</td><td class="text_lf">0.0192·X</td><td class="text_lf">+</td><td class="text_lf">0.0009·X<sup>2</sup></td><td class="text_lf">-</td><td class="text_lf">0.00003·X<sup>3</sup></td><td>(10)</td><td>0.73</td></tr> +<tr><td class="text_lf"> SEE</td><td class="bl"> </td><td class="text_lf">0.3983</td><td class="text_lf"> </td><td class="text_lf">0.0834</td><td class="text_lf"> </td><td class="text_lf">0.0048</td><td class="text_lf"> </td><td class="text_lf">0.00008</td><td> </td><td> </td></tr> +<tr><td class="text_lf"> </td><td class="bl" colspan=10> </td></tr> +<tr><td class="text_lf"> Captive female</td><td class="bl">Y = </td><td class="text_lf">0.0127</td><td class="text_lf">+</td><td class="text_lf">0.0943·X</td><td class="text_lf">- </td><td class="text_lf">0.0060·X<sup>2</sup></td><td class="text_lf">+</td><td class="text_lf">0.00013·X<sup>3</sup></td><td>(31)</td><td>0.64</td></tr> +<tr><td class="text_lf"> SEE</td><td class="bl"> </td><td class="text_lf">0.2218</td><td class="text_lf"> </td><td class="text_lf">0.0547</td><td class="text_lf"> </td><td class="text_lf">0.0036</td><td class="text_lf"> </td><td class="text_lf">0.00006</td><td> </td><td> </td></tr> +<tr><td class="text_lf"> </td><td class="bl" colspan=10> </td></tr> +<tr><td class="text_lf">Winter</td><td colspan=10 class="bl"> </td></tr> +<tr><td class="text_lf"> Captive, both sexes</td><td class="bl">Y = </td><td class="text_lf">0.1550</td><td class="text_lf">+</td><td class="text_lf">0.0426·X</td><td class="text_lf">-</td><td class="text_lf">0.0025·X<sup>2</sup></td><td class="text_lf">+</td><td class="text_lf">0.00006·X<sup>3</sup></td><td>(57)</td><td>0.80</td></tr> +<tr><td class="text_lf bb"> SEE</td><td class="bl bb"> </td><td class="text_lf bb">0.0734</td><td class="text_lf bb"> </td><td class="text_lf bb">0.0192</td><td class="bb text_lf"> </td><td class="text_lf bb">0.0013</td><td class="bb text_lf"> </td><td class="bb text_lf">0.00002</td><td class="bb"> </td><td class="bb"> </td></tr> +</table> +<br> + +<a name="Thermoregulation_at_Low_Temperatures"></a> +<a name="Body_Temperature_1"></a> +<span class="pagenum"><a href="#toc">[↑ TOC]</a></span> +<div class="caption3">Thermoregulation at Low Temperatures</div> +<div class="caption4">Body Temperature</div> + +<p>Body temperatures in <a href="#Fig_6">Figure 6</a> are those recorded during +metabolic measurements from animals equipped with surgically +implanted, temperature-sensitive radio transmitters. Each +point was recorded during the lowest level of oxygen +consumption at each T<sub>a</sub>. In both summer and winter, T<sub>b</sub>'s were +lowest during metabolic measurements at T<sub>a</sub>'s around T<sub>lc</sub>. At +T<sub>a</sub>'s below T<sub>lc</sub>, T<sub>b</sub>'s increased (<a href="#Fig_6">Figure 6</a>), which is an unusual +<span class="pagenum"><a name="Page_13" id="Page_13">[Pg 13]</a></span> +response. Under similar conditions, other procyonids either +maintain a nearly constant T<sub>b</sub> or allow it to fall slightly (Müller +and Kulzer, 1977; Chevillard-Hugot et al., 1980; Müller and +Rost, 1983; Chevalier, 1985). For our raccoons, confinement in +the metabolism chamber at low temperatures must have +stimulated a greater than necessary increase in metabolic rate +such that heat production exceeded heat loss, which caused T<sub>b</sub> +to become elevated.</p> + +<br> +<a name="Fig_6"></a> +<div class="center"> + <img src="images/fig_6.png" width="305" height="520" title="Relationship between body temperature and chamber air temperature" alt="body temp vs air temp"><br><br> + <div class="fig_cap"><span class="smcap">Figure 6.</span>—Relationship between body temperature and chamber air temperature in summer (panel A), and winter (panel B): captive females, open circles and solid lines; captive males, solid circles and dashed lines. Solid vertical lines represent lower critical temperatures.</div> +</div> + +<span class="pagenum"><a name="Page_14" id="Page_14">[Pg 14]</a></span> +<a name="Table_5"></a> +<div class="tab_cap"><span class="smcap">Table 5.</span>—Regression equations describing oxygen consumption <span class="nobreak">(mL O<sub>2</sub>·g<sup>-1</sup>·h<sup>-1</sup>)</span> of <i>Procyon lotor</i> at temperatures below +their lower critical temperature (I = x-intercept (°C), n = number of observations, R<sup>2</sup> = coefficient of determination, SEE + = standard error of estimate for the y-intercept (a) and slope (b), X = chamber temperature (°C), and Y = oxygen consumption).</div> +<br> +<table width="100%" class="center" summary="Oxygen Consumption Regression Equations"> +<tr><th rowspan=2 class="bt bb">Season and sex</th><th rowspan=2 class="bt bl bb">Equation</th><th rowspan=2 class="bt bb">(n)</th><th rowspan=2 class="bt bb">R<sup>2</sup></th><th colspan=2 class="bt bb">SEE</th><th rowspan=2 class="bt bb">I</th></tr> +<tr><th class="bb">a</th><th class="bb">b</th></tr> +<tr><td class="text_lf">Summer</td><td colspan=6 class="bl"> </td></tr> +<tr><td class="text_lf"> Trapped male</td><td class="bl">Y = 1.09 - 0.0281·X</td><td>(30)</td><td>0.64</td><td>0.0353</td><td>0.0040</td><td>38.8</td></tr> +<tr><td class="text_lf"> Captive male</td><td class="bl">Y = 0.97 - 0.0258·X</td><td>(12)</td><td>0.91</td><td>0.0235</td><td>0.0025</td><td>37.6</td></tr> +<tr><td class="text_lf"> Captive female</td><td class="bl">Y = 1.04 - 0.0251·X</td><td>(29)</td><td>0.78</td><td>0.0288</td><td>0.0026</td><td>41.1</td></tr> +<tr><td class="text_lf">Winter</td><td colspan=6 class="bl"> </td></tr> +<tr><td class="text_lf bb"> Captive, both sexes</td><td class="bl bb">Y = 0.68 - 0.0193·X</td><td class="bb">(36)</td><td class="bb">0.68</td><td class="bb">0.0157</td><td class="bb">0.0023</td><td class="bb">35.2</td></tr> +</table> +<br> + +<a name="Summer_1"></a> +<span class="pagenum"><a href="#toc">[↑ TOC]</a></span> +<div class="caption4">Summer</div> + +<p>During summer, T<sub>lc</sub> for male raccoons was 20°C, whereas for +females it was 25°C (<a href="#Fig_2">Figure 2</a>). Regression equations calculated +to describe oxygen consumption at T<sub>a</sub>'s below T<sub>lc</sub> are +presented in <a href="#Table_5">Table 5</a>. For three groups of summer animals, +slopes of regressions are identical. This indicates that minimum +conductances of these three groups were equivalent. Intercepts +of these equations are different, which suggests a difference in +metabolic cost of thermoregulation between these groups +(<a href="#Fig_2">Figure 2</a>); captive males had a lower intercept than either +trapped males (p<0.005) or captive females (p<0.05), but there +was no difference in intercepts of captive females and trapped +males. These regression equations, therefore, also were derived +using values of oxygen consumption expressed in terms of +metabolic body mass (Mellen, 1963). Relationships between +intercepts of these equations are different than those for +regressions in <a href="#Table_5">Table 5</a>. Intercept for females was intermediate +to, and not different from, those of the two groups of males. +However, captive males still had a lower intercept than trapped +males (p<0.025). Thus, in summer, thermoregulatory metabolism +was less expensive for captive than for trapped males, and +in spite of a 5°C difference in their T<sub>lc</sub>'s (<a href="#Fig_2">Figure 2</a>), captive +males and females had similar thermoregulatory costs.</p> + +<p>Regression lines for three groups of animals in summer +extrapolate to zero metabolism at values equivalent to, or +greater than, normal T<sub>b</sub>; 38.8°C for trapped males, 37.6°C for +captive males, and 41.1°C for captive females (<a href="#Table_5">Table 5</a>). Thus, +all three groups had minimized thermal conductance at T<sub>a</sub>'s +below T<sub>lc</sub> (Scholander et al., 1950b; McNab, 1980b). Minimum +wet thermal conductance calculated for raccoons in summer +with <a href="#Eq_4">Eq. 4</a> (<a href="#Table_3">Table 3</a>) is numerically similar to these "slope" +values (<a href="#Table_5">Table 5</a>), and it was, therefore, considered to be the best +estimate of C<sub>mw</sub> for <i>Procyon lotor</i> during that season <span class="nobreak">(0.0256 +mL O<sub>2</sub>·g<sup>-1</sup>·h<sup>-1</sup>·°C<sup>-1</sup>)</span>.</p> + +<a name="Winter_1"></a> +<span class="pagenum"><a href="#toc">[↑ TOC]</a></span> +<div class="caption4">Winter</div> + +<p>During winter T<sub>lc</sub> for both sexes decreased to 11°C (<a href="#Fig_3">Figure 3</a>). +Regression equations of thermoregulatory metabolism for +males and females in winter are not different from each other in +either slope or intercept. These data, therefore, were combined +into a single equation (<a href="#Table_5">Table 5</a>). Slope and intercept of this +equation are both lower (p<0.005 and p<0.05, respectively) +than those for summer animals (<a href="#Table_5">Table 5</a>). Identical results were +obtained from comparisons using regressions derived from +oxygen consumption expressed in terms of metabolic body +mass (Mellen, 1963). Thermoregulatory costs at any temperature +below 20°C were lower for winter than summer animals +(<a href="#Fig_2">Figures 2</a>,<a href="#Fig_3"> 3</a>).</p> + +<a name="Table_6"></a> +<div class="tab_cap"><span class="smcap">Table 6.</span>—Regression equations describing oxygen consumption <span class="nobreak">(mL +O<sub>2</sub>·g<sup>-1</sup>·h<sup>-1</sup>)</span> of <i>Procyon lotor</i> at temperatures below their lower critical +temperature in winter (A = females with radio transmitters, B = females without +radio transmitters, C = males, I = x-intercept (°C), n = number of observations, +R<sup>2</sup> = coefficient of determination, X = chamber temperature (°C), and Y = +oxygen consumption).</div> +<br> +<table width="100%" class="center" summary="Oxygen COnsumpsion Regression Equations"> +<tr><th class="bt bb">Group</th><th class="bt bl bb">Equation</th><th class="bt bb">(n)</th><th class="bt bb">R<sup>2</sup></th><th class="bt bb">I</th></tr> +<tr><td>A</td><td class="bl">Y = 0.63 - 0.0158·X</td><td>(10)</td><td>0.66</td><td>40.1</td></tr> +<tr><td>B</td><td class="bl">Y = 0.72 - 0.0226·X</td><td>(11)</td><td>0.71</td><td>32.1</td></tr> +<tr><td class="bb">C</td><td class="bl bb">Y = 0.69 - 0.0200·X</td><td class="bb">(15)</td><td class="bb">0.79</td><td class="bb">34.7</td></tr> +</table> +<br> +<a name="Fig_7"></a> +<span class="pagenum"><a name="Page_15" id="Page_15">[Pg 15]</a></span> +<div class="center"> + <img src="images/fig_7.png" width="437" height="587" title="Relationship between body temperature and time of day" alt="body temp vs time of day"><br><br> + <div class="fig_cap"><span class="smcap">Figure 7.</span>—Relationship between body temperature and time of day at various months of the year: captive females, open circles; captive males, closed circles. Vertical cross-hatched areas represent civil twilight.</div> +</div> + +<p>The regression line for <i>Procyon lotor</i> in winter (<a href="#Table_5">Table 5</a>) +extrapolates to zero metabolism at 35.2°C, which is below +normal T<sub>b</sub> (<a href="#Fig_6">Figures 6</a>,<a href="#Fig_7"> 7</a>). This suggests that not all raccoons +measured in winter minimized thermoregulatory metabolism or +conductances at T<sub>a</sub>'s below T<sub>lc</sub> (Scholander et al., 1950b; +McNab, 1980b). To assess this possibility, data for these +animals were divided into three groups: (A) females with radio +transmitters, (B) females without radio transmitters, and (C) +males (<a href="#Table_6">Table 6</a>). Regression equations of metabolism below T<sub>lc</sub> +were derived for each group, and based on extrapolated T<sub>b</sub>'s at +zero metabolism, only the two females with implanted radio +transmitters (group A) minimized thermoregulatory metabolism +and conductance. Had animals in groups B and C also +minimized their thermal conductances, while retaining their +measured metabolic rates, their rates of heat production would +have been disproportionately higher than their rates of heat +loss. Equation 4 predicts that under these conditions their body +temperatures would have been elevated to 42.0°C and 40.4°C, +respectively. Thus, in order to avoid such a large increase in +body temperature, animals in groups B and C increased their +thermal conductances in preference to lowering their metabolic +<span class="pagenum"><a name="Page_16" id="Page_16">[Pg 16]</a></span> +rates. The regression equation of thermoregulatory metabolism +for all winter animals (<a href="#Table_5">Table 5</a>), therefore, overestimates +minimum metabolic cost of temperature regulation below T<sub>lc</sub>, +and its slope underestimates C<sub>mw</sub>. Consequently, the best +estimate of C<sub>mw</sub> for <i>Procyon lotor</i> in winter is the value +calculated for group A animals with <a href="#Eq_4">Eq. 4</a> (0.0172 mL O<sub>2</sub>·g<sup>-1</sup>·h<sup>-1</sup>·°C<sup>-1</sup>; +<a href="#Table_3">Table 3</a>), and the minimum cost of thermoregulatory +metabolism at any T<sub>a</sub> below T<sub>lc</sub> is best estimated by +substituting this value into <a href="#Eq_4">Eq. 4</a> and solving for Ḣ<sub>r</sub>.</p> + +<a name="Thermoregulation_at_High_Temperatures"></a> +<a name="Body_Temperature_2"></a> +<span class="pagenum"><a href="#toc">[↑ TOC]</a></span> +<div class="caption3">Thermoregulation at High Temperatures</div> +<div class="caption4">Body Temperature</div> + +<p>In both summer and winter, T<sub>b</sub>'s increased during metabolic +measurements at T<sub>a</sub>'s above T<sub>lc</sub> (<a href="#Fig_6">Figure 6</a>). This response also +was seen during metabolic measurements conducted on other +procyonids (Müller and Kulzer, 1977; Chevillard-Hugot et al., +1980; Müller and Rost, 1983; Chevalier, 1985).</p> + +<a name="Summer_2"></a> +<span class="pagenum"><a href="#toc">[↑ TOC]</a></span> +<div class="caption4">Summer</div> + +<p>During summer our data suggested that the upper critical +temperature (T<sub>uc</sub>) was higher than 35°C. The lowest rates of +oxygen consumption at T<sub>a</sub> = 35°C occurred after 1.5 to 2.5 +hours of exposure to that temperature. Prolonged exposure to +this temperature in summer did not make animals restless, and +their rate of oxygen consumption was very stable throughout +each measurement. Body temperature responses at T<sub>a</sub> = 35°C +were recorded from two males and two females that had +implanted radio transmitters. With the exception of one male, +T<sub>b</sub>'s were maintained near 38°C (<a href="#Fig_6">Figure 6</a>). The one exception +(a male) maintained its T<sub>b</sub> at 39.3°C. At T<sub>a</sub> = 35°C, summer +males had rates of evaporative water loss that were lower than +those of summer females (<a href="#Fig_4">Figure 4</a>). At this temperature, males +dissipated 35% ± 6% and females 56% ± 18% of their metabolic +heat via evaporative water loss. Thus, at T<sub>a</sub> = 35°C, males must +have utilized modes of heat transfer other than evaporative +cooling (convective and conductive heat transfer) to a greater +extent than females.</p> + +<a name="Winter_2"></a> +<span class="pagenum"><a href="#toc">[↑ TOC]</a></span> +<div class="caption4">Winter</div> + +<p>Body temperature, evaporative water loss, and metabolic +data indicated that, in winter, T<sub>uc</sub> was very close to 35°C. In +winter, the lowest level of oxygen consumption was recorded +during the first hour after the chamber had reached T<sub>a</sub> = 35°C. +Unlike summer, animals became restless after the first hour at +35°C, at which point their oxygen consumption increased and +showed a high degree of variability. Body temperature +responses at 35°C were recorded from both females that had +implanted radio transmitters. In one case, T<sub>b</sub> rose from 37.9°C +at the end of the first hour to 40.5°C by the end of the second +hour, and as it did not show signs of leveling off, we terminated +the experiment. We exposed that same animal to T<sub>a</sub> = 35°C one +other time during winter. In that instance, its T<sub>b</sub> rose to 40.0°C +during the first 30 minutes and was maintained at that level for +three hours with no apparent distress. The other female elevated +its T<sub>b</sub> from 37.3°C to 39.0°C during the second hour at +T<sub>a</sub> = 35°C and maintained its T<sub>b</sub> at that level for +two hours. Thus, during winter, prolonged exposure to +T<sub>a</sub> = 35°C stimulated more of an increase in T<sub>b</sub> than it did in +summer. During winter, both males and females increased +evaporative water loss at T<sub>a</sub> = 35°C (<a href="#Fig_5">Figure 5</a>) but only to the +extent that they dissipated 35% ± 10% of their metabolic heat +production. Thus, even in winter, convective and conductive +heat transfers were still the most important modes of heat loss +at this temperature.</p> + +<a name="Daily_Cycle_of_Body_Temperature"></a> +<span class="pagenum"><a href="#toc">[↑ TOC]</a></span> +<div class="caption3">Daily Cycle of Body Temperature</div> + +<p>The daily cycle of raccoon T<sub>b</sub>'s during summer and winter +are presented in <a href="#Fig_7">Figure 7</a>. In general, T<sub>b</sub>'s showed a marked +circadian cycle in phase with photoperiod. T<sub>b</sub>'s rose above +38°C for several hours each night but remained below 38°C +during daytime. During summer, with the exception of one +female whose record was not typical (<a href="#Fig_7">Figure 7</a>), T<sub>b</sub>'s rose above +38°C shortly after sunset, whereas in winter T<sub>b</sub>'s did not rise +above 38°C until several hours after sunset. Once T<sub>b</sub> was +elevated it usually remained so until just before or after sunrise +(<a href="#Fig_7">Figure 7</a>). During summer, T<sub>b</sub> was above 38°C for 85% or +more of the time between sunset and sunrise (87% for the +female with the typical body temperature pattern, and 85% and +98% for males), whereas in winter it was elevated for only +47%-78% of the time between sunset and sunrise (47% and +61% for females, and 67% and 78% for males). During night, +T<sub>b</sub> would oscillate between 38°C and about 39°C, such that two +peak values occurred. These peak values presumably corresponded +to two periods of heightened nighttime activity. +During summer, one of these peaks occurred before and the +other after 24:00 hours, whereas in winter both peaks occurred +after 24:00 hours. With the exception of one female in winter +(<a href="#Fig_7">Figure 7</a>), the lowest T<sub>b</sub> of the day for both sexes was near +37°C, and this typically occurred during daytime (<a href="#Fig_7">Figure 7</a>).</p> + + + +<br> +<a name="Discussion"></a> +<a name="Basal_Metabolic_Rate_3"></a> +<a name="Background_1"></a> +<span class="pagenum"><a href="#toc">[↑ TOC]</a></span> +<div class="caption2">Discussion</div> +<div class="caption3">Basal Metabolic Rate</div> +<div class="caption4">Background</div> + +<p>Basal metabolism represents the minimum energy required +by a mammal to maintain endothermy and basic homeostasis +(Lusk, 1917:141; Kleiber, 1932, 1961:251; Benedict, 1938:191-215; +Brody, 1945:59; Robbins, 1983:105-111). Mammals +with lower than predicted Ḣ<sub>b</sub> maintain endothermy and +enjoy its attendant advantages at a discount, whereas others, +with rates that are higher than predicted, pay a premium +<span class="pagenum"><a name="Page_17" id="Page_17">[Pg 17]</a></span> +(Calder, 1987). Such variation in Ḣ<sub>b</sub> appears to be tied to +ecological circumstances rather than taxonomic affinities +(Vogel, 1980; McNab, 1986a, 1988a, 1989), and depending on +environmental conditions, each rate provides an individual +with various advantages and limitations. During the course of +evolution, therefore, each species' Ḣ<sub>b</sub> evolves to provide it with +the best match between its energy requirements for continuous +endothermy, its food supply, and the thermal characteristics of +its environment.</p> + +<a name="Captive_versus_Wild_Raccoons"></a> +<span class="pagenum"><a href="#toc">[↑ TOC]</a></span> +<div class="caption4">Captive versus Wild Raccoons</div> + +<p>Male raccoons trapped in summer had higher Ḣ<sub>b</sub>'s than our +captive animals in any season (<a href="#Table_2">Table 2</a>). The higher rate of +metabolism of these trapped males could have been due to the +stress of captivity or to the fact that "wild" animals actually +may have higher metabolic rates than those that have adjusted +to captivity. If the latter is true, then our data for captive +animals underestimated the actual energy cost of maintenance +metabolism for <i>Procyon lotor</i> in the wild. At present, we have +no way of determining which of these alternatives is true.</p> + +<a name="Seasonal_Metabolism_of_Raccoons"></a> +<span class="pagenum"><a href="#toc">[↑ TOC]</a></span> +<div class="caption4">Seasonal Metabolism of Raccoons</div> + +<p>In some temperate-zone mammals, Ḣ<sub>b</sub> is elevated in winter, +which presumably increases their "cold-hardiness." Conversely, +lower summer metabolism is considered to be a +mechanism that reduces the potential for heat stress. Such +seasonal variation in Ḣ<sub>b</sub> has been found in several species: +collard peccary, <i>Tayassu tajacu</i> (Zervanos, 1975); antelope +jackrabbit, <i>Lepus alleni</i> (Hinds, 1977); desert cottontail, +<i>Sylvilagus audubonii</i> (Hinds, 1973); and, perhaps, cold-acclimatized +rat, <i>Rattus norvegicus</i> (Hart and Heroux, 1963). +Unlike these species, our captive raccoons showed no seasonal +variation in Ḣ<sub>b</sub> (<a href="#Table_2">Table 2</a>). Instead, raccoons achieved "cold-hardiness" +in winter and reduced their potential for heat stress +in summer with a large seasonal change in thermal conductance +(<a href="#Table_3">Table 3</a>).</p> + +<a name="Table_7"></a> +<div class="tab_cap"> +<span class="smcap">Table 7.</span>—Metabolic characteristics of several procyonid species. +</div> +<br> +<table width="100%" class="center" summary="Metabolic Characteristics"> +<tr><th class="bt bb center" rowspan=2>Species</th><th class="bt bl bb center" rowspan=2>Body<br>Mass<br>(g)</th><th class="bt"> </th><th class="bt bb center" colspan=2>Basal<sup><a name="FNanchor_A_6"></a><a href="#Footnote_A_6">[a]</a></sup><br>metabolism</th><th class="bt"> </th><th class="bt bb center" colspan=2>Minimum<sup><a name="FNanchor_B_7"></a><a href="#Footnote_B_7">[b]</a></sup><br>conductance</th><th class="bt"> </th><th class="bt bb center" colspan=2>T<sub>b</sub><sup><a name="FNanchor_C_8"></a><a href="#Footnote_C_8">[c]</a></sup></th><th class="bt"> </th><th class="bt bb center" colspan=2>T<sub>n</sub><sup><a name="FNanchor_D_9"></a><a href="#Footnote_D_9">[d]</a></sup></th><th class="bt bb" rowspan=2> </th><th class="bt bb center" rowspan=2>References</th></tr> +<tr><th class="bb"> </th><th class="bb center">Meas</th><th class="bb">H<sub>br</sub></th><th class="bb"> </th><th class="bb center"> Meas </th><th class="bb">C<sub>mwr</sub></th><th class="bb"> </th><th class="bb center">α</th><th class="bb">ρ</th><th class="bb"> </th><th class="bb center">T<sub>lc</sub></th><th class="bb">T<sub>uc</sub></th></tr> +<tr><td class="text_lf"><i>Bassariscus astutus</i></td><td class="bl">865</td><td> </td><td>0.43</td><td>0.68</td><td> </td><td>0.0288<sup><a name="FNanchor_E_10"></a><a href="#Footnote_E_10">[e]</a></sup></td><td> 0.85</td><td> </td><td> 37.6 </td><td>23</td><td> </td><td>35.5</td><td colspan=2> </td><td class="text_lf">Chevalier (1985)</td></tr> +<tr><td class="text_lf"><i>Procyon cancrivorus</i></td><td class="bl">1160</td><td> </td><td>0.40</td><td>0.69</td><td> </td><td>0.0368<sup><a href="#Footnote_E_10">[e]</a></sup></td><td>1.25</td><td> </td><td> </td><td> </td><td> </td><td>26</td><td colspan=2> </td><td class="text_lf">Scholander et al. (1950b, c)</td></tr> +<tr><td class="text_lf"><i>Potos flavus</i></td><td class="bl">2030</td><td> </td><td>0.36</td><td>0.51</td><td> </td><td> </td><td> </td><td> </td><td> </td><td colspan=5> </td><td class="text_lf">McNab (1978a)</td></tr> + +<tr><td class="text_lf"><i>Potos flavus</i></td><td class="bl">2400</td><td> </td><td>0.32</td><td>0.65</td><td> </td><td> </td><td> </td><td> </td><td>38.1</td><td>36.0</td><td> </td><td>23</td><td>30</td><td> </td><td class="text_lf">Müller and Kulzer (1977)</td></tr> +<tr><td class="text_lf"><i>Potos flavus</i></td><td class="bl">2600</td><td> </td><td>0.34</td><td>0.71</td><td> </td><td>0.0200<sup><a name="FNanchor_F_11"></a><a href="#Footnote_F_11">[f]</a></sup></td><td>1.02</td><td colspan=4> </td><td>23</td><td>33</td><td> </td><td class="text_lf">Müller and Rost (1983)</tr> +<tr><td class="text_lf"><i>Nasua nasua</i></td><td class="bl">3850</td><td> </td><td>0.26</td><td>0.60</td><td> </td><td>0.0200<sup><a href="#Footnote_F_11">[f]</a></sup></td><td>1.24</td><td> </td><td>38.3</td><td>36.4</td><td> </td><td>25</td><td>33</td><td> </td><td class="text_lf">Chevillard-Hugot et al. (1980)</td></tr> +<tr><td class="text_lf"><i>Nasua nasua</i></td><td class="bl">4847</td><td> </td><td>0.33</td><td>0.79</td><td> </td><td>0.0238<sup><a href="#Footnote_E_10">[e]</a></sup></td><td>1.65</td><td> </td><td>39.1</td><td>37.9</td><td> </td><td>30</td><td>35</td><td> </td><td class="text_lf">Mugaas et al. (in prep.)</td></tr> +<tr><td class="text_lf"><i>Nasua narica</i></td><td class="bl">5554</td><td> </td><td>0.25</td><td>0.62</td><td> </td><td>0.0208<sup><a href="#Footnote_E_10">[e]</a></sup></td><td>1.55</td><td> </td><td>38.9</td><td>37.4</td><td> </td><td>25</td><td>35</td><td> </td></tr> +<tr><td class="text_lf"><i>Nasua narica</i></td><td class="bl">4150</td><td> </td><td>0.42</td><td>1.20</td><td> </td><td>0.0341<sup><a href="#Footnote_E_10">[e]</a></sup></td><td>2.20</td><td colspan=7> </td><td class="text_lf">Scholander et al. (1950b, c)</td></tr> +<tr><td> </td><td class="bl" colspan=5> </td><td>0.0224<sup><a name="FNanchor_G_12"></a><a href="#Footnote_G_12">[g]</a></sup></td><td>1.45</td><td> </td></tr> +<tr><td class="text_lf"><i>Procyon lotor</i></td><td class="bl" colspan=14> </td><td class="text_lf">This study</td></tr> +<tr><td class="text_lf"><i>Summer</i></td><td class="bl" colspan=10> </td></tr> +<tr><td class="text_lf ind2em">Trapped male</td><td class="bl">4400</td><td> </td><td>0.54</td><td>1.28</td><td colspan=6> </td><td> </td><td>20</td><td colspan=3> </td></tr> +<tr><td class="text_lf ind2em">Captive male</td><td class="bl">4790</td><td> </td><td>0.46</td><td>1.07</td><td> </td><td>0.0256<sup><a href="#Footnote_F_11">[f]</a></sup></td><td>1.77</td><td> </td><td>38.4</td><td>37.5</td><td> </td><td>20</td><td colspan=3> </td></tr> +<tr><td class="text_lf ind2em">Captive female</td><td class="bl">4670</td><td> </td><td>0.42</td><td>1.02</td><td> </td><td>0.0256<sup><a href="#Footnote_F_11">[f]</a></sup></td><td>1.79</td><td> </td><td>38.2</td><td>37.6</td><td> </td><td>25</td><td colspan=3> </td></tr> +<tr><td class="text_lf"><i>Winter</i></td><td class="bl" colspan=15> </td></tr> +<tr><td class="text_lf ind2em">Captive male</td><td class="bl">5340</td><td> </td><td>0.47</td><td>1.17</td><td> </td><td colspan=3> </td><td>38.6</td><td>38.6</td><td> </td><td>11</td><td colspan=3> </td></tr> +<tr><td class="bb text_lf ind2em">Captive female</td><td class="bl bb">4490</td><td class="bb"> </td><td class="bb">0.46</td><td class="bb">1.10</td><td class="bb"> </td><td class="bb">0.0172<sup><a href="#Footnote_F_11">[f]</a></sup></td><td class="bb">1.15</td><td class="bb"> </td><td class="bb">38.3</td><td class="bb">37.3</td><td class="bb"> </td><td class="bb">11</td><td class="bb" colspan=3> </td></tr> +</table> +<br> +<div class="footnote"> +<a name="Footnote_A_6" id="Footnote_A_6"></a><a href="#FNanchor_A_6"><span class="label">[a]</span></a> +Meas is measured basal metabolism <span class="nobreak">(mL O<sub>2</sub>·g<sup>-1</sup>·h<sup>-1</sup>).</span> H<sub>br</sub> is the ratio of measured to predicted basal metabolism where the predicted value is calculated from Ḣ<sub>b</sub> + = 3.42·m<sup>-.25</sup> (Kleiber, 1932, 1961:206) and m is body mass in grams.<br> +<br> + +<a name="Footnote_B_7" id="Footnote_B_7"></a><a href="#FNanchor_B_7"><span class="label">[b]</span></a> Meas is measured minimum thermal conductance <span class="nobreak">(mL O<sub>2</sub>·g<sup>-1</sup>·h<sup>-1</sup>·°C<sup>-1</sup>).</span> C<sub>mwr</sub> is the ratio of measured to predicted minimum thermal conductance where the predicted value is calculated from C<sub>m</sub> = 1.0·m<sup>-0.5</sup> (McNab and Morrison, 1963; Herreid and Kessel, 1967), and m is body mass in grams.<br> +<br> + +<a name="Footnote_C_8" id="Footnote_C_8"></a><a href="#FNanchor_C_8"><span class="label">[c]</span></a> T<sub>b</sub> is body temperature during the active (α) and rest (ρ) phases of the daily cycle (°C).<br> +<br> + +<a name="Footnote_D_9" id="Footnote_D_9"></a><a href="#FNanchor_D_9"><span class="label">[d]</span></a> T<sub>n</sub> is the thermoneutral zone as defined by the lower (T<sub>lc</sub>) and upper (T<sub>uc</sub>) critical temperatures (°C).<br> +<br> + +<a name="Footnote_E_10" id="Footnote_E_10"></a><a href="#FNanchor_E_10"><span class="label">[e]</span></a> Conductance calculated as the slope of the line describing oxygen consumption at temperatures below the lower critical temperature.<br> +<br> + +<a name="Footnote_F_11" id="Footnote_F_11"></a><a href="#FNanchor_F_11"><span class="label">[f]</span></a> Conductance calculated from C<sub>mw</sub> = Ḣ<sub>r</sub>/(T<sub>b</sub> - T<sub>a</sub>), where Ḣ<sub>r</sub> is resting metabolic rate at temperatures below T<sub>lc</sub>, and other symbols are as described elsewhere.<br> +<br> + +<a name="Footnote_G_12" id="Footnote_G_12"></a><a href="#FNanchor_G_12"><span class="label">[g]</span></a> Inactive-phase thermal conductance: estimated from Scholander et al. (1950b), assuming that active-phase thermal conductance is 52% higher than values determined during the inactive phase (Aschoff, 1981).<br> +<br> +</div> + + +<a name="Comparison"></a> +<span class="pagenum"><a href="#toc">[↑ TOC]</a></span> +<div class="caption4">Comparison of Procyon lotor with Other Procyonids</div> + +<p><i>Procyon lotor</i> has a much higher mass-specific Ḣ<sub>b</sub> than other +procyonids (<a href="#Table_7">Table 7</a>). To quantify the magnitude of this +difference, we compared the measured value for <i>Procyon lotor</i> +<span class="pagenum"><a name="Page_18" id="Page_18">[Pg 18]</a></span> +with one calculated for it from a mass-specific least-squares +regression equation (<a href="#Eq_6">Eq. 6</a>; R<sup>2</sup> = 0.78) derived from data for +those procyonids with lower than predicted Ḣ<sub>b</sub>: <i>Potos flavus</i>, +<i>Procyon cancrivorus</i>, <i>Nasua nasua</i>, <i>Nasua narica</i>, and +<i>Bassariscus astutus</i> (<a href="#Table_7">Table 7</a>).</p> + +<a name="Eq_6"></a> +<table width="100%" summary="Eq. 6"> +<tr><td class="center">Ḣ<sub>b</sub> = 2.39·m<sup>-0.25</sup></td><td class="text_rt">Eq. 6</td></tr> +</table> + +<p>Ḣ<sub>b</sub> in Eq. 6 is basal metabolism <span class="nobreak">(mL O<sub>2</sub>·g<sup>-1</sup>·h<sup>-1</sup>)</span> and m is body +mass (g). Measured values of Ḣ<sub>b</sub> for <i>Procyon lotor</i> were 1.45 to +1.86 times greater than those predicted for it by Eq. 6 (<a href="#Table_8">Table 8</a>).</p> + +<a name="Table_8"></a> +<div class="tab_cap"><span class="smcap">Table 8.</span>—Basal metabolism <span class="nobreak">(mL O<sub>2</sub>·g<sup>-1</sup>·h<sup>-1</sup>)</span> of <i>Procyon lotor</i> as predicted by +<a href="#Eq_6">Eq. 6</a> (Ḣ<sub>b</sub> = 2.39·m<sup>-0.25</sup>). Body masses, used to calculate predicted values, and +measured values were taken from <a href="#Table_7">Table 7</a>.</div> +<br> +<div class="center"> +<table width="60%" class="center" summary="Basal Metabolism"> +<tr><th class="bt bb">Season and sex</th><th class="bt bl bb">Predicted</th><th class="bt bb"> </th><th class="bt bb">Measured/Predicted</th></tr> +<tr><td class="text_lf">Summer</td><td colspan=3 class="bl"> </td></tr> +<tr><td class="text_lf"> Trapped male</td><td class="bl">0.29</td><td> </td><td>1.86</td></tr> +<tr><td class="text_lf"> Captive male</td><td class="bl">0.29</td><td> </td><td>1.59</td></tr> +<tr><td class="text_lf"> Captive female</td><td class="bl">0.29</td><td> </td><td>1.45</td></tr> +<tr><td class="text_lf">Winter</td><td colspan=3 class="bl"> </td></tr> +<tr><td class="text_lf"> Captive male</td><td class="bl">0.28</td><td> </td><td>1.68</td></tr> +<tr><td class="text_lf bb"> Captive female</td><td class="bl bb">0.29</td><td class="bb"> </td><td class="bb">1.59</td></tr> +</table> +</div> +<br> + +<a name="Influence_of_Diet"></a> +<a name="Background_2"></a> +<span class="pagenum"><a href="#toc">[↑ TOC]</a></span> +<div class="caption4">Influence of Diet on Basal Metabolism</div> + +<p><span class="smcap">Background.</span>—With respect to Ḣ<sub>b</sub>, McNab (1986a:1) +maintains that "the influence of climate is confounded with the +influence of food habits," and that departures from the Kleiber +(1961) "norm" are best correlated with diet. Although this does +appear to be the case for diet specialists, the analysis is not so +clear-cut for omnivorous species (McNab, 1986a). His analysis +also indicates that an animal's "behavior" (i.e., whether it is +terrestrial, arboreal, subterranean, aquatic, etc.), secondarily +modifies the influence of food habits on Ḣ<sub>b</sub>. For example, +terrestrial frugivores have Ḣ<sub>b</sub>'s that are very near predicted +values, whereas arboreal frugivores have rates that are much +lower than predicted (McNab, 1986a).</p> + +<a name="Table_9"></a> +<div class="tab_cap"><span class="smcap">Table 9.</span>—Food habits of some Procyonids. References for foods were as follows: <i>Potos flavus</i>, <i>Procyon cancrivorus</i>, and <i>Nasua nasua</i> taken from Bisbal (1986); <i>Nasua narica</i> taken from Kaufmann (1962:182-198); <i>Bassariscus astutus</i> taken from Martin et al. (1951), Taylor (1954), Wood (1954), Toweill and Teer (1977), and Trapp (1978); <i>Procyon lotor</i> taken from Hamilton (1936), Stuewer (1943:218-220), Stains (1956:39-51), and Greenwood (1981). Symbols represent either qualitative (#) or quantitative (+, †) assessments of feeding habits: # indicates that the animal was observed eating the food; + and † represent volume and frequency, respectively, of food utilization. No attempt was made to account for seasonal variation in the use of these foods.</div> +<br> +<table width="70%" summary="key"> +<tr><td>+</td><td><20% by volume when found.</td><td>†</td><td>1%-19% frequency of occurrence.</td></tr> +<tr><td>++</td><td>>20% by volume when found.</td><td>††</td><td>20%-50% frequency of occurrence.</td></tr> +<tr><td colspan=2> </td><td>†††</td><td>>50% frequency of occurrence.</td></tr> +</table> +<br> + +<table width="100%" class="center" summary="Prey of Procyonids"> +<tr><th class="text_lf bt br bb">Food</th><th colspan=2 class="bt bb">Potos flavus</th><th colspan=2 class="bt bb">Procyon cancrivorus</th><th colspan=2 class="bt bb">Nasua nasua</th><th colspan=2 class="bt bb">Nasua narica</th><th colspan=2 class="bt bb">Bassariscus</th><th colspan=2 class="bt bb">Procyon lotor</th></tr> +<tr><td class="text_lf br">Mammalia</td><td> </td><td> </td><td> </td><td> </td><td>+</td><td>†</td><td> #</td><td> </td><td>++</td><td>†††</td><td>++</td><td>††</td></tr> +<tr><td class="text_lf br">Aves</td><td> </td><td> </td><td> </td><td> </td><td> </td><td> </td><td> </td><td> </td><td>++</td><td>†</td><td>+</td><td>††</td></tr> +<tr><td class="text_lf br">Birds' eggs</td><td> </td><td colspan=10> </td><td>†††</td></tr> +<tr><td class="text_lf br">Reptilia</td><td> </td><td> </td><td>+</td><td>†</td><td>+</td><td>†††</td><td> #</td><td> </td><td>+</td><td>†</td><td>+</td><td>†</td></tr> +<tr><td class="text_lf br">Amphibia</td><td> </td><td> </td><td>+</td><td>†</td><td> </td><td> </td><td> </td><td> </td><td> </td><td> </td><td>+</td><td>†</td></tr> +<tr><td class="text_lf br">Pices</td><td> </td><td> </td><td>++</td><td>††</td><td> </td><td> </td><td> </td><td> </td><td> </td><td> </td><td>++</td><td>††</td></tr> +<tr><td class="text_lf br">Insecta</td><td>++</td><td>†</td><td>+</td><td>†††</td><td>++</td><td>†††</td><td> #</td><td> </td><td>+</td><td>††</td><td>++</td><td>††</td></tr> +<tr><td class="text_lf br">Arachnida</td><td> </td><td> </td><td> </td><td> </td><td>++</td><td>†††</td><td> #</td><td> </td><td>+</td><td>†</td><td>+</td><td>†</td></tr> +<tr><td class="text_lf br">Chilopoda</td><td> </td><td> </td><td> </td><td> </td><td>++</td><td>†††</td><td colspan=6> </td></tr> +<tr><td class="text_lf br">Diplopoda</td><td colspan=6> </td><td> #</td><td> </td><td> </td><td> </td><td>+</td><td>†</td></tr> +<tr><td class="text_lf br">Crustacea</td><td> </td><td> </td><td>++</td><td>†††</td><td> </td><td> </td><td> #</td><td> </td><td> </td><td> </td><td>++</td><td>†††</td></tr> +<tr><td class="text_lf br">Mollusca</td><td> </td><td> </td><td>+</td><td>††</td><td> </td><td> </td><td> #</td><td> </td><td> </td><td> </td><td>+</td><td>††</td></tr> +<tr><td class="text_lf br">Annelida</td><td colspan=6> </td><td> #</td><td colspan=3> </td><td>+</td><td>†</td></tr> +<tr><td class="text_lf br">Nuts</td><td colspan=10> </td><td>++</td><td>††</td></tr> +<tr><td class="text_lf br">Grains</td><td colspan=10> </td><td>++</td><td>††</td></tr> +<tr><td class="text_lf br">Buds</td><td colspan=10> </td><td>+</td><td>†</td></tr> +<tr><td class="text_lf br">Fruit</td><td>++</td><td>†††</td><td> </td><td> </td><td>++</td><td> </td><td> #</td><td> </td><td> </td><td>††</td><td>++</td><td>†††</td></tr> +<tr><td class="text_lf br">Leaves</td><td colspan=10> </td><td>+</td><td>†</td></tr> +<tr><td class="text_lf br bb">Grass</td><td class="bb" colspan=10> </td><td class="bb">+</td><td class="bb"> †</td></tr> +</table> +<br> + +<a name="Food_Habits_of_Procyonids"></a> +<p><span class="smcap">Food Habits of Procyonids.</span>—Food habits of six procyonids +for which metabolic data are available are presented in +<a href="#Table_9">Table 9</a>. All six species clearly have mixed diets. Compared to +other species, <i>Procyon lotor</i> is highly catholic in its diet, taking +food from almost twice as many categories as <i>Nasua narica</i>, +three times as many as <i>Procyon cancrivorus</i>, <i>Nasua nasua</i>, and +<i>Bassariscus astutus</i>, and nine times as many as <i>Potos flavus</i>.</p> + +<p><span class="pagenum"><a name="Page_19" id="Page_19">[Pg 19]</a></span> +For those species for which food habit data are quantified, we +used Eisenberg's (1981:247-251) substrate/feeding matrix +method, where "substrate" is analogous to McNab's (1986a) +"behavior," to construct the following feeding categories that +are based on the major food groups utilized by each species +(<a href="#Table_9">Table 9</a>).</p> + +<div class="blockquot"><p class="noidt"> +1. <i>Potos flavus:</i> (1) arboreal/frugivore, insectivore.<br> +2. <i>Procyon cancrivorus:</i> (1) semiaquatic/crustacivore, molluscivore, insectivore, piscivore, carnivore.<br> +3. <i>Nasua nasua:</i> (1) terrestrial/insectivore, arachnidivore, carnivore, frugivore.<br> +4. <i>Bassariscus astutus:</i> (1) terrestrial/carnivore, insectivore, frugivore.<br> +5. <i>Procyon lotor:</i> (1) terrestrial/carnivore, granivore, frugivore, insectivore; and (2) semiaquatic/crustacivore, molluscivore, insectivore, piscivore, carnivore. +</p></div> + +<a name="Food_Habits_and_Basal_Metabolism"></a> +<p><span class="smcap">Food Habits and Basal Metabolism.</span>—The most important +foods in the diet of <i>Procyon lotor</i> are vertebrates, nuts, +seeds, and fruits (<a href="#Table_9">Table 9</a>). These are the same foods that are +eaten by those dietary specialists that have Ḣ<sub>b</sub>'s equivalent to, +or higher than, values predicted for them by the Kleiber +equation (McNab, 1986a). The most important foods in the +diets of <i>Potos flavus</i>, <i>Procyon cancrivorus</i>, and <i>Nasua nasua</i> +are invertebrates and fruit (<a href="#Table_9">Table 9</a>), and these foods are eaten +by dietary specialists that have lower than predicted Ḣ<sub>b</sub>'s +(McNab, 1986a). Major foods in the diet of <i>Bassariscus astutus</i> +are terrestrial vertebrates, insects, and fruit (<a href="#Table_9">Table 9</a>). Dietary +specialists that eat terrestrial vertebrates have higher than +predicted Ḣ<sub>b</sub>'s, whereas those that feed on insects have Ḣ<sub>b</sub>'s +that are lower than predicted (McNab, 1986a). Year-round +utilization of vertebrates by <i>Bassariscus astutus</i> suggests that it +also should have a metabolic rate that is equivalent to or higher +than predicted, rather than lower (McNab, 1986a). However, +perhaps year-round inclusion of insects in its diet (Martin et al., +1951; Taylor, 1954; Wood, 1954; Toweill and Teer, 1977; +Trapp, 1978), plus water-and energy-conserving advantages of +a low metabolic rate, each exert a stronger selective influence +on Ḣ<sub>b</sub> than do vertebrates in its diet.</p> + +<a name="Summary_1"></a> +<p><span class="smcap">Summary.</span>—The basal metabolic rate of these procyonids +does appear to be influenced by diet. But, it is apparent from +this family's evolutionary history and tropical origins that +climate also has had a profound influence on its member's +metabolism. The history of the family and the data presented +here (<a href="#Table_7">Table 7</a>) suggest that lower than predicted Ḣ<sub>b</sub> is a feature +that evolved very early as the primary metabolic adjustment to +a tropical climate. From this perspective, it could be argued that +climate would have been the major selective force determining +Ḣ<sub>b</sub>, whereas food habits would have had a secondary influence.</p> + +<a name="Basal_Metabolism_1"></a> +<span class="pagenum"><a href="#toc">[↑ TOC]</a></span> +<a name="Background_3"></a> +<div class="caption4">Basal Metabolism and Intrinsic Rate of Natural Increase</div> + +<p><span class="smcap">Background.</span>—McNab (1980a) suggested that if food is +not restricted during an animal's reproductive period, the factor +that will limit growth and reproduction will be the rate at which +energy can be used in growth and development. Under these +conditions, an increase in Ḣ<sub>b</sub> would actually increase r<sub>max</sub> +because it would provide a higher rate of biosynthesis, a faster +growth rate, and a shorter generation time. Hennemann (1983) +tested McNab's (1980a) premise and found a significant +correlation between r<sub>max</sub> and metabolic rate, independent of +body size, for 44 mammal species. A low correlation +coefficient for this relationship, however, indicated to him +(Hennemann, 1983) that factors such as (1) food supply, (2) +thermal characteristics of the environment, and (3) brain size +also contribute toward shaping a species' reproductive potential, +particularly when these factors strongly influence rates of +biosynthesis or growth or for some reason alter generation time. +Results of our estimates of r<sub>max</sub> for procyonids are presented in +<a href="#Table_10">Table 10</a>.</p> + +<a name="Procyon_lotor_1"></a> +<p><i>Procyon lotor.</i>—This species had the highest Ḣ<sub>b</sub> and D<sub>d</sub>, and +also had the highest r<sub>max</sub> (1.34; <a href="#Table_10">Table 10</a>). Such a high r<sub>max</sub> may +infer that this trait evolved under conditions where food and +temperature were not limiting to reproduction. Under these +conditions selection could have favored those reproductive +characteristics sensitive to a higher Ḣ<sub>b</sub> (biosynthesis, growth, +and generation time; McNab, 1980a). <i>Procyon lotor</i>'s high +reproductive potential is due to its early age of first female +reproduction and its large litter size, characteristics that may +reflect metabolically driven increases in both biosynthesis and +growth.</p> + +<a name="Bassariscus_astutus_1"></a> +<p><i>Bassariscus astutus.</i>—This species has a low Ḣ<sub>b</sub> but an r<sub>max</sub> +that was 124% of expected (<a href="#Table_10">Table 10</a>). This suggests that r<sub>max</sub> +evolved under conditions where food and temperature were not +limiting to reproduction. Reduced litter size should restrict this +species' reproductive potential and may be a reflection of its +low Ḣ<sub>b</sub>. The factor that is responsible for increasing its +reproductive potential, however, is its early age of first female +reproduction. <i>Bassariscus astutus</i> is the smallest of these +procyonids, and even though it has a low Ḣ<sub>b</sub>, its small mass +may contribute to its ability to reach adult size and sexual +maturity in its first year. The high quality of its diet (a high +proportion of small vertebrates; <a href="#Table_9">Table 9</a>) also may be a factor +that is permissive to early female reproduction. Thus, small +body size and diet may be factors that have allowed this species +to evolve a higher than expected reproductive potential in spite +of its low Ḣ<sub>b</sub>.</p> + +<a name="Nasua_narica_1"></a> +<p><i>Nasua narica.</i>—This species is one of the largest procyonids +(<a href="#Table_7">Table 7</a>), and it possesses characteristics that should limit its +reproductive potential: lower than predicted Ḣ<sub>b</sub> (<a href="#Table_7">Table 7</a>), a +relatively low-quality diet (Kaufmann, 1962:182-198; <a href="#Table_9">Table +9</a>), and delayed time of first reproduction (<a href="#Table_10">Table 10</a>). In spite of +this, <i>Nasua narica</i> has a higher than expected r<sub>max</sub> (111% of +predicted; <a href="#Table_10">Table 10</a>). The life history feature that enhances +<i>Nasua narica</i>'s reproductive potential, and increases r<sub>max</sub> +beyond expected, is its large litter size. In this species females +live in bands. Each year just before their young are born these +bands break up, and each female seeks out a den for herself and +<span class="pagenum"><a name="Page_20" id="Page_20">[Pg 20]</a></span> +her litter. Once the young are able to leave the den +(approximately five weeks), bands reform. In this situation, +females not only care for their own young but also for those of +other females in the band (Kaufmann, 1962:157-159, 1982, +1987; Russell, 1983). This social structure may contribute to +this species' ability to produce large litters and in this way +increase its reproductive potential.</p> + +<a name="Table_10"></a> +<div class="tab_cap"><span class="smcap">Table 10.</span>—Intrinsic rate of natural increase (r<sub>max</sub>) of several procyonids. (a = potential age of females producing first young; b = potential annual birth rate of female young (= average litter size/2; average litter size was calculated from the published range of litter sizes for each species); n = potential age of females producing their final young; r<sub>maxe</sub> = intrinsic rate of natural increase expected from body mass (Hennemann, 1983); r<sub>maxr</sub> = ratio of calculated to expected intrinsic rate of natural increase (r<sub>max</sub>/r<sub>maxe</sub>).)</div> +<br> +<table width="100%" class="center" summary="Intrinsic rate of natural increase"> +<tr><th class="bt bb">Species</th><th class="bt bl bb">Body<br>mass (g)</th><th class="bt bb"> a </th><th class="bt bb"> b </th><th class="bt bb"> n </th><th class="bt bb"> r<sub>max</sub> </th> +<th class="bt bb">r<sub>maxe</sub><a name="FNanchor_A_13" id="FNanchor_A_13"></a><a href="#Footnote_A_13" class="fnanchor">[a]</a></th> +<th class="bt bb">r<sub>maxr</sub><a name="FNanchor_B_14" id="FNanchor_B_14"></a><a href="#Footnote_B_14" class="fnanchor">[b]</a></th><th class="bt bb">References</th></tr> + +<tr><td class="text_lf vtop"><i>Procyon lotor</i></td><td class="bl vtop">4940</td><td class="vtop">0.83</td><td class="vtop">2.25</td><td class="vtop">16</td><td class="vtop">1.34</td><td class="vtop"> 0.53 </td><td class="vtop"> 2.52 </td><td class="text_lf">Dunn and Chapman (1983); Eisenberg (1981:489); Kaufmann (1987); Lotze and Anderson (1979); Nowak and Paradiso (1983:981); Sanderson(1987); Stains (1956:28-31); This study</td></tr> +<tr><td class="text_lf vtop"><i>Bassariscus astutus</i></td><td class="bl vtop">900</td><td class="vtop">0.83</td><td class="vtop">1.50</td><td class="vtop">14</td><td class="vtop">1.02</td><td class="vtop">0.82</td><td class="vtop">1.24</td><td class="text_lf">Kaufmann (1982, 1987); Nowak and Paradiso(1983:979, 980); Poglayen-Neuwall and Poglayen-Neuwall (1980); Poglayen-Neuwall and Toweill (1988); Russell (1983)</td></tr> +<tr><td class="text_lf vtop"><i>Nasua narica</i></td><td class="bl vtop">3900</td><td class="vtop">2.50</td><td class="vtop">2.25</td><td class="vtop">14</td><td class="vtop">0.62</td><td class="vtop">0.56</td><td class="vtop">1.11</td><td class="text_lf">Kaufmann (1982, 1987); Nowak and Paradiso (1983:983); Sanderson (1983)</td></tr> +<tr><td class="text_lf vtop"><i>Nasua nasua</i></td><td class="bl vtop">3850</td><td colspan=6> </td><td class="text_lf">Chevillard-Hugot et al. (1980)</td></tr> +<tr><td class="text_lf vtop"><i>Procyon cancrivorus</i></td><td class="bl vtop">1160</td><td class="vtop">0.83</td><td class="vtop">1.50</td><td class="vtop">15</td><td class="vtop2"> 1.02<a name="FNanchor_C_15" id="FNanchor_C_15"></a><a href="#Footnote_C_15" class="fnanchor">[c]</a></td><td class="vtop">0.77</td><td class="vtop">1.32</td><td class="text_lf vtop">Crandall (1964:312); Poglayen-Neuwall (1987)</td></tr> +<tr><td class="text_lf"> </td><td class="bl vtop"> </td><td class="vtop">1.75</td><td colspan=2> </td><td class="vtop"> 0.65<a href="#Footnote_C_15" class="fnanchor">[c]</a></td><td class="vtop"> </td><td class="vtop">0.84</td><td> </td></tr> +<tr><td class="text_lf vtop"><i>Potos flavus</i></td><td class="bl vtop">2490</td><td class="vtop">1.75</td><td class="vtop">0.50</td><td class="vtop">12</td><td class="vtop">0.30</td><td class="vtop">0.63</td><td class="vtop">0.48</td><td class="text_lf">Ford and Hoffmann (1988); Nowak and Paradiso (1983:984)</td></tr> +<tr><td class="text_lf bb vtop"><i>Bassaricyon gabbii</i></td><td class="bb bl vtop">1600</td><td class="bb vtop">1.75</td><td class="bb vtop">0.50</td><td class="bb vtop">15</td><td class="bb vtop">0.32</td><td class="bb vtop">0.71</td><td class="bb vtop">0.45</td><td class="bb text_lf">Eisenberg (1981:489); Nowak and Paradiso (1983:985)</td></tr> +</table> +<br> + +<div class="footnote"> +<a name="Footnote_A_13" id="Footnote_A_13"></a><a href="#FNanchor_A_13"><span class="label">[a]</span></a> r<sub>maxe</sub> = 4.9·m<sup>0.2622</sup>, where m is body mass in grams.<br> +<br> +<a name="Footnote_B_14" id="Footnote_B_14"></a><a href="#FNanchor_B_14"><span class="label">[b]</span></a> Regression of r<sub>max</sub> on body mass (m). Assume r<sub>max</sub> = 1.02 for <i>Procyon cancrivorus</i>: r<sub>max</sub> = 0.00005·m + 0.623; R = 0.19; R<sup>2</sup> = 0.03; Regression of r<sub>maxr</sub> (<a href="#Table_10">Table 10</a>) on H<sub>br</sub> (<a href="#Table_7">Table 7</a>); assume <i>Nasua nasua</i> has the same r<sub>maxr</sub> as <i>Nasua narica</i>: r<sub>maxr</sub> = 3.35·H<sub>br</sub> - 1.11; R = 0.93; R<sup>2</sup> = 0.86.<br> +<br> +<a name="Footnote_C_15" id="Footnote_C_15"></a><a href="#FNanchor_C_15"><span class="label">[c]</span></a> Estimate based on females reproducing in their first (a = 0.83) or second (a = 1.75) year.<br> +<br> +</div> + + +<a name="Nasua_nasua_1"></a> +<p><i>Nasua nasua.</i>—Unfortunately, there is not enough reproductive +data to allow calculation of r<sub>max</sub> for <i>Nasua nasua</i> (<a href="#Table_10">Table +10</a>), therefore, it is not possible to compare the reproductive +potential of this South American coati with its North American +relative, <i>Nasua narica</i>. Given its low Ḣ<sub>b</sub> and relatively +low-quality diet of fruit and terrestrial invertebrates (<a href="#Table_9">Table 9</a>), +however, r<sub>max</sub> of <i>Nasua nasua</i> may be very similar to that of +<i>Nasua narica</i>.</p> + +<a name="Procyon_cancrivorus_1"></a> +<p><i>Procyon cancrivorus.</i>—The age of first female reproduction +for <i>Procyon cancrivorus</i> has not been reported. However, if one +assumes females can reproduce in their first year, r<sub>max</sub> for +<i>Procyon cancrivorus</i> would be 1.02 (132% of expected; <a href="#Table_10">Table +10</a>). If, on the other hand, first female reproduction is delayed +until the second year, r<sub>max</sub> would be 0.65 (84% of predicted; +<a href="#Table_10">Table 10</a>). <i>Procyon cancrivorus</i> has a low Ḣ<sub>b</sub>, reduced litter +size, and small body mass. Its low Ḣ<sub>b</sub> may limit litter size, but +as with <i>Bassariscus astutus</i>, the quality of its diet (a high +percentage of small vertebrates; <a href="#Table_9">Table 9</a>) and its small body +size may make it possible for females to reproduce in their first +year and thus increase the species' reproductive potential. This +reasoning would argue that <i>Procyon cancrivorus</i> probably +enjoys higher, rather than lower, than expected r<sub>max</sub>.</p> + +<a name="Potos_flavus_1"></a> +<p><i>Potos flavus.</i>—In addition to a low Ḣ<sub>b</sub>, this species possesses +other characteristics that limit its reproductive potential: +low-quality diet, delayed reproduction, and birth of a single +young each year. Because there does not appear to be any other +feature of its life history that can counteract the influence of +these factors, r<sub>max</sub> in <i>Potos flavus</i> has evolved to be only 48% +of expected (0.30; <a href="#Table_10">Table 10</a>). Its close relative, the olingo, +<i>Bassaricyon gabbii</i>, appears to share the same condition (<a href="#Table_10">Table +10</a>).</p> + +<a name="Summary_2"></a> +<p><span class="smcap">Summary.</span>—This brief survey illustrates that, with the +exception of <i>Potos flavus</i>, procyonids tend to have values of +r<sub>max</sub> that are higher than those predicted for them on the basis of +mass (<a href="#Table_10">Table 10</a>). Regression analysis indicates that, within the +family, body mass accounts for only a small amount (3%) of +the variation in r<sub>max</sub>, whereas the positive slope of the +correlation between r<sub>maxr</sub> and H<sub>br</sub> (R = 0.93) suggests that low +metabolism has a limiting effect on r<sub>max</sub> (see <a href="#Footnote_B_14">Table 10, footnote +b</a>). The implication here is that low Ḣ<sub>b</sub> would be associated with +a lower rate of biosynthesis, a slower growth rate, and a longer +generation time. Procyonids with low Ḣ<sub>b</sub> but higher than +expected r<sub>max</sub> must possess other traits that serve to offset the +effects of low metabolism. Our survey indicates that the +following features compensate for low Ḣ<sub>b</sub> and help increase +r<sub>max</sub>: (1) a high-quality diet may make biosynthesis and growth +more efficient, thus optimizing the time element associated +<span class="pagenum"><a name="Page_21" id="Page_21">[Pg 21]</a></span> +with each of these processes; (2) larger litter sizes and +cooperation in care of the young may increase survivorship in +spite of a slower growth rate; and (3) an early age of first +reproduction, a long reproductive life span, and moderate-size +litters (two to four young) may in the long run add as many +individuals to the population as a shortened generation time. +Our survey also suggests that, at the other extreme, factors such +as a low-quality diet, reduced litter size, absence of cooperative +care of the young, delayed age of first reproduction, and +shortened reproductive life span all serve to decrease r<sub>max</sub>. +Thus, it is obvious that diet, litter size, social structure, +reproductive strategy, and reproductive life span can operate +synergistically with Ḣ<sub>b</sub> to magnify its influence on r<sub>max</sub> (as with +<i>Procyon lotor</i> and <i>Potos flavus</i>), or they can function in +opposition to Ḣ<sub>b</sub> to change the direction of its influence on r<sub>max</sub> +(as with <i>Bassariscus astutus</i>, <i>Procyon cancrivorus</i>, <i>Nasua +narica</i>, and perhaps <i>Nasua nasua</i>).</p> + +<a name="Basal_Metabolism_2"></a> +<a name="Procyon_lotor_2"></a> +<span class="pagenum"><a href="#toc">[↑ TOC]</a></span> +<div class="caption4">Basal Metabolism and Climatic Distribution</div> + +<p><i>Procyon lotor.</i>—The evolution of a higher Ḣ<sub>b</sub> (<a href="#Table_7">Tables 7</a>,<a href="#Table_8"> 8</a>) +may have been the physiological cornerstone that enabled +<i>Procyon lotor</i> to break out of the mold being exploited by other +procyonids and to generalize its use of habitats and climates. +Once this basic physiological change was in place, selection for +appropriate alterations in thermal conductance, capacity for +evaporative cooling, diversity of diet, and energy storage would +have provided this species with the suite of adaptations needed +to extend its distribution into other habitats and climates. +Support for this concept follows from the fact that high levels +of Ḣ<sub>b</sub> are associated with (1) cold-hardiness in mammals that +live in cold-temperate and arctic climates (Scholander et al., +1950c; Irving et al., 1955; Irving, 1972:115, 116; Shield, 1972; +Vogel, 1980; Golightly and Ohmart, 1983); (2) the ability to +utilize a wide variety of food resources and to occupy a large +number of different environments and habitats (McNab, +1980a); and (3) a high intrinsic rate of natural increase (McNab, +1980a; Hennemann, 1983; Lillegraven et al., 1987; Nicoll and +Thompson, 1987; Thompson, 1987).</p> + +<a name="Other_Procyonids"></a> +<p><span class="smcap">Other Procyonids.</span>—Other procyonids (<i>Potos flavus</i>, +<i>Procyon cancrivorus</i>, <i>Nasua narica</i>, and <i>Nasua nasua</i>) have +lower than predicted Ḣ<sub>b</sub>'s (<a href="#Table_7">Table 7</a>), a characteristic that is +considered to be an energy-saving adaptation for those that live +in relatively stable tropical and subtropical habitats (Müller and +Kulzer, 1977; Chevillard-Hugot et al., 1980; Müller and Rost, +1983). However, <i>Bassariscus astutus</i> is found in tropical, +subtropical, and temperate climates. This species is found from +tropical Mexico to temperate regions of the western United +States (Kaufmann, 1982, 1987; Nowak and Paradiso, +1983:979). In the northern part of its distribution, <i>Bassariscus +astutus</i> lives in habitats that are unstable (arid regions), that are +low in productivity, and that characteristically have marked +seasonal changes in temperature. Its lower than predicted Ḣ<sub>b</sub> +could be an important water-conserving adaptation at times +when temperatures are high (McNab and Morrison, 1963; +McNab, 1966; MacMillen and Lee, 1970; Noll-Banholzer, +1979) and an important energy-conserving mechanism when +cold weather may limit food availability and hunting time +(Scholander et al., 1950c; Wang et al., 1973). As will be seen +later, <i>Bassariscus astutus</i> is unique among procyonids with +lower than predicted Ḣ<sub>b</sub>'s in that it also has a lower than +predicted C<sub>mw</sub> (<a href="#Table_7">Table 7</a>). This allows it to use less energy than +expected for thermoregulation at low temperatures. Another +species with a similar set of adaptations (lower than predicted +Ḣ<sub>b</sub> and C<sub>mw</sub>) is the arctic hare, <i>Lepus arcticus</i> (Wang et al., +1973), which lives in one of the coldest and least-productive +regions on earth. Wang et al. (1973) suggest that this +combination of adaptations allows <i>Lepus arcticus</i> to better +match its energy requirements to the low productivity of its +environment. A similar relationship may hold for <i>Bassariscus +astutus</i>, particularly in colder arid portions of its distribution, +and may be the reason that it, but not other procyonids with low +Ḣ<sub>b</sub>'s, has been able to inhabit temperate climates.</p> + +<a name="Minimum_Thermal_Conductance_3"></a> +<a name="Background_4"></a> +<span class="pagenum"><a href="#toc">[↑ TOC]</a></span> +<div class="caption3">Minimum Thermal Conductance</div> +<div class="caption4">Background</div> + +<p>Thermal conductance is a measure of the ease with which +heat is passively transferred to or from a body through its +tissues and pelt. Within T<sub>n</sub>, a mammal is able to vary its thermal +conductance over a wide range of values by changing heat +transfer characteristics of both of these layers. Minimum +thermal conductance occurs when total heat transfer through +these layers is reduced to its lowest possible rate. This +minimum value, which is the reciprocal of maximum resistance, +occurs, theoretically, but not always practically (see +McNab, 1988b), at the animal's T<sub>lc</sub> and is best estimated under +standard conditions in a metabolism chamber (McNab, 1980b; +Aschoff, 1981). Minimum thermal conductance scales to body +mass (McNab and Morrison, 1963; Herreid and Kessel, 1967; +McNab, 1970, 1979b; Bradley and Deavers, 1980; Aschoff, +1981). Therefore, to make comparisons between species of +various sizes, we scaled out body mass by expressing C<sub>mw</sub> as +the ratio of measured to predicted values (C<sub>mwr</sub>; <a href="#Table_7">Table 7</a>). These +ratios were used to make comparisons of heat-transfer +characteristics between species that occupy different habitats or +climates.</p> + +<a name="Effect_of_Molt_on_Thermal_Conductance"></a> +<span class="pagenum"><a href="#toc">[↑ TOC]</a></span> +<div class="caption4">Effect of Molt on Thermal Conductance</div> + +<p>In summer, T<sub>lc</sub>'s of male and female <i>Procyon lotor</i> (<a href="#Fig_2">Figure +2</a>) were very similar to those of other procyonids (22°C-26°C; +<a href="#Table_7">Table 7</a>). In winter, T<sub>lc</sub> of both sexes shifted downward to 11°C +(<a href="#Fig_3">Figure 3</a>). This seasonal shift in T<sub>lc</sub> occurred as the result of a +seasonal change in minimum thermal conductance (<a href="#Table_3">Table 3</a>). +For many northern mammals, a seasonal change in thermal +conductance is partly mediated via cyclic changes in the +insulative quality of their pelt (Scholander et al., 1950a; Irving +et al., 1955; Hart, 1956, 1957; Irving, 1972:165).</p> + +<p><span class="pagenum"><a name="Page_22" id="Page_22">[Pg 22]</a></span> +<i>Procyon lotor</i> begins to shed its heavy winter coat about the +time its young are born. Molt progresses through summer and +by late August the new coat is complete (Stuewer, 1942). +During its summer molt, <i>Procyon lotor</i>'s C<sub>mw</sub> increased by +about 49% over the value for female raccoons in winter (<a href="#Table_3">Table +3</a>). In summer, therefore, it had the highest mass specific C<sub>mw</sub> +of those procyonids considered (C<sub>mwr</sub> = 1.77 and 1.79; <a href="#Table_7">Table 7</a>). +An increase in thermal conductance facilitates passive heat loss +for temperate and arctic species, and this serves as an important +thermoregulatory adaptation during warm summer months +(Scholander et al., 1950c; Irving et al., 1955; Hart, 1956, 1957; +Irving, 1972:165). This adaptation is particularly important to +those temperate- and arctic-zone species (including raccoons) +whose Ḣ<sub>b</sub>'s do not decrease during summer (Irving et al., +1955). From August on, the fur of <i>Procyon lotor</i> becomes +increasingly longer and heavier, with peak, or prime, condition +occurring in late fall and early winter (Stuewer, 1942). +Minimum conductance of our captive raccoons was lowest in +winter (C<sub>mwr</sub> = 1.15) when their pelts were in prime condition +(<a href="#Table_3">Tables 3</a>,<a href="#Table_7"> 7</a>). Because "primeness" of raccoon pelts varies +geographically, thicker pelts being associated with colder +climates (Goldman, 1950:21; Whitney and Underwood, +1952:24-41), the degree of seasonal change in C<sub>mw</sub> must also +vary geographically.</p> + +<p>The only other procyonid for which a seasonal molt has been +described is <i>Bassariscus astutus</i>. Molt in this species extends +from late summer to late fall (Toweill and Toweill, 1978). How +molt effects thermal conductance in <i>Bassariscus astutus</i> is not +known because metabolic data for this species (<a href="#Table_7">Table 7</a>) +apparently were collected only when their pelts were in prime +condition (Chevalier, 1985).</p> + +<p>Goldman (1950:20) reports that <i>Procyon cancrivorus</i> does +not have a seasonal molt. Like other tropical procyonids, +<i>Procyon cancrivorus</i> lives in an environment that has the +following characteristics: high even temperatures throughout +the year (1°C-13°C difference in monthly mean temperature), +a greater range in temperature between day and night than in +mean monthly temperature throughout the year, uniform +lengths of day and night, seasonal variation in rainfall, and +lowest temperatures during the rainy season(s) (Kendeigh, +1961:340). In such a stable environment there would be no +advantage to a sharply defined seasonal molt cycle that could +place an animal in thermoregulatory jeopardy by increasing its +thermal conductance. This would be particularly true for +animals like tropical procyonids that have lower than predicted +Ḣ<sub>b</sub>'s but that maintain typical eutherian body temperatures +(<a href="#Table_7">Table 7</a>). Consequently, molt in all tropical procyonids may +either be prolonged or continuous. This is a feature of their +biology that needs to be examined in more detail.</p> + +<a name="Comparison_of_Thermal_Conductances"></a> +<a name="Procyon_lotor_versus_Tropical_Procyonids"></a> +<span class="pagenum"><a href="#toc">[↑ TOC]</a></span> +<div class="caption4">Comparison of Thermal Conductances</div> + +<p><i>Procyon lotor</i> <span class="smcap">versus Tropical Procyonids</span>.—C<sub>mwr</sub> for +<i>Procyon lotor</i> in winter was 1.15, which is similar to the values +for <i>Potos flavus</i> and <i>Procyon cancrivorus</i>, 1.02 and 1.25, +respectively (<a href="#Table_7">Table 7</a>). These two tropical species, therefore, +have C<sub>mw</sub>'s that are similar on a mass specific basis to the value +for <i>Procyon lotor</i> in winter. However, at their T<sub>lc</sub>'s, the thermal +gradient sustained by these tropical animals is only about 11°C, +whereas for <i>Procyon lotor</i> in winter it was 26.5°C. Examination +of <a href="#Eq_4">Eq. 4</a> with respect to these thermal gradients suggests that +tropical procyonids achieve such low C<sub>mw</sub>'s by virtue of their +lower than predicted Ḣ<sub>b</sub>'s rather than by having pelts that are +exceptionally good insulators. In fact, the insulation afforded +by the pelts of these tropical procyonids is about the same as +that of the 50 g arctic lemming, <i>Dicrostonyx groenlandicus +rubricatus</i>, whose coat has an insulative value that is about half +that of the hare, <i>Lepus americanus</i>, red fox, <i>Vulpes fulva +alascensis</i>, and pine martin, <i>Martes americana</i>, animals +comparable in size to these procyonids (Scholander et al., +1950a). Therefore, pelts of these tropical procyonids do not +have the same insulative value as the prime winter coat of +<i>Procyon lotor</i>.</p> + +<p><i>Nasua narica</i> and <i>Nasua nasua</i> have tropical and subtropical +distributions and they are the only procyonids that are diurnal +(Kaufmann, 1962:103-105, 1982, 1987). Because they are +active during the day they experience a more extreme thermal +environment (higher T<sub>a</sub>'s and solar radiation) than their +nocturnal cousins. Values of C<sub>mwr</sub> for <i>Nasua narica</i> (1.45 and +1.55) and <i>Nasua nasua</i> (1.24 and 1.65) are higher than those +for <i>Procyon cancrivorus</i> or <i>Potos flavus</i> (<a href="#Table_7">Table 7</a>). Thus, these +coatis have higher mass specific C<sub>mw</sub>'s than their nocturnal +tropical cousins. A high C<sub>mw</sub> reduces the cost of thermoregulation +in hot environments because it increases an animal's +ability to lose excess heat passively. The higher C<sub>mw</sub>'s of these +coatis serve as an adaptation that contributes to the success of +their diurnal life style as well as their ability to expand their +habitat use to areas with less thermal stability, such as oak and +pine woodlands and deserts.</p> + +<a name="Bassariscus_astutus_2"></a> +<p><i>Bassariscus astutus.</i>—This species has the lowest mass +specific C<sub>mw</sub> of these procyonids (C<sub>mwr</sub> = 0.85; <a href="#Table_5">Table 7</a>), which +indicates that its pelt has a greater insulative value than the +coats of <i>Potos flavus</i>, <i>Procyon cancrivorus</i>, <i>Nasua nasua</i>, or +<i>Nasua narica</i>. This, coupled with a lower than predicted Ḣ<sub>b</sub>, +allows <i>Bassariscus astutus</i> to maintain T<sub>b</sub> with less energy +expenditure than is possible for any other procyonid of +comparable size; and this combination of adaptations provides +<i>Bassariscus astutus</i> with a distinct energy advantage in +environments that have low productivity (Wang et al., 1973). +The evolution of a pelt that provides better insulation must be +considered an, important contributing factor for the spread of +this species into desert regions of the western United States.</p> + +<a name="Thermoregulation_1"></a> +<a name="Background_5"></a> +<a name="Thermoregulation_2"></a> +<span class="pagenum"><a href="#toc">[↑ TOC]</a></span> +<div class="caption3">Thermoregulation and Use of +Stored Fat at Low Temperatures</div> +<div class="caption4">Background</div> + +<p><span class="smcap">Thermoregulation.</span>—At temperatures below a mammal's +T<sub>n</sub>, heat loss exceeds Ḣ<sub>b</sub>. To maintain T<sub>b</sub> under these +<span class="pagenum"><a name="Page_23" id="Page_23">[Pg 23]</a></span> +conditions, metabolic rate must be increased (<a href="#Eq_4">Eq. 4</a>). <i>Procyon +lotor</i> in summer during its annual molt (<a href="#Table_5">Table 5</a>; <a href="#Fig_2">Figure 2</a>), +<i>Bassariscus astutus</i> (Chevalier, 1985), <i>Nasua nasua</i> (Chevillard-Hugot +et al., 1980; Mugaas et al., in prep.), <i>Nasua narica</i> +(Scholander et al., 1950b; Mugaas et al., in prep.), and <i>Potos +flavus</i> (Müller and Kulzer, 1977; Müller and Rost, 1983) all are +able to elevate their metabolic rates by 130% above basal when +they are exposed to T<sub>a</sub> = 0°C. <i>Procyon cancrivorus</i> responds to +0°C with an increase in metabolic rate of 257% above basal +(Scholander et al., 1950b). All animals listed have about the +same T<sub>lc</sub> and T<sub>b</sub>, so the temperature differential producing this +response is about the same for each species. Metabolic ability +to defend body temperature against low ambient temperatures, +therefore, is well developed in these procyonids. Such large +increases in metabolic rate are energetically expensive, and if +these animals were routinely exposed to T<sub>a</sub> = 0°C, it would be +difficult for them to acquire enough food each day to maintain +endothermy. Raccoons in winter pelage, however, need only +elevate their metabolic rate by 47% above basal to maintain +endothermy at T<sub>a</sub> = 0°C (<a href="#Table_5">Table 5</a>; <a href="#Fig_3">Figure 3</a>). Each year at the +completion of its molt, the raccoon's highly insulative pelt is +renewed. This lowers their T<sub>lc</sub> by 9°C to 15°C below that +measured for them in summer (<a href="#Fig_3">Figure 3</a>) and decreases their +cost of thermoregulation at low temperatures. The increased +insulative capacity of their pelt is one of the primary +adaptations that has allowed <i>Procyon lotor</i> to extend its +distribution into cold climates.</p> + +<a name="Stored_Fat"></a> +<p><span class="smcap">Stored Fat.</span>—Cyclic fattening is an integral and important +part of a raccoon's annual cycle (Mugaas and Seidensticker, +ms); however, it has not been reported for other procyonids. +During winter in parts of the United States and Canada, +raccoons are confined to their dens for variable periods of time +(days to months) depending on the severity of the weather +(Stuewer, 1943:223-225; Whitney and Underwood, 1952:108-116; +Sharp and Sharp, 1956; Mech et al., 1968; Schneider +et al., 1971). During this confinement, they do not hibernate but +rather enter a state of "dormancy" and become inactive. While +dormant they remain endothermic (T<sub>b</sub> > 35°C; Thorkelson, +1972:87-90) and derive most of their energy requirement from +fat reserves accumulated during fall. The rate at which fat stores +are consumed during winter dormancy depends on the +thermoregulatory requirement imposed on them by local +weather conditions, the insulative quality of their pelt, and any +advantage they may gain by seeking shelter in a den.</p> + +<a name="Thermal_Model"></a> +<span class="pagenum"><a href="#toc">[↑ TOC]</a></span> +<div class="caption4">Thermal Model of the Raccoon and Its Den</div> + +<p>Heat transfer between an animal and its environment is a +function of the interaction of its body temperature and thermal +conductance with various environmental variables (air temperature, +wind speed, vapor pressure, and thermal radiation). +When a raccoon is outside its den, its thermal conductance +(C<sub>mw</sub>) is the only barrier to heat transfer with the external +environment. However, when it enters a tree den, a raccoon +imposes two other thermal barriers between itself and the +external environment: (1) conductance of the air space between +its fur and the den's walls (C<sub>a</sub>) and (2) conductance of the den's +walls (C<sub>d</sub>; Thorkelson, 1972:59-63; Thorkelson and Maxwell, +1974). Thorkelson and Maxwell (1974) modeled heat transfer +of a simulated raccoon (a water-filled aluminum cylinder +equipped with a heater and covered with a raccoon pelt) in a +closed tree den. In their system, 65% of resistance to heat flux +was attributable to the pelt, whereas the remainder (35%) was +due to C<sub>a</sub> and C<sub>d</sub>. Because resistance is the inverse of +conductance, and resistances for the raccoon and its den are +arranged in series, we can estimate total conductance (C<sub>t</sub>) of +this system with <a href="#Eq_7">Eq. 7</a>.</p> + +<a name="Eq_7"></a> +<table width="100%" summary="Eq. 1"> +<tr><td class="center">1/C<sub>t</sub> = 1/C<sub>mw</sub> + 1/C<sub>a</sub> + 1/C<sub>d</sub></td><td class="text_rt">Eq. 7</td></tr> +</table> + +<p>Minimum thermal conductance C<sub>mw</sub> for raccoons in winter +was 0.0172 mL O<sub>2</sub>·g<sup>-1</sup>·h<sup>-1</sup>·°C<sup>-1</sup> (<a href="#Table_3">Table 3</a>). Based on Thorkelson +and Maxwell's (1974) model we let 1/C<sub>mw</sub> = 0.65(1/C<sub>t</sub>) = 1/0.0172 mL +O<sub>2</sub>·g<sup>-1</sup>·h<sup>-1</sup>·°C<sup>-1</sup>, and 1/C<sub>a</sub> + 1/C<sub>d</sub> = 0.35(1/C<sub>t</sub>). Substituting +these values into <a href="#Eq_7">Eq. 7</a> and solving for C<sub>t</sub> yields 0.0112 +mL O<sub>2</sub>·g<sup>-1</sup>·h<sup>-1</sup>·°C<sup>-1</sup>, a value that is 35% lower than that of the +animal alone. Substituting this value and the value for basal +metabolism of winter raccoons (0.47 mL O<sub>2</sub>·g<sup>-1</sup>·h<sup>-1</sup>; <a href="#Table_7">Table 7</a>) +into <a href="#Eq_4">Eq. 4</a> and solving for (T<sub>b</sub> - T<sub>a</sub>) yields a new temperature +differential of 42°C. Therefore, by using tree dens, raccoons in +north central Virginia, with T<sub>b</sub> = 37°C (<a href="#Fig_7">Figure 7</a>), could +effectively reduce their T<sub>lc</sub> from 11°C to -5°C and markedly +reduce their metabolic cost of thermoregulation.</p> + +<a name="Metabolic_Advantage_of_the_Den"></a> +<span class="pagenum"><a href="#toc">[↑ TOC]</a></span> +<div class="caption4">Metabolic Advantage of the Den</div> + +<p>Given prevailing winter temperatures in north central +Virginia (see "Materials and Methods"), adult raccoons in that +area should be able to sustain endothermy most of the time they +are in their dens by simply maintaining Ḣ<sub>b</sub>. Depending on the +mass of their stored fat, they could remain in their dens for +several weeks without eating (Mugaas and Seidensticker, ms). +The thermal advantage of a den could be further enhanced +during colder temperatures if two or more raccoons occupied it +at the same time and huddled together, and/or if these animals +could reduce C<sub>mw</sub> even more by lowering T<sub>b</sub> and cooling their +extremities. Although we do not have any data to verify the +second mechanism, there are many accounts in natural history +literature that document raccoons occupying dens together +(Lotze and Anderson, 1979). This habit could be particularly +important for the young of the year and may be one reason why +they often continue to den with their mothers during winter +(Lotze and Anderson, 1979; Seidensticker et al., 1988). +Raccoons that live in colder climates, such as Minnesota, +undoubtedly obtain the same advantage from a den as Virginia +animals, but because of their greater body mass, longer fur, and +potentially lower C<sub>mw</sub>, T<sub>lc</sub> of a Minnesota raccoon in a den +could be even lower than what we calculated for Virginia +raccoons. Therefore, when they are in their dens, raccoons +living in very cold climates also may be able to maintain +homeothermy with a basal level of metabolism.<span class="pagenum"><a name="Page_24" id="Page_24">[Pg 24]</a></span></p> + +<a name="Thermoregulation_3"></a> +<a name="Background_6"></a> +<span class="pagenum"><a href="#toc">[↑ TOC]</a></span> +<div class="caption3">Thermoregulation at High Temperatures</div> +<div class="caption4">Background</div> + +<p>In hot environments mammals depend on behavior to +minimize their thermal load (escape to shaded or cooler +microclimates, use posture and orientation to wind and sun, +restrict activity, become nocturnal, etc.) and on evaporative +water loss to rid themselves of excess heat. With regard to +evaporative heat loss, Calder and King (1974:326) arbitrarily +subdivided the response to various T<sub>a</sub>'s as follows: "(1) cool +temperatures at which water loss should be minimized, both to +reduce heat loss and as an adaptation to terrestriality; (2) an +intermediate temperature range wherein evaporation is gradually +increased as dry heat losses are proportionately reduced +with smaller thermal gradients; and (3) warm to hot temperatures +at which evaporation must be actively increased to +dispose of metabolic and exogenous heat loads." Some +mammals are able to thermoregulate very well at high ambient +temperatures via panting or sweating, whereas others have a +very limited capacity. Hence, there is no general approach to +calculating evaporative water loss under these conditions +(Campbell, 1977:85). However, the ratio of evaporative heat +lost to metabolic heat produced can be used to quantify a +species' capacity for evaporative cooling and to make +comparisons between species.</p> + +<a name="Comparison_of_Procyonid_Responses"></a> +<a name="Potos_flavus_2"></a> +<span class="pagenum"><a href="#toc">[↑ TOC]</a></span> +<div class="caption4">Comparison of Procyonid Responses to Heat Stress</div> + +<p><i>Potos flavus.</i>—This species lives in Neotropical forests of +Central and South America. It is nocturnal, arboreal in habit, +and appears to be the most heat-sensitive of these procyonids. +Its T<sub>uc</sub> is at 30°C to 33°C (<a href="#Table_7">Table 7</a>; Müller and Kulzer, 1977; +Müller and Rost, 1983). It begins to pant at about 30°C, but its +efforts at evaporative cooling are very ineffective. At 33°C +<i>Potos flavus</i> can dissipate 33% of its metabolic heat via +evaporative water loss, but at 35°C the efficiency of this +mechanism falls to 20% (Müller and Rost, 1983). Consequently, +when exposed to T<sub>a</sub>'s above 33°C, any kind of +excitement causes its T<sub>b</sub> to rise rapidly in an uncontrolled +manner (Müller and Kulzer, 1977; Müller and Rost, 1983). +These animals rely on their nocturnal and arboreal habits to +keep them out of situations that could lead to hyperthermia +(Müller and Kulzer, 1977; Müller and Rost, 1983).</p> + +<a name="Nasua_nasua_2"></a> +<p><i>Nasua nasua</i> and <i>Nasua narica</i>.—<i>Nasua nasua</i> is abundant +in tropical and subtropical South America, whereas <i>Nasua +narica</i> occupies the same climates in North America from +southern Arizona and New Mexico south through Panama and +on into Colombia and Ecuador (Hall and Kelson, 1959:892; +Ewer, 1973:391, 392; Poglayen-Neuwall, 1975). Both coatis +are diurnal and forage primarily on the ground (Kaufmann, +1962:185-188, 1987; Poglayen-Neuwall, 1975; Nowak and +Paradiso, 1983:982), consequently they are exposed to a more +severe thermal environment while active (higher T<sub>a</sub>'s and solar +radiation) than are nocturnal procyonids. Both coatis are more +heat-tolerant than <i>Potos flavus</i>; their T<sub>uc</sub>'s are higher (33°C-35°C; +<a href="#Table_7">Table 7</a>), they can tolerate T<sub>a</sub>'s of 35°C without raising +their T<sub>b</sub>'s (Chevillard-Hugot et al., 1980; Mugaas et al., in +prep.), and they have a greater capacity for evaporative cooling +than <i>Potos flavus</i> (Mugaas et al., in prep.). The greater heat +tolerance of these coatis is compatible with their diurnal habits +and widespread distribution in a variety of forest habitats in +both tropical and subtropical areas of the western hemisphere.</p> + +<a name="Bassariscus_astutus_3"></a> +<p><i>Bassariscus astutus.</i>—In addition to living in Neotropical +forests of Mexico, <i>Bassariscus astutus</i> also flourishes in hot +arid climates, and it has extended its range much farther north +than <i>Nasua narica</i> (Hall and Kelson, 1959:881,892; Poglayen-Neuwall, +1975; Kaufmann, 1982). Its T<sub>uc</sub> is higher (35.5°C; +<a href="#Table_7">Table 7</a>) than that of <i>Potos flavus</i>, but it is comparable to those +of <i>Nasua nasua</i> and <i>Nasua narica</i>. Its capacity for evaporative +cooling is well developed; at 40°C <i>Bassariscus astutus</i> is able +to dissipate 100% of its resting metabolic heat via evaporative +water loss, and at 45°C it is able to dissipate 172% (Chevalier, +1985). In spite of its great capacity for evaporative cooling, this +species is nocturnal, a habit that, along with its low Ḣ<sub>b</sub>, should +allow it to keep thermoregulatory water requirements to a minimum.</p> + +<a name="Procyon_lotor_3"></a> +<p><i>Procyon lotor.</i>—Our data suggested that T<sub>uc</sub> for <i>Procyon +lotor</i> in winter was comparable to that for <i>Bassariscus astutus</i> +(35°C), and that in summer it was even higher. When exposed +to temperatures near the upper end of its T<sub>n</sub>, <i>Procyon lotor</i> +increased the gradient for passive heat loss with a controlled +rise in T<sub>b</sub> (<a href="#Fig_6">Figure 6</a>). In summer its capacity for passive heat +loss was enhanced by the molt of its heavy winter fur. <i>Procyon +lotor</i>'s capacity for evaporative cooling also appeared to be +well developed, although our animals were not heated to the +point that evaporative cooling was fully expressed (<a href="#Fig_4">Figures 4</a>, +<a href="#Fig_5"> 5</a>). However, <i>Procyon lotor</i> is nocturnal, and this may allow it +to eliminate, or at least reduce, the need for evaporative +cooling, even in hot climates. Thus, <i>Procyon lotor</i> appears to be +well equipped physiologically and behaviorally to cope with +thermal demands of hot environments in its distribution.</p> + +<a name="Procyon_cancrivorus_2"></a> +<p><i>Procyon cancrivorus.</i>—Unfortunately, data for the crab-eating +raccoon are not complete enough at high temperatures to +include it in this survey.</p> + +<a name="Summary_3"></a> +<p><span class="smcap">Summary.</span>—This comparison demonstrates that capacity +for evaporative cooling, tolerance of an elevated T<sub>b</sub> to enhance +passive heat loss, and behavioral avoidance of thermal stress +are the primary methods used by procyonids to thermoregulate +at high temperatures. <i>Procyon lotor</i> and <i>Bassariscus astutus</i>, +whose distributions extend into temperate regions, have +developed these abilities to a greater extent than other +procyonids. <i>Potos flavus</i>, whose distribution is confined to +lowland tropical forests, has the least ability in this regard. +<i>Nasua nasua</i> and <i>Nasua narica</i> appear to have thermoregulatory +abilities that are intermediate to those of <i>Bassariscus +astutus</i> and <i>Potos flavus</i>. This suggests that ancestral procyonids +<span class="pagenum"><a name="Page_25" id="Page_25">[Pg 25]</a></span> +may have had poor to modest ability to thermoregulate +at high temperatures, a condition that would have limited their +ability to leave the thermal stability afforded by tropical forests. +Dispersal into temperate climates, therefore, required not only +increased cold tolerance but also selective enhancement of +those mechanisms used in thermoregulation at high temperatures.</p> + +<a name="Table_11"></a> +<div class="tab_cap"><span class="smcap">Table 11.</span>—Distribution by climate of selected procyonid species.</div> +<br> +<table width="100%" class="center" summary="Distribution by Climate"> +<tr><th class="bt bb">Species</th><th class="bt bl bb">Tropics</th><th class="bt bb">Subtropics</th><th class="bt bb">Mild<a name="FNanchor_A_16" id="FNanchor_A_16"></a><a href="#Footnote_A_16" class="fnanchor">[a]</a><br>temperate</th><th class="bt bb">Cold<a name="FNanchor_B_17" id="FNanchor_B_17"></a><a href="#Footnote_B_17" class="fnanchor">[b]</a><br>temperate</th></tr> +<tr><td class="text_lf"><i>Procyon lotor</i></td><td class="bl">+</td><td>+</td><td>+</td><td>+</td></tr> +<tr><td class="text_lf"><i>Bassariscus astutus</i></td><td class="bl">+</td><td>+</td><td>+</td><td> </td></tr> +<tr><td class="text_lf"><i>Nasua nasua</i></td><td class="bl">+</td><td>+</td><td><td colspan=2> </td></tr> +<tr><td class="text_lf"><i>Nasua narica</i></td><td class="bl">+</td><td>+</td><td colspan=3> </td></tr> +<tr><td class="text_lf"><i>Procyon cancrivorus</i></td><td class="bl">+</td><td>+</td><td colspan=4> </td></tr> +<tr><td class="text_lf bb"><i>Potos flavus</i></td><td class="bl bb">+</td><td class="bb" colspan=5> </td></tr> +</table> +<br> + +<div class="footnote"> +<a name="Footnote_A_16" id="Footnote_A_16"></a><a href="#FNanchor_A_16"><span class="label">[a]</span></a> Extends from the subtropics north to the northern limit of <i>Bassariscus astutus</i>' distribution (Hall and Kelson, 1959:881), which approximates the 10°C isotherm for average annual temperature in the United States (Kincer, 1941).<br> +<br> +<a name="Footnote_B_17" id="Footnote_B_17"></a><a href="#FNanchor_B_17"><span class="label">[b]</span></a> Extends northward from the 10°C isotherm for average annual temperature in the United States.<br> +<br> +</div> + +<a name="Composite_Scores"></a> +<span class="pagenum"><a href="#toc">[↑ TOC]</a></span> +<div class="caption3">Composite Scores of Adaptive Units and Geographic Distribution</div> + +<p>In <a href="#Table_11">Table 11</a>, procyonid species are arranged in descending +order with respect to the number of major climates that are +included in their geographic distributions (Hall and Kelson, +1959:878-897; Poglayen-Neuwall, 1975; Kortlucke and Ramirez-Pulido, +1982; Nowak and Paradiso, 1983:977-985). +Composite scores ranged from a high of 1.47 for <i>Procyon lotor</i> +to a low of 0.39 for <i>Potos flavus</i>, whereas <i>Nasua nasua</i>, <i>Nasua +narica</i>, <i>Procyon cancrivorus</i>, and <i>Bassariscus astutus</i> had +intermediate values ranging from 0.64 to 0.79 (<a href="#Table_12">Table 12</a>). +<a href="#Fig_8">Figure 8</a> demonstrates that there is a direct relationship between +the number of climates these species occupy and their +composite scores. Regression analysis (Y = 2.68·X + 0.24; +where Y is number of climates, and X is composite score) +demonstrates a high degree of correlation between these +variables (R = 0.94) and indicates that 89% of the variance in +distribution can be explained by composite scores. The various +combinations of adaptations expressed by these species do, +therefore, play a role in delimiting their climatic (latitudinal) +distributions.</p> + +<p><i>Procyon lotor's</i> normalized scores were higher in all +categories than those of other procyonids. <i>Procyon lotor</i>, +therefore, possesses those traits that have allowed it to become +the premier climate generalist of the procyonid family. As an +adaptive unit, these traits provide <i>Procyon lotor</i> with the +physiological and behavioral flexibility required to take full +advantage of a wide range of climates and habitats, and its +distribution verifies that it has done so. Even so, it is probably +not fair to assume that this species represents a perfect +physiological match with climate over its entire distribution. +<i>Procyon lotor</i> is, in many respects, still a forest-dwelling +species, and its ability to expand its distribution into other +habitats such as prairie and desert may well be due, in part, to +its use of behavior to take advantage of favorable microclimates +in otherwise hostile environments (Bartholomew, 1958, +1987). This feature of <i>Procyon lotor's</i> biology needs to be +further examined.</p> + +<a name="Table_12"></a> +<div class="tab_cap"><span class="smcap">Table 12.</span>—Normalized and composite scores for selected procyonids. (H<sub>br</sub> = +ratio of measured to predicted basal metabolism (<a href="#Table_7">Table 7</a>), C<sub>mwr</sub> = ratio of +measured to predicted minimum thermal conductance (<a href="#Table_7">Table 7</a>), D<sub>dr</sub> = ratio of +food categories actually utilized by each species to total food categories eaten +by all six species (calculated from <a href="#Table_9">Table 9</a>), r<sub>maxr</sub> = ratio of calculated to +expected r<sub>max</sub> (<a href="#Table_10">Table 10</a>).)</div> +<br> +<table width="100%" class="center" summary="Normalized and composite scores for selected procyonids"> +<tr><th class="bt bb" rowspan=2>Species</th><th class="bt bl bb" colspan=3>Normalized scores</th><th class="bt bb" rowspan=2>Composite<a name="FNanchor_A_18" id="FNanchor_A_18"></a><a href="#Footnote_A_18" class="fnanchor">[a]</a><br>score</th></tr> +<tr><th class="bl bb">H<sub>br</sub>/C<sub>mwr</sub></th><th class="bb">D<sub>dr</sub></th><th class="bb">r<sub>maxr</sub></th></tr> +<tr><td class="text_lf"><i>Procyon lotor</i></td><td class="bl">0.95</td><td>0.95</td><td>2.52</td><td>1.47<td></tr> +<tr><td class="text_lf"><i>Bassariscus astutus</i></td><td class="bl">0.80</td><td>0.33</td><td>1.24</td><td>0.79<td></tr> +<tr><td class="text_lf"><i>Nasua nasua</i></td><td class="bl">0.48</td><td>0.33</td><td> 1.11<a name="FNanchor_B_19" id="FNanchor_B_19"></a><a href="#Footnote_B_19" class="fnanchor">[b]</a></td><td>0.64<td></tr> +<tr><td class="text_lf"><i>Nasua nasua</i></td><td class="bl">0.48</td><td>0.33</td><td> 1.11<a href="#Footnote_B_19" class="fnanchor">[b]</a></td><td>0.64<td></tr> +<tr><td class="text_lf"><i>Nasua narica</i></td><td class="bl">0.40</td><td>0.53</td><td>1.11</td><td>0.68<td></tr> +<tr><td class="text_lf"><i>Procyon cancrivorus</i></td><td class="bl">0.55</td><td>0.33</td><td>1.32</td><td>0.73<td></tr> +<tr><td class="text_lf bb"><i>Potos flavus</i></td><td class="bl bb">0.60</td><td class="bb">0.11</td><td class="bb">0.48</td><td class="bb">0.39<td></tr> +</table> +<br> +<div class="footnote"> +<a name="Footnote_A_18" id="Footnote_A_18"></a><a href="#FNanchor_A_18"><span class="label">[a]</span></a> Composite score = [(H<sub>br</sub>/C<sub>mwr</sub>) + D<sub>dr</sub> + r<sub>maxr</sub>]/3.<br> +<br> + +<a name="Footnote_B_19" id="Footnote_B_19"></a><a href="#FNanchor_B_19"><span class="label">[b]</span></a> Value calculated for <i>Nasua narica</i> (<a href="#Table_10">Table 10</a>) and used with the assumption that it must be similar to the value for <i>Nasua nasua</i>.<br> +<br> +</div> + +<p>All five species with low Ḣ<sub>b</sub>'s have composite scores less +than 1.0 (<a href="#Table_12">Table 12</a>; <a href="#Fig_8">Figure 8</a>). Four of these five, <i>Nasua nasua</i>, +<i>Nasua narica</i>, <i>Procyon cancrivorus</i>, and <i>Potos flavus</i>, have +H<sub>br</sub>/C<sub>mwr</sub> ratios that are 0.6 or less, which indicates +they are the least cold-tolerant procyonids (McNab, 1966). +These four species also are confined to either tropic, or tropic +and subtropic climates (<a href="#Table_11">Table 11</a>). This suggests that these +species share a common thermoregulatory adaptation that +represents a specialization to these climates. Attendant with +this adaptation, however, is a high cost of thermoregulation at +<span class="pagenum"><a name="Page_26" id="Page_26">[Pg 26]</a></span> +temperatures below their T<sub>lc</sub>, and this must be an important +factor in limiting their distributions to tropic and subtropic +climates. Differences in their distributions within these +climates, therefore, must hinge more on differences in their D<sub>dr</sub> +and r<sub>maxr</sub> values than on differences in their H<sub>br</sub>/C<sub>mwr</sub> ratios. +This is supported by the fact that <i>Potos flavus</i>, which has the +lowest D<sub>dr</sub> and r<sub>maxr</sub> values, is confined to a single climate, +whereas <i>Nasua nasua</i>, <i>Nasua narica</i>, and <i>Procyon cancrivorus</i> +each possess larger D<sub>dr</sub> and r<sub>maxr</sub> values and are found +in two climates. Thus, <i>Potos flavus</i>, with its highly specialized +diet and low reproductive potential, is the most ecologically +specialized of these procyonids, and its distribution is limited to +the single climate that can provide its requirements. <i>Nasua +nasua</i>, <i>Nasua narica</i>, and <i>Procyon cancrivorus</i> are less +specialized and thus show more ecological flexibility in their +distributions.</p> + +<a name="Fig_8"></a> +<div class="center"> + <img src="images/fig_8.png" width="369" height="339" title="Relationship between number of climates in which a species is found and its composite score." alt="number of climates vs composite score"><br><br> + <div class="fig_cap"><span class="smcap">Figure 8.</span>—Relationship between number of climates in which a species is found and its composite score. Symbols for <i>Nasua nasua</i> overlap at coordinates (0.64, 2). Solid line represents linear regression of climates (Y) on composite scores (X): Y = 2.68·X + 0.24; R = 0.94.</div> +</div> + +<p><i>Bassariscus astutus</i>, the other species with low Ḣ<sub>b</sub>, is found +in three climates, which indicates that it has greater ecological +flexibility than <i>Nasua nasua</i>, <i>Nasua narica</i>, or <i>Procyon +cancrivorus</i>. D<sub>dr</sub> and r<sub>maxr</sub> are comparable for these four species +(<a href="#Table_12">Table 12</a>). This suggests that the greater ecological flexibility +of <i>Bassariscus astutus</i> is derived largely from its greater cold +tolerance. <i>Bassariscus astutus</i> has a more insulative pelt than +these other procyonids (C<sub>mwr</sub> = 0.85; <a href="#Table_7">Table 7</a>), so its H<sub>br</sub>/C<sub>mwr</sub> +ratio is higher (0.80; <a href="#Table_12">Table 12</a>). This, and its greater capacity for +evaporative cooling (Chevalier, 1985), allows <i>Bassariscus +astutus</i> to take advantage of a wider range of thermal +environments than these other species. However, even with its +higher H<sub>br</sub>/C<sub>mwr</sub> ratio, the composite score for <i>Bassariscus +astutus</i> is not much different than those for <i>Nasua nasua</i>, +<i>Nasua narica</i>, and <i>Procyon cancrivorus</i> (<a href="#Table_12">Table 12</a>). Consequently, +<i>Bassariscus astutus</i> is found in more climates than +would be predicted for it on the basis of its composite score +(<a href="#Fig_8">Figure 8</a>). This suggests that either the H<sub>br</sub>/C<sub>mwr</sub> ratio carries +greater weight in determining distribution than is reflected in +this analysis, or as has been described for some other species +(Bartholomew, 1958, 1987), <i>Bassariscus astutus</i> may extend +its distribution farther than expected via use of its behavior. In +either case, for procyonids with low Ḣ<sub>b</sub>, <i>Bassariscus astutus</i> +represents the pinnacle of adaptation for climate generalization.</p> + +<a name="Evolution_of_Metabolic_Adaptations"></a> +<a name="Evolution_of_Low_Basal_Metabolic_Rate"></a> +<span class="pagenum"><a href="#toc">[↑ TOC]</a></span> +<div class="caption3">Evolution of Metabolic Adaptations</div> +<div class="caption4">Evolution of Low Basal Metabolic Rate</div> + +<p>A radiation of frugivorous and omnivorous Procyoninae +(<a href="#Table_1">Table 1</a>) occurred in the middle and late Miocene of North +America. It included origins of such terrestrial genera as +<i>Cyonasua</i>, <i>Nasua</i>, and <i>Procyon</i> (Webb, 1985b). The earliest +procyonid genus to find its way to South America was +<i>Cyonasua</i>, an omnivorous carnivore that presumably split, +along with its sister genus <i>Arctonasua</i>, from a common North +American ancestor (Baskin, 1982; Webb, 1985b). <i>Cyonasua</i>, +about the size of present-day raccoons, was adapted to a wide +range of habitats and was probably comparable to modern +raccoons with respect to the breadth of its feeding habits +(Webb, 1985b; Marshall, 1988). Because North American +<i>Arctonasua</i> was about the same size as <i>Cyonasua</i> (Webb, +1985b) and shared a number of characters with it (Baskin, +1982), we speculate that it also may have had similar habits and +occupied similar climates and habitats. <i>Bassariscus</i>, another +member of Procyoninae, had an even earlier origin in tropical +North America (Webb, 1985b). The origin of the small arboreal +forms <i>Potos</i> and <i>Bassaricyon</i> (subfamily Potosinae) is obscure +but is thought to have occurred in the rainforests of Central +America (Webb, 1985b). What were the metabolic capabilities +of these early procyonids? We do not know, but for several +million years, from middle to late Miocene, procyonids lived in +tropical and subtropical forests of Central and North America +(Webb, 1985b; Marshall, 1988). Then, in the Pleistocene, +several modern forms crossed the Panamanian land bridge into +similar habitats and climates in South America; but none of +them appear to have spread far enough northward to have +crossed the Bering land bridge.</p> + +<p>Several million years exposure to a tropical environment, +with its continuous high temperatures and modest range of +thermal extremes, would have favored selection of metabolic +and thermoregulatory traits that would minimize energy +requirements: a lower than predicted basal metabolic rate, a +prolonged or continuous molt resulting in very little annual +change in minimum thermal conductance, and a modest +capacity for evaporative cooling. In addition, we would expect +selection to have favored a diverse diet, good reproductive +<span class="pagenum"><a name="Page_27" id="Page_27">[Pg 27]</a></span> +potential, and behavioral flexibility to utilize a variety of +habitats within these climates. Our analysis has shown that +such characteristics are the norm for extant members of this +family living in tropical and subtropical climates, and we +speculate that these traits also were common to early +procyonids and served to restrict them to these climates. Our +speculation is supported by the fact that their known fossil +history from the Miocene is confined to geographic areas that +had tropical and subtropical climates.</p> + +<p>Later on, during Pleistocene glaciations, tropical and +subtropical forests shrank, savannas expanded, and temperate +climate was pushed toward equatorial regions. The opposite +occurred during interglacial periods (Raven and Axelrod, 1975; +Webb, 1977, 1978; Marshall, 1988). Consequently, mid-latitudes +experienced alternating periods of temperate and +tropical, or at least subtropical, climate change. Selection of +characteristics that would have adapted a species with low Ḣ<sub>b</sub> +to temperate as well as tropic or subtropic climates could have +occurred in mid-latitudes at the temperate edge of these tropical +advances and retreats. Our analysis indicates that, for this +purpose, selection would have favored lower than predicted +thermal conductance, seasonal molt, increased capacity for +evaporative cooling, increased tolerance of elevated T<sub>b</sub>, +increased flexibility of thermoregulatory behavior, food habits +that provided for year-round access to a high-quality diet in all +three climates, and a higher than predicted r<sub>max</sub>.</p> + +<p><i>Bassariscus astutus</i> is the only species with low Ḣ<sub>b</sub> that has +all these characteristics, and it is the only one of them that has +added temperate climate to its distribution (<a href="#Table_11">Table 11</a>). This +suggests that <i>Bassariscus astutus</i> is a species that evolved away +from the norm for procyonids with low Ḣ<sub>b</sub>, toward characteristics +that allowed it to become more of a climate generalist. +<i>Potos flavus</i>, with its dietary specialization, low tolerance to +high temperatures, and arboreal mode of existence, has become +a highly specialized species totally dependent on tropical +forests for its survival. As such, it also represents a species that +has evolved away from the procyonid norm and portrays the +extreme in climate specialization. Olingos, <i>Bassaricyon gabbii</i> +(<a href="#Table_1">Table 1</a>), may be similar to <i>Potos flavus</i> in this respect (see +also <a href="#Table_10">Table 10</a>). This suggests that of the extant procyonids, +<i>Nasua nasua</i>, <i>Nasua narica</i>, and <i>Procyon cancrivorus</i> have +retained metabolic and behavioral characteristics that are +closest to those of their Miocene ancestors.</p> + +<a name="Evolution_of_High_Basal_Metabolic_Rate"></a> +<span class="pagenum"><a href="#toc">[↑ TOC]</a></span> +<div class="caption4">Evolution of High Basal Metabolic Rate</div> + +<p>Between the time that <i>Cyonasua</i> appeared and the Panamanian +land bridge was established in the upper Pliocene (4 to 5 +million years ago), northern climates continued their gradual +cooling. This, along with ongoing elevation of the continents +and continuous modification of their mountain ranges, served +to shrink the tropical forest and create pockets of climatic +instability within it and on its edges (Darlington, 1963:578-596; +Marshall, 1988). In areas of instability, selection would +have favored traits that provided for a broader range of thermal +tolerance: higher Ḣ<sub>b</sub>, improved insulative quality of pelt, a +more sharply defined molt cycle, improved capacity for +evaporative cooling, greater D<sub>d</sub>, and higher r<sub>max</sub>. Consequently, +by the upper Pliocene, two metabolically distinct groups of +procyonids could have been established: those species with low +Ḣ<sub>b</sub> living in climatically stable forests and those with higher Ḣ<sub>b</sub> +living in unstable tropical, subtropical, and perhaps temperate +climates.</p> + +<p><i>Procyon lotor</i> is the only extant procyonid with high Ḣ<sub>b</sub>. +<i>Procyon cancrivorus</i> is its congeneric counterpart in Central +and South America (<a href="#Table_1">Table 1</a>), and the two species are sympatric +in Panama and Costa Rica. However, in terms of its +metabolism, thermal conductance, molt, diversity of diet, r<sub>max</sub>, +and climatic distribution, <i>Procyon cancrivorus</i> shares more in +common with other procyonids than it does with <i>Procyon lotor</i> +(<a href="#Table_7">Tables 7</a>,<a href="#Table_11"> 11</a>,<a href="#Table_12"> 12</a>; <a href="#Fig_8">Figure 8</a>). This suggests that metabolically +<i>Procyon lotor</i> portrays a divergent line of this genus that arose +as the result of a series of mutations that gave rise to different +metabolic characteristics. This view is in keeping with a recent +phylogenetic analysis of this family that shows the genus +<i>Procyon</i> to be highly derived (Decker and Wozencraft, 1991). +Consequently, it would be instructive and would add to our +knowledge of the evolution of climatic adaptation to know +more about the genetic relatedness of these two species as well +as their historical relationship.</p> + +<p>Genus <i>Procyon</i> appears in the fossil record (Hemphillian and +Blancan ages; Baskin, 1982) prior to Pleistocene glaciations. +During the Pleistocene, there were four different glacial +advances and retreats in a relatively short time period (the first +appearing little more than a million years ago; Darlington, +1963:578-596; Webb, 1985a; Marshall, 1988). Glacial retreats +created pulses of time during which subtropic and temperate +climates advanced toward the poles into areas with large +seasonal differences in light/dark cycles, whereas glacial +advances pushed these climates southward into areas having +smaller seasonal differences in light/dark cycles (Raven and +Axelrod, 1975; Webb, 1977, 1978; Marshall, 1988). Those +members of the genus <i>Procyon</i> caught in these wide latitudinal +fluctuations would have experienced conditions favorable to +continued selection for characteristics conducive to physiologic +adaptation to a wide range of climatic conditions. <i>Procyon +lotor</i> is the only member of its genus to have survived this +selective process, and as we have seen, it does possess traits +that adapt it to a wide range of climatic conditions. Primary +among these is its higher Ḣ<sub>b</sub>, which provides it with advantages +not shared with other procyonids (see earlier discussion). Three +other adaptations also have had a profound influence on +<i>Procyon lotor</i>'s ability to generalize its use of climate: (1) the +increased insulative quality of its pelt coupled with its sharply +defined molt cycle, which allows for a large annual change in +thermal conductance; (2) its annual cycle of fat storage; and (3) +a diverse high-quality diet. The first two of these adaptations +<span class="pagenum"><a name="Page_28" id="Page_28">[Pg 28]</a></span> +required evolution of neuroendocrine pathways capable of +responding to time-dependent environmental cues such as +changing day length, changing temperature, etc. Such conditions +would have been available as selective stimuli in +high-latitude forests and savannas of interglacial periods. +<i>Procyon lotor</i>'s elevated basal metabolic rate would have +increased its overall energy requirement, and it makes good +intuitive sense, therefore, that evolution during the Pleistocene +also would have favored selection of a diverse diet containing +many items of high nutritive value.</p> + +<a name="Summary_4"></a> +<span class="pagenum"><a href="#toc">[↑ TOC]</a></span> +<div class="caption3">Summary</div> + +<p>Our analysis has illustrated that within Procyonidae there are +two distinct modes of metabolic adaptation to climate. One is +typified by those species with low Ḣ<sub>b</sub>'s (<i>Bassariscus astutus</i>, +<i>Nasua nasua</i>, <i>Nasua narica</i>, <i>Procyon cancrivorus</i>, and <i>Potos +flavus</i>), and the other by <i>Procyon lotor</i> with its higher Ḣ<sub>b</sub>. +Those with low Ḣ<sub>b</sub>'s have more restricted geographic distributions, +and, with the exception of <i>Bassariscus astutus</i>, they are +all confined to tropical and subtropical areas. The fossil history +of this family indicates that it had its origins in tropical forests +of North and Central America. This indicates that those +procyonids whose distributions are still primarily restricted to +tropical forests share many of the metabolic adaptations +characteristic of their ancestors. We speculate, therefore, that +ancestral procyonids had a lower than predicted Ḣ<sub>b</sub>, a pelt with +modest to poor insulative quality, good thermogenic ability but +poor heat tolerance, modest to poor capacity for evaporative +cooling, no well-defined molt cycle, no cyclic period of +fattening, nocturnal habits, and a modestly diverse diet of +high-enough quality to provide for an average reproductive +potential. Although this pedigree contributed to the success of +this family in tropical and subtropical forests, it limited the +ability of its members to expand their distributions into cooler, +less stable climates. Viewed in this perspective, <i>Procyon +lotor</i>'s high basal metabolic rate, extraordinarily diverse diet, +well-defined cyclic changes in fat content and thermal +conductance, high level of heat tolerance, high capacity for +evaporative cooling, and high reproductive potential all stand +out in sharp contrast to the condition described for other +procyonids. This suggests that the North American raccoon +represents culmination of a divergent evolutionary event that +has given this species the ability to break out of the old +procyonid mold and carry the family into new habitats and +climates.</p> + + +<br> +<a name="Appendix_List_of_Symbols"></a> +<p><span class="pagenum"><a href="#toc">[↑ TOC]</a></span></p> +<br> +<p><span class="pagenum"><a name="Page_29" id="Page_29">[Pg 29]</a></span></p> +<div class="caption2">Appendix: List of Symbols</div> + +<table width="100%" summary="List of Symbols" cellpadding=7> +<tr><td>a</td><td>potential age of females first producing young</td></tr> +<tr><td>b</td><td>potential annual birth rate of female young</td></tr> +<tr><td>C<sub>a</sub></td><td>conductance of air</td></tr> +<tr><td>C<sub>d</sub></td><td>conductance of den walls</td></tr> +<tr><td>C<sub>m</sub></td><td>minimum thermal conductance</td></tr> +<tr><td>C<sub>md</sub></td><td>minimum dry thermal conductance</td></tr> +<tr><td>C<sub>mw</sub></td><td> minimum wet thermal conductance</td></tr> +<tr><td>C<sub>mwr</sub></td><td>ratio of measured to predicted minimum wet thermal conductance</td></tr> +<tr><td>C<sub>t</sub></td><td>total conductance</td></tr> +<tr><td>D<sub>d</sub></td><td> diversity of diet</td></tr> +<tr><td>D<sub>dr</sub></td><td> ratio of food categories actually used by a species to the total number of food categories taken by all species tested</td></tr> +<tr><td>Ė</td><td>evaporative water loss</td></tr> +<tr><td>E<sub>c</sub></td><td>ratio of evaporative heat lost to metabolic heat produced</td></tr> +<tr><td>Ė<sub>eq</sub></td><td>oxygen equivalent for heat lost by evaporation</td></tr> +<tr><td>Ḣ<sub>b</sub></td><td> basal metabolic rate</td></tr> +<tr><td>Ḣ<sub>r</sub></td><td> lowest resting metabolic rate at each temperature</td></tr> +<tr><td>H<sub>br</sub></td><td> ratio of measured to predicted basal metabolic rate</td></tr> +<tr><td>m</td><td>mass of animal</td></tr> +<tr><td>m<sub>w</sub></td><td> mass of water</td></tr> +<tr><td>n</td><td>potential age of females producing their final young</td></tr> +<tr><td>r<sub>max</sub></td><td> intrinsic rate of natural increase</td></tr> +<tr><td>r<sub>maxe</sub></td><td> expected intrinsic rate of natural increase</td></tr> +<tr><td>r<sub>maxr</sub></td><td> ratio of calculated to expected intrinsic rate of natural increase</td></tr> +<tr><td>RQ</td><td> respiratory quotient</td></tr> +<tr><td>T<sub>a</sub></td><td>chamber air temperature</td></tr> +<tr><td>T<sub>b</sub></td><td> body temperature</td></tr> +<tr><td>T<sub>lc</sub></td><td> lower critical temperature</td></tr> +<tr><td>T<sub>n</sub></td><td> thermoneutral zone</td></tr> +<tr><td>T<sub>uc</sub></td><td> upper critical temperature</td></tr> +<tr><td>t</td><td>time</td></tr> +<tr><td><span class="dot">.</span><span class="V">V</span><sub>a</sub></td><td>rate of air flow through <img src="images/u_tube.png" width="14" height="14" border="0" alt="U-tube" title="U-tube">-tubes</td></tr> +<tr><td><span class="dot">.</span><span class="V">V</span><sub>e</sub></td><td>rate of air flow into metabolism chamber</td></tr> +<tr><td>α</td><td>active phase of the daily cycle</td></tr> +<tr><td>γ</td><td>heat equivalent of oxygen</td></tr> +<tr><td>λ</td><td>heat of vaporization of water</td></tr> +<tr><td>ρ</td><td>rest phase of the daily cycle</td></tr> +</table> +<br> + +<p><span class="pagenum"><a name="Page_30" id="Page_30">[Pg 30]</a></span></p> +<br> +<a name="Literature_Cited"></a> +<p><span class="pagenum"><a href="#toc">[↑ TOC]</a></span></p> +<div class="caption2">Literature Cited</div> + +Aschoff, Jürgen<br> + +<div class="reference">1981. 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In his formal plan for the institution, Joseph Henry outlined a +program that included the following statement: "It is proposed to publish a series of reports, +giving an account of the new discoveries in science, and of the changes made from year to year +in all branches of knowledge." This theme of basic research has been adhered to through the +years by thousands of titles issued in series publications under the Smithsonian imprint, +commencing with <i>Smithsonian Contributions to Knowledge</i> in 1848 and continuing with the +following active series:</p> + +<div class="center"> +<i>Smithsonian Contributions to Anthropology</i><br> +<i>Smithsonian Contributions to Botany</i><br> +<i>Smithsonian Contributions to the Earth Sciences</i><br> +<i>Smithsonian Contributions to the Marine Sciences</i><br> +<i>Smithsonian Contributions to Paleobiology</i><br> +<i>Smithsonian Contributions to Zoology</i><br> +<i>Smithsonian Folklife Studies</i><br> +<i>Smithsonian Studies in Air and Space</i><br> +<i>Smithsonian Studies in History and Technology</i> +</div> + +<p>In these series, the Institution publishes small papers and full-scale monographs that report +the research and collections of its various museums and bureaux or of professional colleagues +in the world of science and scholarship. The publications are distributed by mailing lists to +libraries, universities, and similar institutions throughout the world.</p> + +<p>Papers or monographs submitted for series publication are received by the Smithsonian +Institution Press, subject to its own review for format and style, only through departments of the +various Smithsonian museums or bureaux, where the manuscripts are given substantive review. +Press requirements for manuscript and art preparation are outlined on the inside back cover.</p> + + +<div style="margin-left: 60%"> +Robert McC. Adams<br> +<i>Secretary</i><br> +Smithsonian Institution +</div> +<br> +<br> + + + +<div class="trans_notes"> +<div class="caption2">Transcriber's Notes</div> + +<p>With the exception of the typographical corrections listed below and some minor +changes that may have been made in moving tables or illustrations so that they +are rejoined, the text presented is that published in the original printed media.</p> + +<div class="caption4">Typographical Corrections</div> +<div class="center"> +<table width="86%" summary="typo list"> +<tr><td>Page ii, LOC Data</td><td> : </td><td>Instituion's</td><td> => </td><td>Institution's</td></tr> +<tr><td>Page 1, Introduction</td><td> : </td><td>linages</td><td> => </td><td>lineages</td></tr> +<tr><td>Page 4, The Atypical Procyonid</td><td> : </td><td>consumate</td><td> => </td><td>consummate</td></tr> +<tr><td>Page 21,Summary</td><td> : </td><td>Table 10, footnote f</td><td> => </td><td>Table 10, footnote b</td></tr> +<tr><td>Page 26, first paragraph</td><td> : </td><td>Nassua</td><td> => </td><td>Nasua</td></tr> +<tr><td>Page 31, Literature Cited</td><td> : </td><td>Incoporated</td><td> => </td><td>Incorporated</td></tr> +<tr><td>Page 34, Literature Cited</td><td> : </td><td>Gettleman</td><td> => </td><td>Gittleman</td></tr> +</table> +</div> +</div> + + + + + + + + + + + +<pre> + + + + + +End of the Project Gutenberg EBook of Metabolic Adaptation to Climate and +Distribution of the Raccoon Procyon Lotor and Other Procyonidae, by John N. 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Mugaas and John Seidensticker and Kathleen P. Mahlke-Johnson + +This eBook is for the use of anyone anywhere at no cost and with +almost no restrictions whatsoever. You may copy it, give it away or +re-use it under the terms of the Project Gutenberg License included +with this eBook or online at www.gutenberg.org + + +Title: Metabolic Adaptation to Climate and Distribution of the Raccoon Procyon Lotor and Other Procyonidae + +Author: John N. Mugaas + John Seidensticker + Kathleen P. Mahlke-Johnson + +Release Date: May 5, 2011 [EBook #36036] + +Language: English + +Character set encoding: ASCII + +*** START OF THIS PROJECT GUTENBERG EBOOK THE RACCOON PROCYON LOTOR *** + + + + +Produced by Colin Bell, Tom Cosmas, Joseph Cooper and the +Online Distributed Proofreading Team at http://www.pgdp.net + + + + + + + + + + SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY . NUMBER 542 + + + Metabolic Adaptation to Climate + and Distribution of the Raccoon + _Procyon lotor_ and Other Procyonidae + + + _John N. Mugaas, John Seidensticker, + and Kathleen P. Mahlke-Johnson_ + + + [Smithsonian Institution logo] + + + SMITHSONIAN INSTITUTION PRESS + Washington, D.C. + 1993 + + + + +ABSTRACT + + +Mugaas, J. N., J. Seidensticker, and K. Mahlke-Johnson. Metabolic +Adaptation to Climate and Distribution of the Raccoon _Procyon lotor_ +and Other Procyonidae. _Smithsonian Contributions to Zoology_, number +542, 34 pages, 8 figures, 12 tables, 1993.--Although the family +Procyonidae is largely a Neotropical group, the North American +raccoon, _Procyon lotor_, is more versatile in its use of climate, and +it is found in nearly every habitat from Panama to 60 deg.N in Canada. We +hypothesized that most contemporary procyonids have remained in tropic +and subtropic climates because they have retained the metabolic +characteristics of their warm-adapted ancestors, whereas _Procyon +lotor_ evolved a different set of adaptations that have enabled it to +generalize its use of habitats and climates. To test this hypothesis +we compared _Procyon lotor_ with several other procyonids +(_Bassariscus astutus_, _Nasua nasua_, _Nasua narica_, _Procyon +cancrivorus_, and _Potos flavus_) with respect to (1) basal metabolic +rate ([.H]_{b}), (2) minimum wet thermal conductance (C_{mw}), (3) +diversity of diet (D_{d}), (4) intrinsic rate of natural increase +(r_{max}), and, where possible, (5) capacity for evaporative cooling +(E_{c}). We measured basal and thermoregulatory metabolism, +evaporative water loss, and body temperature of both sexes of _Procyon +lotor_ from north central Virginia, in summer and winter. Metabolic +data for other procyonids were from literature, as were dietary and +reproductive data for all species. + +Procyon lotor differed from other procyonids in all five +variables. (1) _Procyon lotor_'s mass specific [.H]_{b} (0.46 +mL O_{2}.g^{-1}.h^{-1}) was 1.45 to 1.86 times greater than values for +other procyonids. (2) Because of its annual molt, _Procyon lotor_'s +C_{mw} was about 49% higher in summer than winter, 0.0256 and 0.0172 +mL O_{2}.g^{-1}.h^{-1}. deg.C^{-1}, respectively. The ratio of measured to +predicted C_{mw} for _Procyon lotor_ in winter (1.15) was similar to +values calculated for _Potos flavus_ (1.02) and _Procyon cancrivorus_ +(1.25). Values for other procyonids were higher than this, but less +than the value for _Procyon lotor_ (1.76) in summer. On a mass +specific basis, _Bassariscus astutus_ had the lowest C_{mw} with a +ratio of 0.85. (3) _Procyon lotor_ utilized three times as many food +categories as _Procyon cancrivorus_, _Nasua nasua_, and _Bassariscus +astutus_; about two times as many as _Nasua narica_; and nine times as +many as _Potos flavus_. (4) Intrinsic rate of natural increase +correlated positively with [.H]_{b}. _Procyon lotor_ had the highest +r_{max} (2.52 of expected) and _Potos flavus_ the lowest (0.48 of +expected). The other procyonids examined also had low [.H]_{b}, but +their r_{max}'s were higher than predicted (1.11-1.32 of expected). +Early age of first female reproduction, fairly large litter size, long +life span, high-quality diet, and, in one case, female social +organization all compensated for low [.H]_{b} and elevated r_{max}. +(5) Although data on the capacity for evaporative cooling were +incomplete, this variable appeared to be best developed in _Procyon +lotor_ and _Bassariscus astutus_, the two species that have been most +successful at including temperate climates in their distributions. + +These five variables are functionally interrelated, and have +co-evolved in each species to form a unique adaptive unit that +regulates body temperature and energy balance throughout each annual +cycle. The first four variables were converted into normalized +dimensionless numbers, which were used to derive a composite score +that represented each species' adaptive unit. _Procyon lotor_ had the +highest composite score (1.47) and _Potos flavus_ the lowest (0.39). +Scores for the other procyonids were intermediate to these extremes +(0.64-0.79). There was a positive correlation between the number of +climates a species occupies and the magnitude of its composite score. +Linear regression of this relationship indicated that 89% of the +variance in climatic distribution was attributed to the composite +scores. Differences in metabolic adaptation, therefore, have played a +role in delimiting climatic distribution of these species. + +It was clear that _Procyon lotor_ differed from the other procyonids +with respect to thermoregulatory ability, diet, and reproductive +potential. These differences have enabled it to become a highly +successful climate generalist, and its evolution of an [.H]_{b} that +is higher than the procyonid norm appears to be the cornerstone of its +success. + + +OFFICIAL PUBLICATION DATE is handstamped in a limited number of +initial copies and is recorded in the Institution's annual report, +_Smithsonian Year_. SERIES COVER DESIGN: The coral _Montastrea +cavernosa_ (Linnaeus). + + + Library of Congress Cataloging-in-Publication Data + + Mugaas, John N. + + Metabolic adaptation to climate and distribution of the raccoon + Procyon lotor and other Procyonidae / John N. Mugaas, John + Seidensticker, and Kathleen P. Mahlke-Johnson. + + p. cm.--(Smithsonian contributions to zoology; no. 542) + + Includes bibliographical references (p. ) + + 1. Raccoons-Metabolism-Climatic factors. + 2. Procyonidae-Metabolism-Climatic factors. + 3. Raccoons-Geographical distribution. + 4. Procyonidae-Geographical distribution. + I. Seidensticker, John. + II. Mahlke-Johnson, Kathleen. + III. Title. + IV. Series. + + QL1.S54 no. 542 [QL737.C26] 591 s-dc20 [599.74'443'04542] 93-3119 + + +[permanent paper symbol] The paper used in this publication meets the +minimum requirements of the American National Standard for Permanence +of Paper for Printed Library Materials z39.48--1984. + + + + +Contents + + + _Page_ + Introduction 1 + Defining the Problem 1 + Procyonid Origins 1 + Typical Procyonids 2 + The Atypical Procyonid 3 + The Hypothesis 4 + Hypothesis Testing 4 + Adaptive Significance of the Variables 4 + Basal Metabolic Rate and Intrinsic Rate of Natural Increase 4 + Minimum Thermal Conductance 4 + Capacity for Evaporative Cooling 5 + Diet 5 + Experimental Design and Summary 5 + Acknowledgments 5 + + Materials and Methods 6 + Live-trapping 6 + Metabolic Studies 6 + Basal and Thermoregulatory Metabolism 6 + Evaporative Water Loss 7 + Body Temperature 7 + Calibrations 7 + Calorimeter 7 + Body Temperature Transmitters 8 + Statistical Methods 8 + Estimating Intrinsic Rate of Natural Increase 8 + Comparison of Adaptive Units 8 + + Results 8 + Body Mass 8 + Basal Metabolic Rate 9 + Minimum Thermal Conductance 9 + Evaporative Water Loss 11 + Thermoregulation at Low Temperatures 12 + Body Temperature 12 + Summer 14 + Winter 14 + Thermoregulation at High Temperatures 16 + Body Temperature 16 + Summer 16 + Winter 16 + Daily Cycle of Body Temperature 16 + + Discussion 16 + Basal Metabolic Rate 16 + Background 16 + Captive versus Wild Raccoons 17 + Seasonal Metabolism of Raccoons 17 + Comparison of _Procyon lotor_ with Other Procyonids 17 + Influence of Diet on Basal Metabolism 18 + Background 18 + Food Habits of Procyonids 18 + Food Habits and Basal Metabolism 19 + Summary 19 + Basal Metabolism and Intrinsic Rate of Natural Increase 19 + Background 19 + _Procyon lotor_ 19 + _Bassariscus astutus_ 19 + _Nasua narica_ 19 + _Nasua nasua_ 20 + _Procyon cancrivorus_ 20 + _Potos flavus_ 20 + Summary 20 + Basal Metabolism and Climatic Distribution 21 + _Procyon lotor_ 21 + Other Procyonids 21 + Minimum Thermal Conductance 21 + Background 21 + Effect of Molt on Thermal Conductance 21 + Comparison of Thermal Conductances 22 + _Procyon lotor_ versus Tropical Procyonids 22 + _Bassariscus astutus_ 22 + Thermoregulation and Use of Stored Fat at Low Temperatures 22 + Background 22 + Thermoregulation 22 + Stored Fat 23 + Thermal Model of the Raccoon and Its Den 23 + Metabolic Advantage of the Den 23 + Thermoregulation at High Temperatures 24 + Background 24 + Comparison of Procyonid Responses to Heat Stress 24 + _Potos flavus_ 24 + _Nasua nasua and Nasua narica_ 24 + _Bassariscus astutus_ 24 + _Procyon lotor_ 24 + _Procyon cancrivorus_ 24 + Summary 24 + Composite Scores of Adaptive Units and Geographic Distribution 25 + Evolution of Metabolic Adaptations 26 + Evolution of Low Basal Metabolic Rate 26 + Evolution of High Basal Metabolic Rate 27 + Summary 28 + + Appendix: List of Symbols 29 + + Literature Cited 30 + + + + + Metabolic Adaptation to Climate + and Distribution of the Raccoon + _Procyon lotor_ and Other Procyonidae + + + _John N. Mugaas, John Seidensticker, + and Kathleen P. Mahlke-Johnson_ + + + _John N. Mugaas, Department of Physiology, Division of Functional + Biology, West Virginia School of Osteopathic Medicine, Lewisburg, + West Virginia 24901. John Seidensticker and Kathleen P. + Mahlke-Johnson, National Zoological Park, Smithsonian Institution, + Washington, D.C. 20008._ + + + + +$Introduction$ + + + +DEFINING THE PROBLEM + + +_Procyonid Origins_ + +The major carnivore radiations took place about 40 million years +before present (MYBP) in the late Eocene and early Oligocene (Ewer, +1973:363; Wayne et al., 1989). Between 30 and 40 MYBP, a progenitor +split into the ursid and procyonid lineages, which evolved into +present-day bears, pandas, and raccoons (Wayne et al., 1989). The +taxonomic relatedness of pandas to bears and raccoons has been tested +extensively and a number of authors have summarized current thinking +on the problem (Martin, 1989; Wayne et al., 1989; Wozencraft, 1989a, +1989b; Decker and Wozencraft, 1991). Davis (1964:322-327) and others +(Leone and Wiens, 1956; Todd and Pressman, 1968; Sarich, 1976; O'Brien +et al., 1985) place the giant panda, _Ailuropoda melanoleuca_, with +the ursids. The taxonomic status of the red panda, _Ailurus fulgens_, +appears to be less certain. Some current investigations align the red +panda with bears (Segall, 1943; Todd and Pressman, 1968; Hunt, 1974; +Ginsburg, 1982; Wozencraft, 1984:56-110; 1989a), whereas others place +them intermediate to procyonids and bears (Wurster and Benirschke, +1968; Sarich, 1976; O'Brien et al., 1985), or in close relationship to +the giant panda (Tagle et al., 1986). + +The procyonid radiation took place in North America and produced forms +that were mostly arboreal and omnivorous (Eisenberg, 1981:122; Martin, +1989). The center of this diversification occurred in Middle America +(Baskin, 1982; Webb, 1985b) during the Miocene (Darlington, 1963:367; +Webb, 1985b). Fossil procyonids from the late Miocene are represented +in Florida, California, Texas, Nebraska, Kansas, and South Dakota +(Baskin, 1982; Martin, 1989) and include such genera as _Bassariscus_, +_Arctonasua_, _Cyonasua_, _Paranasua_, _Nasua_, and _Procyon_ (Baskin, +1982; Webb, 1985b). During the Miocene procyonids underwent a modest +radiation within tropical and subtropical climates of North America's +central and middle latitudes. _Cyonasua_, which has close affinities +to _Arctonasua_ (Baskin, 1982), appears in tropical South America in +the late Miocene and immigrated there either by rafting across the +Bolivar Trough or by island-hopping through the Antilles archipelagoes +(Marshall et al., 1982; Marshall, 1988). Thus, procyonids were found +on both continents prior to formation of the Panamanian land bridge +(Darlington, 1963:367, 395; Marshall et al., 1982; Marshall, 1988). +Origins of _Bassaricyon_ and _Potos_ are obscure but probably occurred +in tropical rainforests of Middle America (Baskin, 1982; Webb, 1985b). +A subsequent Pleistocene dispersal carried several modern genera +(Table 1) across the Panamanian land bridge into South America (Webb, +1985b). _Bassariscus_ and _Bassaricyon_ represent the most primitive +genera in Procyoninae and Potosinae subfamilies, respectively (Table 1; +Wozencraft, 1989a; Decker and Wozencraft, 1991). + +In the early Tertiary, mid-latitudes of North America were much warmer +than they are now, but not fully tropical, and temperate deciduous +forests, associated with strongly seasonal climates, occurred only in +the far north (Barghoorn, 1953; Colbert, 1953; Darlington, 1963:589, +590). Major climatic deteriorations, with their attendant cooling of +northern continents, occurred during the Eo-Oligocene transition, in +the middle Miocene, at the end of the Miocene, and at about 3 MYBP +(late Pliocene). This last deterioration corresponds with closure of +the Panamanian isthmus (Berggren, 1982; Webb, 1985a). Climatic +deterioration went on at an accelerating rate during the late +Tertiary, with glacial conditions developing at the poles by the +mid-Pliocene (Barghoorn, 1953). Therefore, throughout the Tertiary, as +continents cooled, northern climate zones moved toward the tropics +(Barghoorn, 1953; Colbert, 1953; Darlington, 1963:589, 590, 594, 595; +Webb, 1985a). + + + TABLE 1.--Classification of recent Procyonidae after Wozencraft + (1989a) and Decker and Wozencraft (1991). Information in + parenthesis indicates general geographic distribution (modified from + Kortlucke and Ramirez-Pulido (1982) and Poglayen-Neuwall (1975)): + S.A. = South America; C.A. = Central America; M. = Mexico; + U.S. = United States; C. = Canada. Lower case letters preceding + geographic areas signify north (n), south (s), and west (w). + + Order CARNIVORA Bowdich, 1821 + Suborder CANIFORMIA Kretzoi, 1945 + Family PROCYONIDAE Gray, 1825 + Subfamily POTOSINAE Trouessart, 1904 + Genus _Potos_ E. Geoffroy and G. Cuvier, 1795 + _P. flavus_ (S.A., C.A., M.) + Genus _Bassaricyon_ Allen, 1876 + _B. alleni_[a] (S.A.) + _B. beddardi_[a] (S.A.) + _B. gabbii_[a] (nS.A., C.A.) + _B. lasius_[a] (C.A.) + _B. pauli_[a] (C.A.) + Subfamily PROCYONINAE Gray, 1825 + Genus _Bassariscus_ Coues, 1887 + _B. astutus_ (M., wU.S.) + _B. sumichrasti_ (C.A., M.) + Genus _Nasua_ Storr, 1780 + _N. narica_[b] (nS.A., C.A., M., swU.S.) + _N. nasua_[b] (S.A., sC.A.) + Genus _Nasuella_ Hollister, 1915 + _N. olivacea_ (S.A.) + Genus _Procyon_ Storr, 1780 + _P. cancrivorus_ (S.A., sC.A.) + _P. gloveralleni_[c] (Barbados) + _P. insularis_[c] (Maria Madre Is., Maria Magdalene Is.) + _P. lotor_[c] (C.A., M., U.S., sC.) + _P. maynardi_[c] (Bahamas, New Providence Is.) + _P. minor_[c] (Guadeloupe Is.) + _P. pygmaeus_[c] (M., Quintana Roo, Cozumel Is.) + + [a] The several named forms of _Bassaricyon_ are a single + species, _Bassaricyon gabbii_ (Wozencraft, 1989a). + + [b] These are considered conspecific in some current + taxonomies (Kortlucke and Ramirez-Pulido, 1982); however, + the scheme followed here maintains them as separate species + (Decker, 1991). + + [c] Several named forms of _Procyon_ are a single species, + _Procyon lotor_ (Wozencraft, 1989a). + + +During the late Miocene, late Pliocene, and Pleistocene, the Bering +land bridge between North America and Asia formed periodically, +offering an avenue for dispersal between northern continents +(Darlington, 1963:366; Webb, 1985a). However, by the late Tertiary, +northern continents had cooled to the extent that climate, with its +attendant sharply defined vegetative zones, became the major factor +limiting dispersal by this route (Darlington, 1963:366; Webb, 1985a). +Those Holarctic mammals that did cross the Bering land bridge in the +late Tertiary were "cold-adapted" species associated with relatively +cool, but not alpine, climates (Darlington, 1963:366; Ewer, 1973:369). +Among carnivores this included some canids, ursids, mustelids, and +felids (Darlington, 1963:393-395, 397; Webb, 1985a). Procyonids, +however, did not cross the Bering land bridge into Asia, and Ewer +(1973:369) ascribes this to their being an "essentially tropical +group." Miocene radiation of procyonids occurred at a time when two of +the four major climatic deteriorations (middle and late Miocene) were +taking place (Webb, 1985a, 1985b). These deteriorations had the effect +of cooling the middle latitudes to the extent that temperate forest +forms began to appear in mid-latitude floras, along with a rapid +influx of herbaceous plants (Barghoorn, 1953). The procyonid radiation +did not penetrate beyond these climatically changing middle latitudes, +which implies that these animals were "warm-adapted," and were, +therefore, physiologically excluded from reaching the Bering land +bridge. Today, three of the six genera and over half of the 18 species +that comprise Procyonidae (Table 1; Wozencraft, 1989b) remain confined +to tropical regions of North and South America (Hall and Kelson, +1959:878-897; Poglayen-Neuwall, 1975; Kortlucke and Ramirez-Pulido, +1982; Nowak and Paradiso, 1983:977-985). + + +_Typical Procyonids_ + +McNab (1988a) contends that basal metabolism is a highly plastic +character in evolution, and he has amply shown that ecologically +uniform species are more apt to share common metabolic rates than +taxonomically allied species from drastically different environments +(McNab, 1984a, 1986a, 1986b, 1988a). Procyonids represent a +taxonomically allied group that shared a common ecological situation +for millions of years; consequently, members of this family might be +expected to show some uniformity in their [.H]_{b}. Basal and +thermoregulatory metabolism of several procyonids have been measured: +kinkajou, _Potos flavus_ (Mueller and Kulzer, 1977; McNab, 1978a; +Mueller and Rost, 1983), coatis, _Nasua nasua_ (Chevillard-Hugot et +al., 1980; Mugaas et al., in prep.), and _Nasua narica_ (Scholander et +al., 1950c; Mugaas et al., in prep.), ringtail, _Bassariscus astutus_ +(Chevalier, 1985), and crab-eating raccoon, _Procyon cancrivorus_ +(Scholander et al., 1950c). In general, these species have [.H]_{b}'s +that are 40%-80% of the values predicted for them by the Kleiber +(1961:206) equation. Lower than predicted [.H]_{b} is viewed as an +energy-saving adaptation for procyonids living in relatively stable +tropical climates (Mueller and Kulzer, 1977; Chevillard-Hugot et al., +1980; Mueller and Rost, 1983). This implies that lower than predicted +[.H]_{b} is a general procyonid condition and that it represents a +characteristic that evolved in response to the family's long +association with tropical and subtropical forest environments. + + +_The Atypical Procyonid_ + +Although most procyonids are found in only tropical to subtropical +climates, the North American raccoon, _Procyon lotor_, (Figure 1) has +a much broader distribution that extends from tropical Panama (8 deg.N) to +southern Canada. In Alberta, Canada, its range reaches the edge of the +Hudsonian Life Zone at 60 deg.N (for distribution maps see Hall and +Kelson, 1959:878-897, and Poglayen-Neuwall, 1975). Range extensions +and an increase in numbers have been noted in Canada and in parts of +the United States since the 19th century (Lotze and Anderson, 1979; +Kaufmann, 1982; Nowak and Paradiso, 1983:977-985). Thus, _Procyon +lotor_ is more complex ecologically than other procyonids, +particularly when one takes into account its highly generalized food +habits (Hamilton, 1936; Stuewer, 1943; Stains, 1956:39-51; Greenwood, +1981) and the wide range of habitat types (forest, prairie, desert, +mountain, coastal marsh, freshwater marsh) and climates (tropical to +north temperate) in which it is successful (Whitney and Underwood, +1952:1; Hall and Kelson, 1959:885; Lotze and Anderson, 1979; +Kaufmann, 1982). On this basis it is clear that _Procyon lotor_ has +deviated from the typical procyonid portrait and has become the +consummate generalist of the Procyonidae. + + [Illustration: FIGURE 1.--North American raccoon, _Procyon lotor_.] + + +_The Hypothesis_ + +Our general hypothesis was that whereas most contemporary procyonids +have retained the metabolic characteristics of their warm-adapted +ancestors, _Procyon lotor_ possesses a different set of adaptations, +which either evolved as characteristics unique to this species or were +acquired from its ancestral stock. In either case, its unique +adaptations have given _Procyon lotor_ the physiological flexibility +to generalize its use of habitats and climates and expand its +geographic distribution to a much greater extent than other +procyonids. + + +_Hypothesis Testing_ + +We tested our hypothesis by comparing _Procyon lotor_ with several +other procyonids (_Bassariscus astutus_, _Nasua nasua_, _Nasua +narica_, _Procyon cancrivorus_, and _Potos flavus_) on the basis of +their (1) basal metabolic rate ([.H]_{b}), (2) minimum wet thermal +conductance (C_{mw}), (3) diversity of diet (D_{d}), (4) intrinsic +rate of natural increase (r_{max}), and, when data were available, (5) +capacity for evaporative cooling (E_{c}). In a genetic sense each one +of these variables is a complex adaptive characteristic, expression of +which is determined by the interaction of several genes (Prosser, +1986:110-165). Experience has shown that a given species will express +each one of these variables in a specific manner that is relevant to +its mass, physiology, behavior, and environmental circumstance. Thus, +different expressions of these variables may represent specific +climatic adaptations (Prosser, 1986:16) that have been selected-for by +evolutionary process. Because these variables are interrelated with +respect to regulation of body temperature and energy balance, they +have co-evolved in each species to form an adaptive unit. For each +species, measured and calculated values for the first four variables +were converted into dimensionless numbers and used to derive a +composite score that represented its adaptive unit. Climatic +distributions of these species were then compared relative to their +composite scores. + + + +ADAPTIVE SIGNIFICANCE OF THE VARIABLES + + +_Basal Metabolic Rate and Intrinsic Rate of Natural Increase_ + +Basal metabolic rate represents the minimum energy required by an +animal to maintain basic homeostasis (Lusk, 1917:141; Kleiber, 1932, +1961:251; Benedict, 1938; Brody, 1945:59; Robbins, 1983:105-111). For +mammals, [.H]_b appears to be determined by complex interactions +between their body size (Kleiber, 1932, 1961:206; Benedict, 1938; +Brody, 1945:368-374; Hemmingsen, 1960:15-36; McNab, 1983b; Calder, +1987), the climate in which they live (Scholander et al., 1950c; McNab +and Morrison, 1963; Hulbert and Dawson, 1974; Shkolnik and +Schmidt-Nielsen, 1976; McNab, 1979a; Vogel, 1980), their food habits +(McNab, 1978a, 1978b, 1980a, 1983a, 1984a, 1986a, 1986b, 1988a, 1989), +and their circadian period (Aschoff and Pohl, 1970; Prothero, 1984). +Some species have higher mass-specific [.H]_{b} than others, and this +variation appears to be tied to ecological circumstances rather than +taxonomic affinities (McNab, 1988a, 1989). Basal metabolic rate is +important ecologically because it serves as a measure of a species' +minimum "obligatory" energy requirement, and under many circumstances, +it represents the largest energy demand associated with a daily energy +budget (King, 1974:38-55; McNab, 1980a; Mugaas and King, 1981:37-40). +Recently it also has been implicated as a permissive factor with +respect to r_{max} of mammals (Hennemann, 1983; Lillegraven et al., +1987; Nicoll and Thompson, 1987; Thompson, 1987) via its direct effect +on their rates of development and fecundity (McNab, 1980a, 1983a, +1986b; Hennemann, 1983; Schmitz and Lavigne, 1984; Glazier, 1985a, +1985b). The implication of this latter point is that those species +with higher [.H]_{b}'s also have faster rates of development and +greater fecundity and hence enjoy the competitive advantage of a +higher r_{max}. Basal metabolism is, therefore, "a highly plastic +character in the course of evolution" (McNab, 1988a:25) that has a +profound influence on each species' life history. + + +_Minimum Thermal Conductance_ + +Whole-body resistance to passive heat transfer is equal to tissue +resistance plus coat resistance. Within limits, these resistances can +be altered; tissue resistance can be varied by changes in blood flow, +whereas coat resistance can be changed by piloerection, molt, and +behavior. When whole-body resistance is maximized (maximum tissue and +coat resistances), passive heat transfer is minimized. The inverse of +resistance is conductance; therefore, maximum whole-body resistance is +the inverse of minimum thermal conductance (C_{m}). Minimum thermal +conductance is readily derived from metabolic chamber data, and it is +commonly used to describe an animal's capacity to minimize passive +heat transfer. Minimum thermal conductance interacts with [.H]_{b} and +body mass to set the maximum temperature differential a mammal can +maintain without increasing its basal level of heat production. The +low temperature in this differential is the lower critical temperature +(T_{lc}). + +Mass-specific C_{m} for mammals is negatively correlated with body +mass (McNab and Morrison, 1963; Herreid and Kessel, 1967; McNab, 1970, +1979b; Bradley and Deavers, 1980; Aschoff, 1981), and for any given +mass its magnitude is 52% higher during the active, rather than the +inactive, phase of the daily cycle (Aschoff, 1981). However, some +mammals have C_{m}'s that are higher or lower than would be predicted +for them on the basis of body mass and circadian phase. Seasonal +variation in C_{m} (higher values during summer than winter) has been +reported for many northern mammals that experience large annual +variations in air temperature (Scholander et al., 1950a; Irving et +al., 1955; Hart, 1956, 1957; Irving, 1972:165). Some tropical mammals +with very thin fur coats, and others with nearly hairless bodies, have +high C_{m}'s (McNab, 1984a), as do burrowing mammals (McNab, 1966, +1979b, 1984a) and the kit fox, _Vulpes macrotis_ (Golightly and +Ohmart, 1983). Some small mammals with low basal metabolic rates tend +to have lower than predicted C_{m}'s: small marsupials (McNab, 1978a), +heteromyid rodents (McNab, 1979a), several ant eaters (McNab, 1984a), +the arctic hare, _Lepus arcticus_ (Wang et al., 1973), the ringtail, +_Bassariscus astutus_ (Chevalier, 1985), and the fennec, _Fennecus +zerda_ (Noll-Banholzer, 1979). Thus, in spite of its mass dependence, +C_{m} also has been modified during the course of evolution by +selective factors in the environment and by the animal's own metabolic +characteristics. + + +_Capacity for Evaporative Cooling_ + +Latent heat loss occurs as a result of evaporation from the +respiratory tract and through the skin, and except under conditions of +heat stress, it "is a liability in thermal and osmotic homeostasis" +(Calder and King, 1974:302). E_{c}, defined as the ratio of +evaporative heat lost to metabolic heat produced, can be used to +quantify thermoregulatory effectiveness of evaporative cooling and to +make comparisons of heat tolerance between species. Thermoregulatory +effectiveness of latent heat loss is not just a function of the rate +of evaporative water loss but also of the rate of metabolic heat +production (Lasiewski and Seymour, 1972). For example, a low metabolic +rate minimizes endogenous heat load and thus conserves water, whereas +the opposite is true of high metabolic rates (Lasiewski and Seymour, +1972). Some mammals that live in arid regions have evolved low +metabolic rates and thus capitalize on this relationship to reduce +their thermoregulatory water requirement (McNab and Morrison, 1963; +McNab, 1966; MacMillen and Lee, 1970; Noll-Banholzer, 1979). What is +evident, therefore, is that an animal's capacity for increasing latent +heat loss must evolve together with its [.H]_{b} and C_{m} in response +to specific environmental demands. + + +_Diet_ + +McNab (1986a, 1988a, 1989) demonstrated that, for mammals, departures +of [.H]_{b} from the Kleiber (1961:206) "norm" are highly correlated +with diet and independent of phylogenetic relationships. McNab's +analysis indicates that for mammals that feed on invertebrates, those +species with body mass less than 100 g have [.H]_{b}'s that are equal +to or greater than values predicted by the Kleiber equation, whereas +those with body mass greater than 100 g have metabolic rates that are +lower than predicted. Grazers, vertebrate eaters, nut eaters, and +terrestrial frugivores also have [.H]_{b}'s that are equal to or +greater than predicted, whereas insectivorous bats, arboreal +folivores, arboreal frugivores, and terrestrial folivores all have +rates that are lower than predicted. McNab (1986a) found animals with +mixed diets harder to categorize, but in general he predicted that +their [.H]_{b}'s would be related to (1) a food item that is +constantly available throughout the year, (2) a food item that is most +available during the worst conditions of the year, or (3) a mix of +foods available during the worst time of the year. Although these +correlations do not establish cause and effect between food habits and +[.H]_{b}, McNab's analysis does make it clear that the relationship +between these variables has very real consequences for an animal's +physiology, ecology, and evolution. + + + +EXPERIMENTAL DESIGN AND SUMMARY + + +In this investigation we measured basal and thermoregulatory +metabolism, evaporative water loss, and body temperature of raccoons +from north central Virginia. Measurements were conducted on both sexes +in summer and winter to determine how season and sex influenced these +variables. We then compared the data for this widely distributed +generalist with data from literature for its ecologically more +restricted relatives. Dietary data for all species were taken from +literature, as were reproductive data for calculation of r_{max}. + +Our analysis demonstrated clear differences between _Procyon lotor_ +and other procyonids with respect to [.H]_{b}, C_{mw}, D_{d}, and +r_{max}. The composite score calculated from these variables for +_Procyon lotor_ was much higher than those derived for other species, +and there was a positive correlation between the number of climates a +species occupies and the magnitude of its composite score. Data on +evaporative water loss, although not complete for all species, +suggested that tropical and subtropical procyonids have less capacity +for evaporative cooling than _Procyon lotor_ or _Bassariscus astutus_. +It was clear, therefore, that with respect to its thermal physiology, +_Procyon lotor_ differed markedly from other procyonids, and we +contend that these differences have allowed this species to become a +highly successful climate generalist and to expand its distribution +into many different habitats and climates. Our analysis also suggested +that the cornerstone of _Procyon lotor_'s success as a climate +generalist is its [.H]_{b}, which is higher than the procyonid norm. + + + +ACKNOWLEDGMENTS + + +The authors would like to thank John Eisenberg and Devra Kleiman for +their support and encouragement throughout the study. This +investigation was supported by research grants from the West Virginia +School of Osteopathic Medicine (WVSOM), and Friends of the National +Zoo (FONZ). Logistic support was provided by the National Zoological +Park's Conservation and Research Center (CRC), and the departments of +Mammalogy and Zoological Research. Our ability to conduct +physiological research at CRC was made possible by the thoughtful +support and encouragement provided by Chris Wemmer. His excellent +staff at CRC, especially Jack Williams, Junior Allison, and Red +McDaniel, were very helpful in providing hospitality and logistical +support to the senior author and his family during their various +visits to the Center. The assistance of several people at the National +Zoo also is gratefully acknowledged: Mitch Bush and Lyndsay Phillips +not only provided veterinary support throughout the investigation, +but also performed surgical procedures required to implant +temperature-sensitive radio transmitters in several raccoons; Olav +Oftedal made his laboratory available to us at various times and +loaned us equipment to use at CRC; Miles Roberts and his staff +provided care for our captive raccoons in the Department of Zoological +Research during various parts of the investigation. Greg Sanders and +Ken Halama, supported by FONZ assistantships, cared for our captive +raccoons at CRC, provided assistance in the laboratory whenever +needed, and were an invaluable source of aid. Their friendship and +help is gratefully acknowledged. Ellen Broudy and Andy Meyer, +supported by WVSOM and a student work study grant, respectively, +provided assistance in the laboratory. David Brown, John Eisenberg, +Mary Etta Hight, Brian McNab, Steve Thompson, and W. Chris Wozencraft +critically reviewed various phases of the manuscript and provided many +helpful suggestions. We deeply appreciate the work of Jean B. +McConville, whose beneficial editorial suggestions helped us improve +several early versions of the manuscript. We also gratefully +acknowledge Diane M. Tyler, our editor at the Smithsonian Institution +Press, whose expertise helped us mold the manuscript into its final +form. Jill Mellon and Sriyanie Miththalapa, supported by FONZ +traineeships, assisted in measuring the daily cycle of body +temperature in raccoons. The Virginia Commission of Game and Inland +Fisheries gave us permission to use wild-caught raccoons in this +project. + + + + +$Materials and Methods$ + + + +LIVE-TRAPPING + + +Raccoons were caught from May 1980 through December 1984 on a trapping +grid of 30 to 35 stations (one or two "live traps" per station) that +covered about one-third of the National Zoological Park's Conservation +and Research Center (CRC) near Front Royal, Virginia (Seidensticker et +al., 1988; Hallett et al., 1991). Animals were trapped during 10 +consecutive days each month, and in this five-year interval 407 +raccoons were captured and marked with tattoos and ear tags. All +captured animals were individualized with respect to age, reproductive +status, physical condition, parasite load, and mass and body +dimensions. These data characterized the structure and dynamics of the +raccoon population at CRC and provided information on the annual +cycle of fattening for raccoons in north central Virginia. + +Animals used for metabolic measurements were captured at CRC about +1.5 km south of the trapping grid and thus were genetically +representative of the area. Six males were captured and measured during +the summer of 1983. These animals were kept isolated for a week before +being measured and were released later that summer at the site of their +capture. The other seven animals used in our study were from the +collection of the National Zoological Park and all of them had their +origins at CRC. + + + +METABOLIC STUDIES + + +_Basal and Thermoregulatory Metabolism_ + +Metabolic measurements, conducted at CRC, were carried out on eight +males during July and August 1983, on four females and three males +from November 1983 through March 1984, and on four females during June +and July 1984. + +Raccoons were housed throughout the study such that they were +constantly exposed to a natural cycle of temperature and photoperiod. +Weather records for the Front Royal area indicate that average +temperatures are around -0.5 deg.C in January and 23.3 deg.C in July +(Crockett, 1972). Light:dark (L:D) periods for the latitude of CRC +(48 deg.55'N; United States Department of the Interior Geological Survey, +1972), calculated from duration of daylight tables (List, 1971:506-512), +were 14.9:9.1 and 9.4:14.6 hours L:D for summer and winter solstices, +respectively, and 12.2:11.8 hours L:D for vernal and autumnal equinoxes. + +Our animals were fed a measured amount of food daily, and they usually +ate most of what was provided. Occasionally these animals would eat +very little or none of their ration, and on some days they would eat +all that was given to them. We fed them either feline diet (ground +horse meat) or canned mackerel (Star-kist(R)[1]) along with +high-protein dog chow (Purina(R)). When available, fresh fruit also +was added to their diet. Water was always provided ad libitum. + + [1] _The use of product brand names in this publication is + not intended as an endorsement of the products by the + Smithsonian Institution._ + +Measurements were conducted during the raccoons' daily inactive period +(sunrise to sunset) in both summer and winter. Oxygen consumption was +measured in a flow-through metabolism chamber at 5 deg.C intervals from +-10 deg.C to 35 deg.C. Animals were held at each temperature until the lowest +rate of oxygen consumption had been obtained and maintained for at +least 15 minutes. During each determination, oxygen consumption was +monitored for 30 minutes to one hour beyond a suspected minimum value +to see if an even lower reading could be obtained. Raccoons attained +minimum levels of oxygen consumption more quickly at warm (>10 deg.C) than +at cold temperatures. Depending on the temperature, therefore, each +measurement took from two to five hours to complete. On days when two +measurements could be completed, the second trial was always at a +temperature 10 deg.C warmer than the first. + +The metabolism chamber was constructed from galvanized sheet metal +(77.5 x 45.5 x 51.0 cm = 180 liters) and was painted black inside. +Within the chamber, the animal was held in a cage (71 x 39 x 33 cm) +constructed from turkey wire that also was painted black. This cage +prevented the raccoons from coming into contact with the walls of the +chamber, yet it was large enough to allow them to stand and freely +move about. The bottom of the cage was 11 cm above the chamber floor, +which was covered to a depth of one cm with mineral oil to trap urine +and feces. + +During measurements, the metabolism chamber was placed in a +controlled-temperature cabinet (modified Montgomery Ward model 8969 +freezer). Air temperature (T_{a}) in the metabolism chamber was +regulated with a Yellow Springs Instrument model 74 temperature +controller. T_{a} was controlled to +-1.0 deg.C at temperatures below +freezing, and to +-0.5 deg.C at temperatures above freezing. The chamber +air and wall temperatures were recorded continuously (Linseis model +LS-64 recorder) during each experiment, and, except during temperature +changes, they were always within 0.5 deg.C of each other. + +Columns of Drierite(R) and Ascarite(R) removed water vapor and carbon +dioxide, respectively, from air entering and leaving the chamber. Dry +carbon-dioxide-free room air was pumped into the chamber (Gilman model +13152 pressure/vacuum pump) at a rate of 3.0 L/min (Gilmont model +K3203-20 flow meter). Downstream from the chemical absorbents, an +aliquot (0.1 L/min) of dry carbon-dioxide-free air was drawn off the +chamber exhaust line and analyzed for oxygen content (Applied +Electrochemistry model S-3A oxygen analyzer, model 22M analysis cell, +and model R-1 flow control). All gas values were corrected to standard +temperature and pressure for dry gas. Oxygen consumption was +calculated from the difference in oxygen content between inlet and +outlet air using Eq. 8 of Depocas and Hart (1957). + +Each raccoon was fasted for at least 12 hours before oxygen +consumption measurements began. At the start and end of each metabolic +trial the animal was weighed to the nearest 10 g (Doctors Infant +Scale, Detecto Scales, Inc., Brooklyn, N.Y., U.S.A.). The body mass +used in calculating minimum oxygen consumption and evaporative water +loss was estimated from timed extrapolations of the difference between +starting and ending weights, and the time at which these variables +were measured. + + +_Evaporative Water Loss_ + +During metabolic measurements at temperatures above freezing, +evaporative water loss was determined gravimetrically. Upstream from +the chemical columns, an aliquot of air (0.1 L/min) was drawn off the +exhaust line and diverted for a timed interval through a series of +preweighed (0.1 mg) U-tubes containing Drierite(R). The aliquot then +passed through a second series of U-tubes containing Ascarite(R) +before entering the oxygen analysis system. Evaporative water loss was +calculated using Eq. 1 + + [.E] = (m_{w}.[.V]_{e})/([.V]_{a}.t.m) Eq. 1 + +where [.E] is evaporative water loss (mg.g^{-1}.h^{-1}), m_{w} is mass +of water collected (mg), [.V]_{e} is rate of air flow into the chamber +(3.0 L/min), [.V]_{a} is rate of air flow through the U-tubes +(0.1 L/min), t is length of the timed interval (h), and m is the +estimated mass of the raccoon at the time of sampling (g). + + +_Body Temperature_ + +Veterinarians at the National Zoological Park surgically implanted +calibrated temperature-sensitive radio transmitters (Telonics, Inc., +Mesa, AZ, U.S.A.) into abdominal cavities of two female and two male +raccoons. Transmitter pulse periods were monitored with a digital +processor (Telonics TDP-2) coupled to a receiver (Telonics +TR-2-164/166). During some metabolic measurements, body temperatures +of these animals were recorded to the nearest 0.1 deg.C at 30-minute +intervals. The daily cycle of body temperature of these raccoons also +was measured once a month. + + + +CALIBRATIONS + + +_Calorimeter_ + +At the conclusion of these experiments, the accuracy of our +calorimetry apparatus was tested by burning an ethanol lamp in the +metabolism chamber. During these tests a CO_{2} analyzer was +incorporated into the system (Beckman, LB-2). Results demonstrated +that we measured 84% of the oxygen consumed by the lamp as well as 84% +of the water and CO_{2} it produced; standard deviation = +-2.6, +-5.0, +and +-3.6, respectively (n = 27). Average respiratory quotient (RQ) +calculated from these data was O.657 +-0.008 (n = 27), which is 99.5% +of that predicted (0.66). McNab (1988b) reports that the accuracy of +open-flow indirect calorimetry systems, such as ours, depends on the +rate of air flow through the animal chamber. If flow rates are too +low, there is inadequate mixing of air within the chamber, and the +rate of oxygen consumption, as calculated from the difference in +oxygen content of air flowing into and out of the chamber (Depocas and +Hart, 1957), is underestimated. At some critical rate of air flow, +which is unique to each combination of chamber and animal, this +situation changes such that measured rates of oxygen consumption +become independent of any further increase in flow rate (McNab, +1988b). In recent tests of our system, where we burned the ethanol +lamp at a variety of chamber flow rates, the efficiency of +measurement increased linearly as flow rate increased, and the +critical rate of air flow was about 6.7 L/min. This appeared to +explain why a flow rate of 3.0 L/min underestimated oxygen consumption +of the ethanol lamp. + +Our earlier tests of the efficiency of our system indicated that +although we underestimated actual oxygen consumption of the ethanol +lamp, we did so with a fair degree of precision; probably because flow +rates were closely controlled. During our metabolic measurements, +chamber flow rates also were closely controlled at 3.0 L/min, and we +believe, therefore, that these measurements also were carried out with +a high degree of precision. Consequently, all measured values of +oxygen consumption and water production were considered to be 84% of +their actual value and were adjusted to 100% before being included in +this report. + + +_Body Temperature Transmitters_ + +The calibration of all temperature-sensitive radio transmitters +drifted over time. Transmitters were calibrated before they were +surgically implanted and again after they were removed from the +animals. Although the drift of each transmitter was unique, it was +also linear (S. Tomkiewicz, Telonics, Inc., pers. com.). All body +temperature measurements were corrected from timed extrapolations of +the difference between starting and ending calibrations. + + + +STATISTICAL METHODS + + +Values of oxygen consumption, evaporative water loss, and body +temperature were plotted as a function of chamber air temperature. +Linear regressions of oxygen consumption at temperatures below the +thermoneutral zone (T_{n}), and evaporative water loss at temperatures +above freezing, were determined with the SAS (1982) GLM procedure. +Lower critical temperature (T_{lc}) was determined graphically from +intersection of the line representing [.H]_{b} and the regression line +representing oxygen consumption below T_{n}. Slopes and intercepts of +regression lines, as well as other mean values, were compared with +_t_-tests (Statistical Analysis System, 1982; Ott, 1984:138-175). +Unless indicated otherwise, data are expressed as mean +- standard +deviation (s.d.). + + + +ESTIMATING INTRINSIC RATE OF NATURAL INCREASE + + +We employed the method first described by Cole (1954) to calculate +r_{max}: + + 1 = e^{-r_{max}} + b.e^{-r_{max}(a)} - b.e^{r_{max}(n+1)} Eq. 2 + +where a is potential age of females first producing young, b is +potential annual birth rate of female young, and n is potential age of +females producing their final young. After life-history data were +substituted into Eq. 2, r_{max} was determined by trial and error +substitution (Hennemann, 1983). + +Because r_{max} represents the genetically fixed, physiologically +determined maximum possible rate of increase, data on earliest +possible age of female reproduction, highest possible birth rate of +female young, and longest possible female reproductive life span were +used for a, b, and n, respectively. Calculated values, therefore, +represent physiologically possible, not ecologically possible, +intrinsic rates of increase (Hennemann, 1983, 1984; Hayssen, 1984; +McNab, 1984b). Values of n were derived from longevity records for +captive animals, and as these were all large values of similar +duration (14-16 years), they had very little effect on r_{max}. All +species considered have one litter per year, and because their sex +ratios at birth are about 50:50, variation in b was due to differences +in litter size. Therefore, age of first reproduction and litter size +had the greatest effect on r_{max}. Intrinsic rate of increase scales +to body mass (Fenchel, 1974), and we removed this effect by comparing +each calculated r_{max} with the value expected (r_{maxe}) on the +basis of body mass (Hennemann, 1983). + + + +COMPARISON OF ADAPTIVE UNITS + + +Dimensionless numbers for each of the four variables used in +calculating composite scores were derived as follows. Ratios of +measured to predicted values were used for basal metabolism (H_{br}) +and minimum wet thermal conductance (C_{mwr}). Thermoregulatory +ability at low temperatures is closely related to the ratio +H_{br}/C_{mwr} (McNab, 1966). This ratio was used, therefore, to gauge +each species' cold tolerance. For D_{d} we used the ratio of food +categories actually used by a species to the total number of food +categories taken by all species tested (D_{dr}). The ratio of +calculated to expected intrinsic rates of natural increase was used to +derive r_{maxr}. Composite scores were calculated as + + Composite score = [(H_{br}/C_{mwr}) + D_{dr} + r_{maxr}]/3 Eq. 3 + +The correlation between number of climates these species occupy and +their composite scores was tested by linear regression. + + + + +$Results$ + + + +BODY MASS + + +According to monthly live-trapping records, the body mass of +free-ranging female raccoons increased from 3.6 +-0.6 kg during summer +to 5.6 +-0.8 kg in early winter, and the mass of free-ranging males +increased from 4.0 +-0.5 to 6.7 +-0.9 kg during the same interval. These +seasonal changes in body mass were due to fluctuations in the amount +of body fat and represent a mechanism for storing energy during fall +for use in winter. In summer, captive and trapped male and captive +female raccoons had the same body mass (4.73 +-0.61, 4.41 +-0.70, and +4.67 +-0.88 kg, respectively, Table 2). Mass of captive females did +not change between seasons, whereas captive males were heavier in +winter than summer (p<0.005; Table 2). This seasonal change in mass of +our captive males was of a much smaller magnitude (0.6 kg) than that +observed for wild males (2.7 kg). During winter, captive males (5.34 ++-1.39 kg) were heavier than captive females (4.49 +-0.98 kg; p<0.005; +Table 2). Thus, our captive animals maintained a body mass throughout +the year that was intermediate to the range of values found for wild +raccoons in the same area. + + + TABLE 2.--Body mass in kg and basal metabolism + (mL O_{2}.kg^{-0.75}.h^{-1}) of _Procyon lotor_ + in summer and winter (s.d. = standard deviation + and n = number of observations). + + ----------------+----------------------------------------------------- + Season and sex | Body mass, +-s.d., (n) Basal metabolism, +-s.d., (n) + ----------------+----------------------------------------------------- + Summer | + Trapped male | 4.41 +-0.70 (52) 780 +-112 (20) + Captive male | 4.73 +-0.61 (22) 680 +-102 (8) + Captive female| 4.67 +-0.88 (41) 618 +- 92 (13) + Winter | + Captive male | 5.34 +-1.39 (31) 704 +- 81 (19) + Captive female| 4.49 +-0.98 (42) 667 +-139 (25) + ----------------+----------------------------------------------------- + + + +BASAL METABOLIC RATE + + +Within thermoneutrality, [.H]_{b} (mL O_{2}.g^{-1}.h^{-1}) was +0.54 +-0.09 for trapped males in summer, 0.46 +-0.07 for captive males +in summer, 0.42 +-0.07 for captive females in summer, 0.47 +-0.06 for +captive males in winter, and 0.46 +-0.10 for captive females in winter +(Figures 2, 3). Ratios of these measured values to those predicted by +the Kleiber (1932, 1961:206) equation are 1.28, 1.12, 1.02, 1.17, and +1.09, respectively. To minimize the effect of body size (Mellen, 1963) +and to facilitate comparisons between sexes and seasons and between +captive and trapped animals, basal metabolism also was calculated as a +function of metabolic body size (mL O_{2}.kg^{-0.75}.h^{-1}; Table 2). +Based on this analysis, trapped summer males had a higher basal +metabolism than captive males (p<0.025) or females (p<0.005) in either +season (Table 2). There was no difference in basal metabolism between +captive males and females in either summer or winter, and there was no +seasonal difference in their basal metabolic rates (Table 2). + + + +MINIMUM THERMAL CONDUCTANCE + + +Minimum wet and dry thermal conductances were calculated using Eqs. 4 +and 5 + + C_{mw} = [.H]_{r} / (T_{b} - T_{a}) Eq. 4 + + C_{md} = ([.H]_{r} - [.E]_{eq}) / (T_{b} - T_{a}) Eq. 5 + +where C_{mw} is wet and C_{md} is dry conductance +(mL O_{2}.g^{-1}.h^{-1}. deg.C^{-1}); [.H]_{r} is the lowest resting +metabolic rate measured at each temperature (mL O_{2}.g^{-1}.h^{-1}); +[.E]_{eq} is oxygen equivalent for heat lost by evaporation +[[.E]_{eq} = mL O_{2}.g^{-1}.h^{-1} = [.E].[lambda]/[gamma], where +[.E] is evaporative water loss (mg.g^{-1}.h^{-1}), [lambda] is heat of +vaporization for water (2.43 J/mg), and [gamma] is heat equivalent for +oxygen (20.097 J/mL)]; T_{b} is body temperature ( deg.C); and T_{a} is +chamber air temperature ( deg.C). Only data from animals equipped with +temperature-sensitive radio transmitters were used for these +calculations. + + + TABLE 3.--Minimum wet and dry thermal conductances + (mL O_{2}.g^{-1}.h^{-1}. deg.C^{-1}) of _Procyon lotor_ in summer + and winter. Means of values were calculated from equations 3 + and 4 (s.d. = standard deviation and n = number of observations). + + ----------------------+---------------------------------------- + | Thermal conductance + Season and sex |---------------------------------------- + | Wet +-s.d. (n) Dry +-s.d. (n) + ----------------------+---------------------------------------- + Summer | + Captive, both sexes | 0.0256 +-0.0028 (18) 0.0246 +-0.0019 (12) + Winter | + Captive, female | 0.0172 +-0.0023 (10) 0.0161 +-0.0027 (6) + ----------------------+---------------------------------------- + + + [Illustration: FIGURE 2.--Relationship between oxygen consumption + and chamber air temperature for raccoons in summer: captive + females, open circles; captive males, closed circles; trapped + males, open squares. Sloping lines represent regressions of oxygen + consumption on chamber air temperature, and horizontal lines, basal + metabolism.] + + [Illustration: FIGURE 3.--Relationship between oxygen consumption + and chamber air temperature for raccoons in winter: captive + females, open circles; captive males, closed circles. Solid sloping + line represents regression of oxygen consumption on chamber air + temperature for males and females, and the horizontal line, basal + metabolism for males and females.] + +C_{mw} was calculated for each season from metabolic measurements made +at all air temperatures below T_{lc} (Table 3). Because evaporative +water loss was not measured at temperatures below freezing, C_{md} +was calculated only from metabolic determinations made at air +temperatures between T_{lc} and 0 deg.C. There was no difference +between males and females in summer for either C_{mw} or C_{md} +(mL O_{2}.g^{-1}.h^{-1}. deg.C^{-1}). Data for each sex were combined +to give a summer average of 0.0256 +-0.0028 for C_{mw}, and 0.0246 ++-0.0019 for C_{md} (Table 3). These summer conductances were 49% higher +(p<0.005) than those calculated for winter females (0.0172 +-0.0023, and +0.0161 +-0.0027 for C_{mw} and C_{md}, respectively; Table 3). C_{mw} and +C_{md} were not different from each other in either summer or winter, +which indicated that in both seasons evaporative water loss contributed +very little to heat dissipation at temperatures below T_{n}. Comparisons +of thermal conductances calculated on the basis of metabolic body size +(Mellen, 1963) gave the same results. + + + +EVAPORATIVE WATER LOSS + + +Evaporative water loss increased as chamber temperature increased in +both summer and winter (Figures 4, 5). In summer, the pattern of +increase was different for females and males. Polynomial regressions +for trapped and captive males produced equations that describe a +concave relationship between T_{a} and evaporative water loss, whereas +the equation for females describes a sigmoid curve (Table 4; Figure 4). +For females, water loss increased rapidly at temperatures above 25 deg.C +(Figure 4). The intercepts and coefficients of the X, X squared, and X cubed terms +of the polynomial regression equations (Table 4) were compared +(_t_-tests) to determine if they differed from each other. The +coefficients in the equation for trapped males differed from those for +captive females in the X squared (p<0.05) and X cubed (p<0.025) terms. The +intercept and coefficients of the equation for captive males, however, +were not different from those for either captive females or trapped +males. Although this lack of difference is understandable in the case +of trapped males, where the shape of the two curves is similar +(concave), it is not so clear for the sigmoid curve of captive females +(Figure 4). Perhaps the lack of difference in this case is simply due +to the small number of observations available for captive males (n = 10; +Table 4). Nonetheless, in summer at 35 deg.C, both captive and trapped +males relied less on evaporative cooling than did captive females +(Figure 4). + +In winter, males and females had similar rates of evaporative water +loss across the full range of temperatures tested (Figure 5). +Therefore, data for both sexes were combined. The intercept and +coefficients of this equation (Table 4) did not differ from those for +summer females, but they did differ from those in the regression for +trapped males in the X squared (p<0.05) and X cubed (p<0.025) terms. As was +the case for females in summer, rates of water loss for winter animals +increased most rapidly at temperatures above 25 deg.C (Figure 5). + + [Illustration: FIGURE 4.--Relationship between evaporative water + loss and chamber air temperature for raccoons in summer: captive + females, open circles; captive males, closed circles; trapped + males, open squares. Lines represent polynomial regressions of + evaporative water loss on chamber air temperature.] + + [Illustration: FIGURE 5.--Relationship between evaporative water + loss and chamber air temperature for raccoons in winter: captive + females, open circles; captive males, closed circles. Lines + represent polynomial regressions of evaporative water loss on + chamber air temperature.] + + + TABLE 4.--Polynomial regression equations describing evaporative + water loss (mg.g^{-1}.h^{-1}) of _Procyon lotor_ in summer and + winter (X = chamber temperature ( deg.C), Y = evaporative water loss, + n = number of observations, R squared = coefficient of determination, and + SEE = standard error of estimate). + + --------------+-------------------------------------------------------- + Season and sex| Equation (n) R squared + --------------+-------------------------------------------------------- + Summer | + Trapped male |Y = 0.1899 + 0.0114.X + 0.0011.X squared - 0.00002.X cubed (32) 0.86 + SEE | 0.0885 0.0223 0.0015 0.00003 + Captive male |Y = 0.2174 + 0.0192.X + 0.0009.X squared - 0.00003.X cubed (10) 0.73 + SEE | 0.3983 0.0834 0.0048 0.00008 + Captive | + female |Y = 0.0127 + 0.0943.X - 0.0060.X squared + 0.00013.X cubed (31) 0.64 + SEE | 0.2218 0.0547 0.0036 0.00006 + Winter | + Captive, | + both sexes |Y = 0.1550 + 0.0426.X - 0.0025.X squared + 0.00006.X cubed (57) 0.80 + SEE | 0.0734 0.0192 0.0013 0.00002 + --------------+-------------------------------------------------------- + + + +THERMOREGULATION AT LOW TEMPERATURES + + +_Body Temperature_ + +Body temperatures in Figure 6 are those recorded during metabolic +measurements from animals equipped with surgically implanted, +temperature-sensitive radio transmitters. Each point was recorded +during the lowest level of oxygen consumption at each T_{a}. In both +summer and winter, T_{b}'s were lowest during metabolic measurements +at T_{a}'s around T_{lc}. At T_{a}'s below T_{lc}, T_{b}'s increased +(Figure 6), which is an unusual response. Under similar conditions, +other procyonids either maintain a nearly constant T_{b} or allow it +to fall slightly (Mueller and Kulzer, 1977; Chevillard-Hugot et al., +1980; Mueller and Rost, 1983; Chevalier, 1985). For our raccoons, +confinement in the metabolism chamber at low temperatures must have +stimulated a greater than necessary increase in metabolic rate such +that heat production exceeded heat loss, which caused T_{b} to become +elevated. + + [Illustration: FIGURE 6.--Relationship between body temperature and + chamber air temperature in summer (panel A), and winter (panel B): + captive females, open circles and solid lines; captive males, solid + circles and dashed lines. Solid vertical lines represent lower + critical temperatures.] + + + TABLE 5.--Regression equations describing oxygen consumption + (mL O_{2}.g^{-1}.h^{-1}) of _Procyon lotor_ at temperatures below + their lower critical temperature (I = x-intercept ( deg.C), n = number + of observations, R squared = coefficient of determination, SEE = standard + error of estimate for the y-intercept (a) and slope (b), X = chamber + temperature ( deg.C), and Y = oxygen consumption). + + ----------------+------------------------------------------------------ + Season | SEE + and sex | ----------- + | Equation (n) R squared a b I + ----------------+------------------------------------------------------ + Summer | + Trapped male | Y = 1.09 - 0.0281.X (30) 0.64 0.0353 0.0040 38.8 + Captive male | Y = 0.97 - 0.0258.X (12) 0.91 0.0235 0.0025 37.6 + Captive female| Y = 1.04 - 0.0251.X (29) 0.78 0.0288 0.0026 41.1 + Winter | + Captive, | + both sexes | Y = 0.68 - 0.0193.X (36) 0.68 0.0157 0.0023 35.2 + ----------------+------------------------------------------------------ + + +_Summer_ + +During summer, T_{lc} for male raccoons was 20 deg.C, whereas for females +it was 25 deg.C (Figure 2). Regression equations calculated to describe +oxygen consumption at T_{a}'s below T_{lc} are presented in Table 5. +For three groups of summer animals, slopes of regressions are +identical. This indicates that minimum conductances of these three +groups were equivalent. Intercepts of these equations are different, +which suggests a difference in metabolic cost of thermoregulation +between these groups (Figure 2); captive males had a lower intercept +than either trapped males (p<0.005) or captive females (p<0.05), but +there was no difference in intercepts of captive females and trapped +males. These regression equations, therefore, also were derived using +values of oxygen consumption expressed in terms of metabolic body mass +(Mellen, 1963). Relationships between intercepts of these equations +are different than those for regressions in Table 5. Intercept for +females was intermediate to, and not different from, those of the two +groups of males. However, captive males still had a lower intercept +than trapped males (p<0.025). Thus, in summer, thermoregulatory +metabolism was less expensive for captive than for trapped males, and +in spite of a 5 deg.C difference in their T_{lc}'s (Figure 2), captive +males and females had similar thermoregulatory costs. + +Regression lines for three groups of animals in summer extrapolate to +zero metabolism at values equivalent to, or greater than, normal +T_{b}; 38.8 deg.C for trapped males, 37.6 deg.C for captive males, and 41.1 deg.C +for captive females (Table 5). Thus, all three groups had minimized +thermal conductance at T_{a}'s below T_{lc} (Scholander et al., 1950b; +McNab, 1980b). Minimum wet thermal conductance calculated for raccoons +in summer with Eq. 4 (Table 3) is numerically similar to these "slope" +values (Table 5), and it was, therefore, considered to be the best +estimate of C_{mw} for _Procyon lotor_ during that season +(0.0256 mL O_{2}.g^{-1}.h^{-1}. deg.C^{-1}). + + +_Winter_ + +During winter T_{lc} for both sexes decreased to 11 deg.C (Figure 3). +Regression equations of thermoregulatory metabolism for males and +females in winter are not different from each other in either slope or +intercept. These data, therefore, were combined into a single equation +(Table 5). Slope and intercept of this equation are both lower +(p<0.005 and p<0.05, respectively) than those for summer animals +(Table 5). Identical results were obtained from comparisons using +regressions derived from oxygen consumption expressed in terms of +metabolic body mass (Mellen, 1963). Thermoregulatory costs at any +temperature below 20 deg.C were lower for winter than summer animals +(Figures 2, 3). + + + TABLE 6.--Regression equations describing oxygen consumption + (mL O_{2}.g^{-1}.h^{-1}) of _Procyon lotor_ at temperatures below + their lower critical temperature in winter (A = females with radio + transmitters, B = females without radio transmitters, C = males, + I = x-intercept ( deg.C), n = number of observations, R squared = coefficient + of determination, X = chamber temperature ( deg.C), and Y = oxygen + consumption). + + -----+-------------------------------------- + Group| Equation (n) R squared I + -----+-------------------------------------- + A | Y = 0.63 - 0.0158.X (10) 0.66 40.1 + B | Y = 0.72 - 0.0226.X (11) 0.71 32.1 + C | Y = 0.69 - 0.0200.X (15) 0.79 34.7 + -----+-------------------------------------- + + + [Illustration: FIGURE 7.--Relationship between body temperature and + time of day at various months of the year: captive females, open + circles; captive males, closed circles. Vertical cross-hatched + areas represent civil twilight.] + +The regression line for _Procyon lotor_ in winter (Table 5) +extrapolates to zero metabolism at 35.2 deg.C, which is below normal T_{b} +(Figures 6, 7). This suggests that not all raccoons measured in winter +minimized thermoregulatory metabolism or conductances at T_{a}'s below +T_{lc} (Scholander et al., 1950b; McNab, 1980b). To assess this +possibility, data for these animals were divided into three groups: +(A) females with radio transmitters, (B) females without radio +transmitters, and (C) males (Table 6). Regression equations of +metabolism below T_{lc} were derived for each group, and based on +extrapolated T_{b}'s at zero metabolism, only the two females with +implanted radio transmitters (group A) minimized thermoregulatory +metabolism and conductance. Had animals in groups B and C also +minimized their thermal conductances, while retaining their measured +metabolic rates, their rates of heat production would have been +disproportionately higher than their rates of heat loss. Equation 4 +predicts that under these conditions their body temperatures would +have been elevated to 42.0 deg.C and 40.4 deg.C, respectively. Thus, in order +to avoid such a large increase in body temperature, animals in groups +B and C increased their thermal conductances in preference to lowering +their metabolic rates. The regression equation of thermoregulatory +metabolism for all winter animals (Table 5), therefore, overestimates +minimum metabolic cost of temperature regulation below T_{lc}, and its +slope underestimates C_{mw}. Consequently, the best estimate of C_{mw} +for _Procyon lotor_ in winter is the value calculated for group A +animals with Eq. 4 (0.0172 mL O_{2}.g^{-1}.h^{-1}. deg.C^{-1}; Table 3), +and the minimum cost of thermoregulatory metabolism at any T_{a} below +T_{lc} is best estimated by substituting this value into Eq. 4 and +solving for [.H]_{r}. + + + +THERMOREGULATION AT HIGH TEMPERATURES + + +_Body Temperature_ + +In both summer and winter, T_{b}'s increased during metabolic +measurements at T_{a}'s above T_{lc} (Figure 6). This response also +was seen during metabolic measurements conducted on other procyonids +(Mueller and Kulzer, 1977; Chevillard-Hugot et al., 1980; Mueller and +Rost, 1983; Chevalier, 1985). + + +_Summer_ + +During summer our data suggested that the upper critical temperature +(T_{uc}) was higher than 35 deg.C. The lowest rates of oxygen consumption +at T_{a} = 35 deg.C occurred after 1.5 to 2.5 hours of exposure to that +temperature. Prolonged exposure to this temperature in summer did not +make animals restless, and their rate of oxygen consumption was very +stable throughout each measurement. Body temperature responses at +T_{a} = 35 deg.C were recorded from two males and two females that had +implanted radio transmitters. With the exception of one male, T_{b}'s +were maintained near 38 deg.C (Figure 6). The one exception (a male) +maintained its T_{b} at 39.3 deg.C. At T_{a} = 35 deg.C, summer males had +rates of evaporative water loss that were lower than those of summer +females (Figure 4). At this temperature, males dissipated 35% +-6% and +females 56% +-18% of their metabolic heat via evaporative water loss. +Thus, at T_{a} = 35 deg.C, males must have utilized modes of heat transfer +other than evaporative cooling (convective and conductive heat +transfer) to a greater extent than females. + + +_Winter_ + +Body temperature, evaporative water loss, and metabolic data indicated +that, in winter, T_{uc} was very close to 35 deg.C. In winter, the lowest +level of oxygen consumption was recorded during the first hour after +the chamber had reached T_{a} = 35 deg.C. Unlike summer, animals became +restless after the first hour at 35 deg.C, at which point their oxygen +consumption increased and showed a high degree of variability. Body +temperature responses at 35 deg.C were recorded from both females that had +implanted radio transmitters. In one case, T_{b} rose from 37.9 deg.C at +the end of the first hour to 40.5 deg.C by the end of the second hour, and +as it did not show signs of leveling off, we terminated the +experiment. We exposed that same animal to T_{a} = 35 deg.C one other time +during winter. In that instance, its T_{b} rose to 40.0 deg.C during the +first 30 minutes and was maintained at that level for three hours with +no apparent distress. The other female elevated its T_{b} from 37.3 deg.C +to 39.0 deg.C during the second hour at T_{a} = 35 deg.C and maintained its +T_{b} at that level for two hours. Thus, during winter, prolonged +exposure to T_{a} = 35 deg.C stimulated more of an increase in T_{b} than +it did in summer. During winter, both males and females increased +evaporative water loss at T_{a} = 35 deg.C (Figure 5) but only to the +extent that they dissipated 35% +-10% of their metabolic heat +production. Thus, even in winter, convective and conductive heat +transfers were still the most important modes of heat loss at this +temperature. + + + +DAILY CYCLE OF BODY TEMPERATURE + + +The daily cycle of raccoon T_{b}'s during summer and winter are +presented in Figure 7. In general, T_{b}'s showed a marked circadian +cycle in phase with photoperiod. T_{b}'s rose above 38 deg.C for several +hours each night but remained below 38 deg.C during daytime. During +summer, with the exception of one female whose record was not typical +(Figure 7), T_{b}'s rose above 38 deg.C shortly after sunset, whereas in +winter T_{b}'s did not rise above 38 deg.C until several hours after +sunset. Once T_{b} was elevated it usually remained so until just +before or after sunrise (Figure 7). During summer, T_{b} was above +38 deg.C for 85% or more of the time between sunset and sunrise (87% for +the female with the typical body temperature pattern, and 85% and 98% +for males), whereas in winter it was elevated for only 47%-78% of the +time between sunset and sunrise (47% and 61% for females, and 67% and +78% for males). During night, T_{b} would oscillate between 38 deg.C and +about 39 deg.C, such that two peak values occurred. These peak values +presumably corresponded to two periods of heightened nighttime +activity. During summer, one of these peaks occurred before and the +other after 24:00 hours, whereas in winter both peaks occurred after +24:00 hours. With the exception of one female in winter (Figure 7), +the lowest T_{b} of the day for both sexes was near 37 deg.C, and this +typically occurred during daytime (Figure 7). + + + + +$Discussion$ + + + +BASAL METABOLIC RATE + + +_Background_ + +Basal metabolism represents the minimum energy required by a mammal to +maintain endothermy and basic homeostasis (Lusk, 1917:141; Kleiber, +1932, 1961:251; Benedict, 1938:191-215; Brody, 1945:59; Robbins, +1983:105-111). Mammals with lower than predicted [.H]_{b} maintain +endothermy and enjoy its attendant advantages at a discount, whereas +others, with rates that are higher than predicted, pay a premium +(Calder, 1987). Such variation in [.H]_{b} appears to be tied to +ecological circumstances rather than taxonomic affinities (Vogel, +1980; McNab, 1986a, 1988a, 1989), and depending on environmental +conditions, each rate provides an individual with various advantages +and limitations. During the course of evolution, therefore, each +species' [.H]_{b} evolves to provide it with the best match between +its energy requirements for continuous endothermy, its food supply, +and the thermal characteristics of its environment. + + +_Captive versus Wild Raccoons_ + +Male raccoons trapped in summer had higher [.H]_{b}'s than our captive +animals in any season (Table 2). The higher rate of metabolism of +these trapped males could have been due to the stress of captivity or +to the fact that "wild" animals actually may have higher metabolic +rates than those that have adjusted to captivity. If the latter is +true, then our data for captive animals underestimated the actual +energy cost of maintenance metabolism for _Procyon lotor_ in the wild. +At present, we have no way of determining which of these alternatives +is true. + + +_Seasonal Metabolism of Raccoons_ + +In some temperate-zone mammals, [.H]_{b} is elevated in winter, which +presumably increases their "cold-hardiness." Conversely, lower summer +metabolism is considered to be a mechanism that reduces the potential +for heat stress. Such seasonal variation in [.H]_{b} has been found in +several species: collard peccary, _Tayassu tajacu_ (Zervanos, 1975); +antelope jackrabbit, _Lepus alleni_ (Hinds, 1977); desert cottontail, +_Sylvilagus audubonii_ (Hinds, 1973); and, perhaps, cold-acclimatized +rat, _Rattus norvegicus_ (Hart and Heroux, 1963). Unlike these +species, our captive raccoons showed no seasonal variation in [.H]_{b} +(Table 2). Instead, raccoons achieved "cold-hardiness" in winter and +reduced their potential for heat stress in summer with a large +seasonal change in thermal conductance (Table 3). + + + TABLE 7.--Metabolic characteristics of several procyonid species. + + ---------------------+------------------------------------------------ + |Body Basal[a] Minimum[b] + Species |mass metabolism conductance T_{b}[c] + |(g) ------------ ------------- ------------- + | Meas H_{br} Meas C_{mwr} [alpha] [rho] + ---------------------+------------------------------------------------ + _Bassariscus astutus_| 865 0.43 0.68 0.0288[e] 0.85 37.6 23 + _Procyon cancrivorus_|1160 0.40 0.69 0.0368[e] 1.25 + _Potos flavus_ |2030 0.36 0.51 + _Potos flavus_ |2400 0.32 0.65 38.1 36.0 + _Potos flavus_ |2600 0.34 0.71 0.0200[f] 1.02 + _Nasua nasua_ |3850 0.26 0.60 0.0200[f] 1.24 38.3 36.4 + _Nasua nasua_ |4847 0.33 0.79 0.0238[e] 1.65 39.1 37.9 + _Nasua narica_ |5554 0.25 0.62 0.0208[e] 1.55 38.9 37.4 + _Nasua narica_ |4150 0.42 1.20 0.0341[e] 2.20 + | 0.0224[g] 1.45 + _Procyon lotor_ | + Summer | + Trapped male |4400 0.54 1.28 + Captive male |4790 0.46 1.07 0.0256[f] 1.77 38.4 37.5 + Captive female |4670 0.42 1.02 0.0256[f] 1.79 38.2 37.6 + Winter | + Captive male |5340 0.47 1.17 38.6 37.6 + Captive female |4490 0.46 1.10 0.0172[f] 1.15 38.3 37.3 + ---------------------+------------------------------------------------ + + ---------------------+----------------------------------------------- + | + Species | T_{n}[d] + |--------------- + | T_{lc} T_{uc} References + ---------------------+----------------------------------------------- + _Bassariscus astutus_| 35.5 Chevalier (1985) + _Procyon cancrivorus_| 26 Scholander et al. (1950b, c) + _Potos flavus_ | McNab (1978a) + _Potos flavus_ | 23 30 Mueller and Kulzer (1977) + _Potos flavus_ | 23 33 Mueller and Rost (1983) + _Nasua nasua_ | 25 33 Chevillard-Hugot et al. (1980) + _Nasua nasua_ | 30 35 Mugaas et al. (in prep.) + _Nasua narica_ | 25 35 + _Nasua narica_ | Scholander et al. (1950b, c) + | + _Procyon lotor_ | This study + Summer | + Trapped male | 20 + Captive male | 20 + Captive female | 25 + Winter | + Captive male | 11 + Captive female | 11 + ---------------------+----------------------------------------------- + + [a] Meas is measured basal metabolism (mL O_{2}.g^{-1}.h^{-1}). H_{br} + is the ratio of measured to predicted basal metabolism where the + predicted value is calculated from [.H]_{b} = 3.42.m^{-.25} + (Kleiber, 1932, 1961:206) and m is body mass in grams. + + [b] Meas is measured minimum thermal conductance + (mL O_{2}.g^{-1}.h^{-1}. deg.C^{-1}). C_{mwr} is the ratio of measured + to predicted minimum thermal conductance where the predicted value + is calculated from C_{m} = 1.0.m^{-0.5} (McNab and Morrison, 1963; + Herreid and Kessel, 1967), and m is body mass in grams. + + [c] T_{b} is body temperature during the active ([alpha]) and rest + ([rho]) phases of the daily cycle ( deg.C). + + [d] T_{n} is the thermoneutral zone as defined by the lower (T_{lc}) + and upper (T_{uc}) critical temperatures ( deg.C). + + [e] Conductance calculated as the slope of the line describing oxygen + consumption at temperatures below the lower critical temperature. + + [f] Conductance calculated from C_{mw} = [.H]_{r}/(T_{b} - T_{a}), + where [.H]_{r} is resting metabolic rate at temperatures below + T_{lc}, and other symbols are as described elsewhere. + + [g] Inactive-phase thermal conductance: estimated from Scholander et + al. (1950b), assuming that active-phase thermal conductance is 52% + higher than values determined during the inactive phase (Aschoff, + 1981). + + +_Comparison of Procyon lotor with Other Procyonids_ + +_Procyon lotor_ has a much higher mass-specific [.H]_{b} than other +procyonids (Table 7). To quantify the magnitude of this difference, +we compared the measured value for _Procyon lotor_ with one calculated +for it from a mass-specific least-squares regression equation (Eq. 6; +R squared = 0.78) derived from data for those procyonids with lower than +predicted [.H]_{b}: _Potos flavus_, _Procyon cancrivorus_, +_Nasua nasua_, _Nasua narica_, and _Bassariscus astutus_ (Table 7). + + [.H]_{b} = 2.39.m^{-0.25} Eq. 6 + +[.H]_{b} in Eq. 6 is basal metabolism (mL O_{2}.g^{-1}.h^{-1}) and m +is body mass (g). Measured values of [.H]_{b} for _Procyon lotor_ were +1.45 to 1.86 times greater than those predicted for it by Eq. 6 +(Table 8). + + + TABLE 8.--Basal metabolism (mL O_{2}.g^{-1}.h^{-1}) of _Procyon + lotor_ as predicted by Eq. 6 ([.H]_{b} = 2.39.m^{-0.25}). Body + masses, used to calculate predicted values, and measured values + were taken from Table 7. + + ----------------+-------------------------------- + Season and sex | Predicted Measured/Predicted + ----------------+-------------------------------- + Summer | + Trapped male | 0.29 1.86 + Captive male | 0.29 1.59 + Captive female| 0.29 1.45 + Winter | + Captive male | 0.28 1.68 + Captive female| 0.29 1.59 + ----------------+-------------------------------- + + +_Influence of Diet on Basal Metabolism_ + +BACKGROUND.--With respect to [.H]_{b}, McNab (1986a:1) maintains that +"the influence of climate is confounded with the influence of food +habits," and that departures from the Kleiber (1961) "norm" are best +correlated with diet. Although this does appear to be the case for +diet specialists, the analysis is not so clear-cut for omnivorous +species (McNab, 1986a). His analysis also indicates that an animal's +"behavior" (i.e., whether it is terrestrial, arboreal, subterranean, +aquatic, etc.), secondarily modifies the influence of food habits on +[.H]_{b}. For example, terrestrial frugivores have [.H]_{b}'s that are +very near predicted values, whereas arboreal frugivores have rates +that are much lower than predicted (McNab, 1986a). + + + TABLE 9.--Food habits of some Procyonids. References for foods were + as follows: _Potos flavus_, _Procyon cancrivorus_, and _Nasua nasua_ + taken from Bisbal (1986); _Nasua narica_ taken from Kaufmann + (1962:182-198); _Bassariscus astutus_ taken from Martin et al. + (1951), Taylor (1954), Wood (1954), Toweill and Teer (1977), and + Trapp (1978); _Procyon lotor_ taken from Hamilton (1936), Stuewer + (1943:218-220), Stains (1956:39-51), and Greenwood (1981). Symbols + represent either qualitative (#) or quantitative (+,|) assessments + of feeding habits: # indicates that the animal was observed eating + the food; + and | represent volume and frequency, respectively, of + food utilization. No attempt was made to account for seasonal + variation in the use of these foods. + + + <20% by volume when found. | 1%-19% frequency of occurrence. + ++ >20% by volume when found. || 20%-50% frequency of occurrence. + ||| >50% frequency of occurrence. + + -----------+-------------------------------------------------------------- + |_Potos_ _Procyon_ _Nasua_ _Nasua_ _Bassariscus_ _Procyon_ + Food |_flavus_ _cancrivorus_ _nasua_ _narica_ _astutus_ _lotor_ + -----------+-------------------------------------------------------------- + Mammalia | + | # ++ ||| ++ || + Aves | ++ | + || + Birds' eggs| ||| + Reptilia | + | + ||| # + | + | + Amphibia | + | # + | + Pices | ++ || ++ || + Insecta |++ | + ||| ++ ||| # + || ++ || + Arachnida | ++ ||| # + | + | + Chilopoda | ++ ||| + Diplopoda | # + | + Crustacea | ++ ||| # ++ ||| + Mollusca | + || # + || + Annelida | # + | + Nuts | ++ || + Grains | ++ || + Buds | + | + Fruit |++ ||| ++ # || ++ ||| + Leaves | + | + Grass | + | + -----------+-------------------------------------------------------------- + + +FOOD HABITS OF PROCYONIDS.--Food habits of six procyonids for which +metabolic data are available are presented in Table 9. All six species +clearly have mixed diets. Compared to other species, _Procyon lotor_ +is highly catholic in its diet, taking food from almost twice as +many categories as _Nasua narica_, three times as many as _Procyon +cancrivorus_, _Nasua nasua_, and _Bassariscus astutus_, and nine times +as many as _Potos flavus_. + +For those species for which food habit data are quantified, we used +Eisenberg's (1981:247-251) substrate/feeding matrix method, where +"substrate" is analogous to McNab's (1986a) "behavior," to construct +the following feeding categories that are based on the major food +groups utilized by each species (Table 9). + + 1. _Potos flavus:_ (1) arboreal/frugivore, insectivore. + + 2. _Procyon cancrivorus:_ (1) semiaquatic/crustacivore, + molluscivore, insectivore, piscivore, carnivore. + + 3. _Nasua nasua:_ (1) terrestrial/insectivore, arachnidivore, + carnivore, frugivore. + + 4. _Bassariscus astutus:_ (1) terrestrial/carnivore, insectivore, + frugivore. + + 5. _Procyon lotor:_ (1) terrestrial/carnivore, granivore, + frugivore, insectivore; and (2) semiaquatic/crustacivore, + molluscivore, insectivore, piscivore, carnivore. + +FOOD HABITS AND BASAL METABOLISM.--The most important foods in the +diet of _Procyon lotor_ are vertebrates, nuts, seeds, and fruits +(Table 9). These are the same foods that are eaten by those dietary +specialists that have [.H]_{b}'s equivalent to, or higher than, values +predicted for them by the Kleiber equation (McNab, 1986a). The most +important foods in the diets of _Potos flavus_, _Procyon cancrivorus_, +and _Nasua nasua_ are invertebrates and fruit (Table 9), and these +foods are eaten by dietary specialists that have lower than predicted +[.H]_{b}'s (McNab, 1986a). Major foods in the diet of _Bassariscus +astutus_ are terrestrial vertebrates, insects, and fruit (Table 9). +Dietary specialists that eat terrestrial vertebrates have higher than +predicted [.H]_{b}'s, whereas those that feed on insects have [.H]_{b}'s +that are lower than predicted (McNab, 1986a). Year-round utilization of +vertebrates by _Bassariscus astutus_ suggests that it also should have +a metabolic rate that is equivalent to or higher than predicted, rather +than lower (McNab, 1986a). However, perhaps year-round inclusion of +insects in its diet (Martin et al., 1951; Taylor, 1954; Wood, 1954; +Toweill and Teer, 1977; Trapp, 1978), plus water- and energy-conserving +advantages of a low metabolic rate, each exert a stronger selective +influence on [.H]_{b} than do vertebrates in its diet. + +SUMMARY.--The basal metabolic rate of these procyonids does appear to +be influenced by diet. But, it is apparent from this family's +evolutionary history and tropical origins that climate also has had a +profound influence on its member's metabolism. The history of the +family and the data presented here (Table 7) suggest that lower than +predicted [.H]_{b} is a feature that evolved very early as the primary +metabolic adjustment to a tropical climate. From this perspective, it +could be argued that climate would have been the major selective force +determining [.H]_{b}, whereas food habits would have had a secondary +influence. + + +_Basal Metabolism and Intrinsic Rate of Natural Increase_ + +BACKGROUND.--McNab (1980a) suggested that if food is not restricted +during an animal's reproductive period, the factor that will limit +growth and reproduction will be the rate at which energy can be used +in growth and development. Under these conditions, an increase in +[.H]_{b} would actually increase r_{max} because it would provide a +higher rate of biosynthesis, a faster growth rate, and a shorter +generation time. Hennemann (1983) tested McNab's (1980a) premise and +found a significant correlation between r_{max} and metabolic rate, +independent of body size, for 44 mammal species. A low correlation +coefficient for this relationship, however, indicated to him +(Hennemann, 1983) that factors such as (1) food supply, (2) thermal +characteristics of the environment, and (3) brain size also contribute +toward shaping a species' reproductive potential, particularly when +these factors strongly influence rates of biosynthesis or growth or +for some reason alter generation time. Results of our estimates of +r_{max} for procyonids are presented in Table 10. + +_Procyon lotor._--This species had the highest [.H]_{b} and D_{d}, and +also had the highest r_{max} (1.34; Table 10). Such a high r_{max} may +infer that this trait evolved under conditions where food and +temperature were not limiting to reproduction. Under these conditions +selection could have favored those reproductive characteristics +sensitive to a higher [.H]_{b} (biosynthesis, growth, and generation +time; McNab, 1980a). _Procyon lotor_'s high reproductive potential is +due to its early age of first female reproduction and its large litter +size, characteristics that may reflect metabolically driven increases +in both biosynthesis and growth. + +_Bassariscus astutus._--This species has a low [.H]_{b} but an r_{max} +that was 124% of expected (Table 10). This suggests that r_{max} +evolved under conditions where food and temperature were not +limiting to reproduction. Reduced litter size should restrict this +species' reproductive potential and may be a reflection of its low +[.H]_{b}. The factor that is responsible for increasing its +reproductive potential, however, is its early age of first female +reproduction. _Bassariscus astutus_ is the smallest of these +procyonids, and even though it has a low [.H]_{b}, its small mass +may contribute to its ability to reach adult size and sexual maturity +in its first year. The high quality of its diet (a high proportion of +small vertebrates; Table 9) also may be a factor that is permissive to +early female reproduction. Thus, small body size and diet may be +factors that have allowed this species to evolve a higher than expected +reproductive potential in spite of its low [.H]_{b}. + +_Nasua narica._--This species is one of the largest procyonids +(Table 7), and it possesses characteristics that should limit its +reproductive potential: lower than predicted [.H]_{b} (Table 7), a +relatively low-quality diet (Kaufmann, 1962:182-198; Table 9), and +delayed time of first reproduction (Table 10). In spite of this, +_Nasua narica_ has a higher than expected r_{max} (111% of predicted; +Table 10). The life history feature that enhances _Nasua narica_'s +reproductive potential, and increases r_{max} beyond expected, is its +large litter size. In this species females live in bands. Each year +just before their young are born these bands break up, and each female +seeks out a den for herself and her litter. Once the young are able to +leave the den (approximately five weeks), bands reform. In this +situation, females not only care for their own young but also for those +of other females in the band (Kaufmann, 1962:157-159, 1982, 1987; +Russell, 1983). This social structure may contribute to this species' +ability to produce large litters and in this way increase its +reproductive potential. + + + TABLE 10.--Intrinsic rate of natural increase (r_{max}) of several + procyonids. (a = potential age of females producing first young; + b = potential annual birth rate of female young (= average litter + size/2; average litter size was calculated from the published range + of litter sizes for each species); n = potential age of females + producing their final young; r_{maxe} = intrinsic rate of natural + increase expected from body mass (Hennemann, 1983); r_{maxr} = ratio + of calculated to expected intrinsic rate of natural increase + (r_{max}/r_{maxe}).) + + ---------------------+------------------------------------------------ + | + Species |Body mass a b n r r [a] r [b] + | (g) max maxe maxr + ---------------------+------------------------------------------------ + _Procyon lotor_ | 4940 0.83 2.25 16 1.34 0.53 2.52 + | + | + | + _Bassariscus astutus_| 900 0.83 1.50 14 1.02 0.82 1.24 + | + | + | + _Nasua narica_ | 3900 2.50 2.25 14 0.62 0.56 1.11 + | + _Nasua nasua_ | 3850 + _Procyon cancrivorus_| 1160 0.83 1.50 15 1.02[c] 0.77 1.32 + | 1.75 0.65[c] 0.84 + _Potos flavus_ | 2490 1.75 0.50 12 0.30 0.63 0.48 + | + _Bassaricyon gabbii_ | 1600 1.75 0.50 15 0.32 0.71 0.45 + | + ---------------------+------------------------------------------------ + + ---------------------+------------------------------------------------ + | + Species | References + ---------------------+------------------------------------------------ + _Procyon lotor_ | Dunn and Chapman (1983); Eisenberg (1981:489); + | Kaufmann (1987); Lotze and Anderson (1979); + | Nowak and Paradiso (1983:981); Sanderson + | (1987); Stains (1956:28-31); This study + _Bassariscus astutus_| Kaufmann (1982, 1987); Nowak and Paradiso + | (1983:979, 980); Poglayen-Neuwall and + | Poglayen-Neuwall (1980); Poglayen-Neuwall + | and Toweill (1988); Russell (1983) + _Nasua narica_ | Kaufmann (1982, 1987); Nowak and Paradiso + | (1983:983); Sanderson (1983) + _Nasua nasua_ | Chevillard-Hugot et al. (1980) + _Procyon cancrivorus_| Crandall (1964:312); Poglayen-Neuwall (1987) + | + _Potos flavus_ | Ford and Hoffmann (1988); Nowak and Paradiso + | (1983:984) + _Bassaricyon gabbii_ | Eisenberg (1981:489); Nowak and Paradiso + | (1983:985) + ---------------------+------------------------------------------------ + + [a] r_{maxe} = 4.9.m^{0.2622}, where m is body mass in grams. + + [b] Regression of r_{max} on body mass (m). Assume r_{max} = 1.02 for + _Procyon cancrivorus_: r_{max} = 0.00005.m + 0.623; R = 0.19; + R squared = 0.03; Regression of r_{maxr} (Table 10) on H_{br} (Table 7); + assume _Nasua nasua_ has the same r_{maxr} as _Nasua narica_: + r_{maxr} = 3.35.H_{br} - 1.11; R = 0.93; R squared = 0.86. + + [c] Estimate based on females reproducing in their first (a = 0.83) or + second (a = 1.75) year. + + +_Nasua nasua._--Unfortunately, there is not enough reproductive data +to allow calculation of r_{max} for _Nasua nasua_ (Table 10), therefore, +it is not possible to compare the reproductive potential of this South +American coati with its North American relative, _Nasua narica_. Given +its low [.H]_{b} and relatively low-quality diet of fruit and +terrestrial invertebrates (Table 9), however, r_{max} of _Nasua nasua_ +may be very similar to that of _Nasua narica_. + +_Procyon cancrivorus._--The age of first female reproduction for +_Procyon cancrivorus_ has not been reported. However, if one assumes +females can reproduce in their first year, r_{max} for _Procyon +cancrivorus_ would be 1.02 (132% of expected; Table 10). If, on the +other hand, first female reproduction is delayed until the second year, +r_{max} would be 0.65 (84% of predicted; Table 10). _Procyon +cancrivorus_ has a low [.H]_{b}, reduced litter size, and small body +mass. Its low [.H]_{b} may limit litter size, but as with _Bassariscus +astutus_, the quality of its diet (a high percentage of small +vertebrates; Table 9) and its small body size may make it possible for +females to reproduce in their first year and thus increase the species' +reproductive potential. This reasoning would argue that _Procyon +cancrivorus_ probably enjoys higher, rather than lower, than expected +r_{max}. + +_Potos flavus._--In addition to a low [.H]_{b}, this species possesses +other characteristics that limit its reproductive potential: +low-quality diet, delayed reproduction, and birth of a single young +each year. Because there does not appear to be any other feature of +its life history that can counteract the influence of these factors, +r_{max} in _Potos flavus_ has evolved to be only 48% of expected +(0.30; Table 10). Its close relative, the olingo, _Bassaricyon gabbii_, +appears to share the same condition (Table 10). + +SUMMARY.--This brief survey illustrates that, with the exception of +_Potos flavus_, procyonids tend to have values of r_{max} that are +higher than those predicted for them on the basis of mass (Table 10). +Regression analysis indicates that, within the family, body mass +accounts for only a small amount (3%) of the variation in r_{max}, +whereas the positive slope of the correlation between r_{maxr} and +H_{br} (R = 0.93) suggests that low metabolism has a limiting effect +on r_{max} (see Table 10, footnote f). The implication here is that +low [.H]_{b} would be associated with a lower rate of biosynthesis, +a slower growth rate, and a longer generation time. Procyonids with +low [.H]_{b} but higher than expected r_{max} must possess other +traits that serve to offset the effects of low metabolism. Our survey +indicates that the following features compensate for low [.H]_{b} and +help increase r_{max}: (1) a high-quality diet may make biosynthesis +and growth more efficient, thus optimizing the time element associated +with each of these processes; (2) larger litter sizes and cooperation +in care of the young may increase survivorship in spite of a slower +growth rate; and (3) an early age of first reproduction, a long +reproductive life span, and moderate-size litters (two to four young) +may in the long run add as many individuals to the population as a +shortened generation time. Our survey also suggests that, at the other +extreme, factors such as a low-quality diet, reduced litter size, +absence of cooperative care of the young, delayed age of first +reproduction, and shortened reproductive life span all serve to +decrease r_{max}. Thus, it is obvious that diet, litter size, social +structure, reproductive strategy, and reproductive life span can operate +synergistically with [.H]_{b} to magnify its influence on r_{max} (as +with _Procyon lotor_ and _Potos flavus_), or they can function in +opposition to [.H]_{b} to change the direction of its influence on +r_{max} (as with _Bassariscus astutus_, _Procyon cancrivorus_, _Nasua +narica_, and perhaps _Nasua nasua_). + + +_Basal Metabolism and Climatic Distribution_ + +_Procyon lotor._--The evolution of a higher [.H]_{b} (Tables 7, 8) may +have been the physiological cornerstone that enabled _Procyon lotor_ +to break out of the mold being exploited by other procyonids and to +generalize its use of habitats and climates. Once this basic +physiological change was in place, selection for appropriate +alterations in thermal conductance, capacity for evaporative cooling, +diversity of diet, and energy storage would have provided this species +with the suite of adaptations needed to extend its distribution into +other habitats and climates. Support for this concept follows from the +fact that high levels of [.H]_{b} are associated with (1) cold-hardiness +in mammals that live in cold-temperate and arctic climates (Scholander +et al., 1950c; Irving et al., 1955; Irving, 1972:115, 116; Shield, 1972; +Vogel, 1980; Golightly and Ohmart, 1983); (2) the ability to utilize a +wide variety of food resources and to occupy a large number of different +environments and habitats (McNab, 1980a); and (3) a high intrinsic rate +of natural increase (McNab, 1980a; Hennemann, 1983; Lillegraven et al., +1987; Nicoll and Thompson, 1987; Thompson, 1987). + +OTHER PROCYONIDS.--Other procyonids (_Potos flavus_, _Procyon +cancrivorus_, _Nasua narica_, and _Nasua nasua_) have lower than +predicted [.H]_{b}'s (Table 7), a characteristic that is considered to +be an energy-saving adaptation for those that live in relatively stable +tropical and subtropical habitats (Mueller and Kulzer, 1977; +Chevillard-Hugot et al., 1980; Mueller and Rost, 1983). However, +_Bassariscus astutus_ is found in tropical, subtropical, and temperate +climates. This species is found from tropical Mexico to temperate +regions of the western United States (Kaufmann, 1982, 1987; Nowak and +Paradiso, 1983:979). In the northern part of its distribution, +_Bassariscus astutus_ lives in habitats that are unstable (arid +regions), that are low in productivity, and that characteristically +have marked seasonal changes in temperature. Its lower than predicted +[.H]_{b} could be an important water-conserving adaptation at times when +temperatures are high (McNab and Morrison, 1963; McNab, 1966; MacMillen +and Lee, 1970; Noll-Banholzer, 1979) and an important energy-conserving +mechanism when cold weather may limit food availability and hunting time +(Scholander et al., 1950c; Wang et al., 1973). As will be seen later, +_Bassariscus astutus_ is unique among procyonids with lower than +predicted [.H]_{b}'s in that it also has a lower than predicted +C_{mw} (Table 7). This allows it to use less energy than expected for +thermoregulation at low temperatures. Another species with a similar set +of adaptations (lower than predicted [.H]_{b} and C_{mw}) is the arctic +hare, _Lepus arcticus_ (Wang et al., 1973), which lives in one of +the coldest and least-productive regions on earth. Wang et al. (1973) +suggest that this combination of adaptations allows _Lepus arcticus_ +to better match its energy requirements to the low productivity of its +environment. A similar relationship may hold for _Bassariscus astutus_, +particularly in colder arid portions of its distribution, and may be the +reason that it, but not other procyonids with low [.H]_{b}'s, has been +able to inhabit temperate climates. + + + +MINIMUM THERMAL CONDUCTANCE + + +_Background_ + +Thermal conductance is a measure of the ease with which heat is +passively transferred to or from a body through its tissues and pelt. +Within T_{n}, a mammal is able to vary its thermal conductance over a +wide range of values by changing heat transfer characteristics of both +of these layers. Minimum thermal conductance occurs when total heat +transfer through these layers is reduced to its lowest possible rate. +This minimum value, which is the reciprocal of maximum resistance, +occurs, theoretically, but not always practically (see McNab, 1988b), +at the animal's T_{lc} and is best estimated under standard conditions +in a metabolism chamber (McNab, 1980b; Aschoff, 1981). Minimum thermal +conductance scales to body mass (McNab and Morrison, 1963; Herreid and +Kessel, 1967; McNab, 1970, 1979b; Bradley and Deavers, 1980; Aschoff, +1981). Therefore, to make comparisons between species of various sizes, +we scaled out body mass by expressing C_{mw} as the ratio of measured +to predicted values (C_{mwr}; Table 7). These ratios were used to make +comparisons of heat-transfer characteristics between species that +occupy different habitats or climates. + + +_Effect of Molt on Thermal Conductance_ + +In summer, T_{lc}'s of male and female _Procyon lotor_ (Figure 2) were +very similar to those of other procyonids (22 deg.C-26 deg.C; Table 7). In +winter, T_{lc} of both sexes shifted downward to 11 deg.C (Figure 3). This +seasonal shift in T_{lc} occurred as the result of a seasonal change +in minimum thermal conductance (Table 3). For many northern mammals, a +seasonal change in thermal conductance is partly mediated via cyclic +changes in the insulative quality of their pelt (Scholander et al., +1950a; Irving et al., 1955; Hart, 1956, 1957; Irving, 1972:165). + +_Procyon lotor_ begins to shed its heavy winter coat about the time +its young are born. Molt progresses through summer and by late August +the new coat is complete (Stuewer, 1942). During its summer molt, +_Procyon lotor_'s C_{mw} increased by about 49% over the value for +female raccoons in winter (Table 3). In summer, therefore, it had the +highest mass specific C_{mw} of those procyonids considered +(C_{mwr} = 1.77 and 1.79; Table 7). An increase in thermal conductance +facilitates passive heat loss for temperate and arctic species, and +this serves as an important thermoregulatory adaptation during warm +summer months (Scholander et al., 1950c; Irving et al., 1955; +Hart, 1956, 1957; Irving, 1972:165). This adaptation is particularly +important to those temperate- and arctic-zone species (including +raccoons) whose [.H]_{b}'s do not decrease during summer (Irving et al., +1955). From August on, the fur of _Procyon lotor_ becomes increasingly +longer and heavier, with peak, or prime, condition occurring in late +fall and early winter (Stuewer, 1942). Minimum conductance of our +captive raccoons was lowest in winter (C_{mwr} = 1.15) when their pelts +were in prime condition (Tables 3, 7). Because "primeness" of raccoon +pelts varies geographically, thicker pelts being associated with colder +climates (Goldman, 1950:21; Whitney and Underwood, 1952:24-41), the +degree of seasonal change in C_{mw} must also vary geographically. + +The only other procyonid for which a seasonal molt has been described +is _Bassariscus astutus_. Molt in this species extends from late summer +to late fall (Toweill and Toweill, 1978). How molt effects thermal +conductance in _Bassariscus astutus_ is not known because metabolic +data for this species (Table 7) apparently were collected only when +their pelts were in prime condition (Chevalier, 1985). + +Goldman (1950:20) reports that _Procyon cancrivorus_ does not have a +seasonal molt. Like other tropical procyonids, _Procyon cancrivorus_ +lives in an environment that has the following characteristics: high +even temperatures throughout the year (1 deg.C-13 deg.C difference in monthly +mean temperature), a greater range in temperature between day and +night than in mean monthly temperature throughout the year, uniform +lengths of day and night, seasonal variation in rainfall, and lowest +temperatures during the rainy season(s) (Kendeigh, 1961:340). In such +a stable environment there would be no advantage to a sharply defined +seasonal molt cycle that could place an animal in thermoregulatory +jeopardy by increasing its thermal conductance. This would be +particularly true for animals like tropical procyonids that have lower +than predicted [.H]_{b}'s but that maintain typical eutherian body +temperatures (Table 7). Consequently, molt in all tropical procyonids +may either be prolonged or continuous. This is a feature of their +biology that needs to be examined in more detail. + + +_Comparison of Thermal Conductances_ + +_Procyon lotor_ VERSUS TROPICAL PROCYONIDS.--C_{mwr} for _Procyon +lotor_ in winter was 1.15, which is similar to the values for _Potos +flavus_ and _Procyon cancrivorus_, 1.02 and 1.25, respectively +(Table 7). These two tropical species, therefore, have C_{mw}'s that +are similar on a mass specific basis to the value for _Procyon lotor_ +in winter. However, at their T_{lc}'s, the thermal gradient sustained +by these tropical animals is only about 11 deg.C, whereas for _Procyon +lotor_ in winter it was 26.5 deg.C. Examination of Eq. 4 with respect to +these thermal gradients suggests that tropical procyonids achieve such +low C_{mw}'s by virtue of their lower than predicted [.H]_{b}'s rather +than by having pelts that are exceptionally good insulators. In fact, +the insulation afforded by the pelts of these tropical procyonids is +about the same as that of the 50 g arctic lemming, _Dicrostonyx +groenlandicus rubricatus_, whose coat has an insulative value that is +about half that of the hare, _Lepus americanus_, red fox, _Vulpes fulva +alascensis_, and pine martin, _Martes americana_, animals comparable in +size to these procyonids (Scholander et al., 1950a). Therefore, pelts +of these tropical procyonids do not have the same insulative value as +the prime winter coat of _Procyon lotor_. + +_Nasua narica_ and _Nasua nasua_ have tropical and subtropical +distributions and they are the only procyonids that are diurnal +(Kaufmann, 1962:103-105, 1982, 1987). Because they are active during +the day they experience a more extreme thermal environment (higher +T_{a}'s and solar radiation) than their nocturnal cousins. Values of +C_{mwr} for _Nasua narica_ (1.45 and 1.55) and _Nasua nasua_ (1.24 and +1.65) are higher than those for _Procyon cancrivorus_ or _Potos flavus_ +(Table 7). Thus, these coatis have higher mass specific C_{mw}'s than +their nocturnal tropical cousins. A high C_{mw} reduces the cost of +thermoregulation in hot environments because it increases an animal's +ability to lose excess heat passively. The higher C_{mw}'s of these +coatis serve as an adaptation that contributes to the success of their +diurnal life style as well as their ability to expand their habitat use +to areas with less thermal stability, such as oak and pine woodlands +and deserts. + +_Bassariscus astutus._--This species has the lowest mass specific +C_{mw} of these procyonids (C_{mwr} = 0.85; Table 7), which indicates +that its pelt has a greater insulative value than the coats of _Potos +flavus_, _Procyon cancrivorus_, _Nasua nasua_, or _Nasua narica_. This, +coupled with a lower than predicted [.H]_{b}, allows _Bassariscus +astutus_ to maintain T_{b} with less energy expenditure than is +possible for any other procyonid of comparable size; and this +combination of adaptations provides _Bassariscus astutus_ with a +distinct energy advantage in environments that have low productivity +(Wang et al., 1973). The evolution of a pelt that provides better +insulation must be considered an important contributing factor for +the spread of this species into desert regions of the western United +States. + + + +THERMOREGULATION AND USE OF STORED FAT AT LOW TEMPERATURES + + +_Background_ + +THERMOREGULATION.--At temperatures below a mammal's T_{n}, heat loss +exceeds [.H]_{b}. To maintain T_{b} under these conditions, metabolic +rate must be increased (Eq. 4). _Procyon lotor_ in summer during its +annual molt (Table 5; Figure 2), _Bassariscus astutus_ (Chevalier, +1985), _Nasua nasua_ (Chevillard-Hugot et al., 1980; Mugaas et al., +in prep.), _Nasua narica_ (Scholander et al., 1950b; Mugaas et al., +in prep.), and _Potos flavus_ (Mueller and Kulzer, 1977; Mueller and Rost, +1983) all are able to elevate their metabolic rates by 130% above basal +when they are exposed to T_{a} = 0 deg.C. _Procyon cancrivorus_ responds to +0 deg.C with an increase in metabolic rate of 257% above basal (Scholander +et al., 1950b). All animals listed have about the same T_{lc} and T_{b}, +so the temperature differential producing this response is about the +same for each species. Metabolic ability to defend body temperature +against low ambient temperatures, therefore, is well developed in these +procyonids. Such large increases in metabolic rate are energetically +expensive, and if these animals were routinely exposed to T_{a} = 0 deg.C, +it would be difficult for them to acquire enough food each day to +maintain endothermy. Raccoons in winter pelage, however, need only +elevate their metabolic rate by 47% above basal to maintain endothermy +at T_{a} = 0 deg.C (Table 5; Figure 3). Each year at the completion of its +molt, the raccoon's highly insulative pelt is renewed. This lowers +their T_{lc} by 9 deg.C to 15 deg.C below that measured for them in summer +(Figure 3) and decreases their cost of thermoregulation at low +temperatures. The increased insulative capacity of their pelt is one +of the primary adaptations that has allowed _Procyon lotor_ to extend +its distribution into cold climates. + +STORED FAT.--Cyclic fattening is an integral and important part of a +raccoon's annual cycle (Mugaas and Seidensticker, ms); however, it has +not been reported for other procyonids. During winter in parts of the +United States and Canada, raccoons are confined to their dens for +variable periods of time (days to months) depending on the severity of +the weather (Stuewer, 1943:223-225; Whitney and Underwood, 1952:108-116; +Sharp and Sharp, 1956; Mech et al., 1968; Schneider et al., 1971). +During this confinement, they do not hibernate but rather enter a state +of "dormancy" and become inactive. While dormant they remain endothermic +(T_{b} > 35 deg.C; Thorkelson, 1972:87-90) and derive most of their energy +requirement from fat reserves accumulated during fall. The rate at which +fat stores are consumed during winter dormancy depends on the +thermoregulatory requirement imposed on them by local weather +conditions, the insulative quality of their pelt, and any advantage +they may gain by seeking shelter in a den. + + +_Thermal Model of the Raccoon and Its Den_ + +Heat transfer between an animal and its environment is a function of +the interaction of its body temperature and thermal conductance with +various environmental variables (air temperature, wind speed, vapor +pressure, and thermal radiation). When a raccoon is outside its den, +its thermal conductance (C_{mw}) is the only barrier to heat transfer +with the external environment. However, when it enters a tree den, a +raccoon imposes two other thermal barriers between itself and the +external environment: (1) conductance of the air space between its fur +and the den's walls (C_{a}) and (2) conductance of the den's walls +(C_{d}; Thorkelson, 1972:59-63; Thorkelson and Maxwell, 1974). +Thorkelson and Maxwell (1974) modeled heat transfer of a simulated +raccoon (a water-filled aluminum cylinder equipped with a heater and +covered with a raccoon pelt) in a closed tree den. In their system, +65% of resistance to heat flux was attributable to the pelt, whereas +the remainder (35%) was due to C_{a} and C_{d}. Because resistance is +the inverse of conductance, and resistances for the raccoon and its +den are arranged in series, we can estimate total conductance (C_{t}) +of this system with Eq. 7. + + 1/C_{t} = 1/C_{mw} + 1/C_{a} + 1/C_{d} Eq. 7 + +Minimum thermal conductance C_{mw} for raccoons in winter was +0.0172 mL O_{2}.g^{-1}.h^{-1}. deg.C^{-1} (Table 3). Based on Thorkelson +and Maxwell's (1974) model we let 1/C_{mw} = 0.65(1/C_{t}) = +1/0.0172 mL O_{2}.g^{-1}.h^{-1}. deg.C^{-1}, and 1/C_{a} + 1/C_{d} = +0.35(1/C_{t}). Substituting these values into Eq. 7 and solving +for C_{t} yields 0.0112 mL O_{2}.g^{-1}.h^{-1}. deg.C^{-1}, a value +that is 35% lower than that of the animal alone. Substituting +this value and the value for basal metabolism of winter raccoons +(0.47 mL O_{2}.g^{-1}.h^{-1}; Table 7) into Eq. 4 and solving for +(T_{b} - T_{a}) yields a new temperature differential of 42 deg.C. +Therefore, by using tree dens, raccoons in north central Virginia, +with T_{b} = 37 deg.C (Figure 7), could effectively reduce their T_{lc} +from 11 deg.C to -5 deg.C and markedly reduce their metabolic cost of +thermoregulation. + + +_Metabolic Advantage of the Den_ + +Given prevailing winter temperatures in north central Virginia (see +"Materials and Methods"), adult raccoons in that area should be able +to sustain endothermy most of the time they are in their dens by simply +maintaining [.H]_{b}. Depending on the mass of their stored fat, they +could remain in their dens for several weeks without eating (Mugaas and +Seidensticker, ms). The thermal advantage of a den could be further +enhanced during colder temperatures if two or more raccoons occupied it +at the same time and huddled together, and/or if these animals could +reduce C_{mw} even more by lowering T_{b} and cooling their extremities. +Although we do not have any data to verify the second mechanism, there +are many accounts in natural history literature that document raccoons +occupying dens together (Lotze and Anderson, 1979). This habit could be +particularly important for the young of the year and may be one reason +why they often continue to den with their mothers during winter (Lotze +and Anderson, 1979; Seidensticker et al., 1988). Raccoons that live in +colder climates, such as Minnesota, undoubtedly obtain the same +advantage from a den as Virginia animals, but because of their greater +body mass, longer fur, and potentially lower C_{mw}, T_{lc} of a +Minnesota raccoon in a den could be even lower than what we calculated +for Virginia raccoons. Therefore, when they are in their dens, raccoons +living in very cold climates also may be able to maintain homeothermy +with a basal level of metabolism. + + + +THERMOREGULATION AT HIGH TEMPERATURES + + +_Background_ + +In hot environments mammals depend on behavior to minimize their +thermal load (escape to shaded or cooler microclimates, use posture +and orientation to wind and sun, restrict activity, become nocturnal, +etc.) and on evaporative water loss to rid themselves of excess heat. +With regard to evaporative heat loss, Calder and King (1974:326) +arbitrarily subdivided the response to various T_{a}'s as follows: +"(1) cool temperatures at which water loss should be minimized, both +to reduce heat loss and as an adaptation to terrestriality; (2) an +intermediate temperature range wherein evaporation is gradually +increased as dry heat losses are proportionately reduced with smaller +thermal gradients; and (3) warm to hot temperatures at which +evaporation must be actively increased to dispose of metabolic and +exogenous heat loads." Some mammals are able to thermoregulate very +well at high ambient temperatures via panting or sweating, whereas +others have a very limited capacity. Hence, there is no general +approach to calculating evaporative water loss under these conditions +(Campbell, 1977:85). However, the ratio of evaporative heat lost to +metabolic heat produced can be used to quantify a species' capacity +for evaporative cooling and to make comparisons between species. + + +_Comparison of Procyonid Responses to Heat Stress_ + +_Potos flavus._--This species lives in Neotropical forests of Central +and South America. It is nocturnal, arboreal in habit, and appears to +be the most heat-sensitive of these procyonids. Its T_{uc} is at 30 deg.C +to 33 deg.C (Table 7; Mueller and Kulzer, 1977; Mueller and Rost, 1983). It +begins to pant at about 30 deg.C, but its efforts at evaporative cooling +are very ineffective. At 33 deg.C _Potos flavus_ can dissipate 33% of its +metabolic heat via evaporative water loss, but at 35 deg.C the efficiency +of this mechanism falls to 20% (Mueller and Rost, 1983). Consequently, +when exposed to T_{a}'s above 33 deg.C, any kind of excitement causes its +T_{b} to rise rapidly in an uncontrolled manner (Mueller and Kulzer, +1977; Mueller and Rost, 1983). These animals rely on their nocturnal +and arboreal habits to keep them out of situations that could lead to +hyperthermia (Mueller and Kulzer, 1977; Mueller and Rost, 1983). + +_Nasua nasua_ and _Nasua narica_.--_Nasua nasua_ is abundant in +tropical and subtropical South America, whereas _Nasua narica_ occupies +the same climates in North America from southern Arizona and New Mexico +south through Panama and on into Colombia and Ecuador (Hall and Kelson, +1959:892; Ewer, 1973:391, 392; Poglayen-Neuwall, 1975). Both coatis +are diurnal and forage primarily on the ground (Kaufmann, 1962:185-188, +1987; Poglayen-Neuwall, 1975; Nowak and Paradiso, 1983:982), +consequently they are exposed to a more severe thermal environment +while active (higher T_{a}'s and solar radiation) than are nocturnal +procyonids. Both coatis are more heat-tolerant than _Potos flavus_; +their T_{uc}'s are higher (33 deg.C-35 deg.C; Table 7), they can tolerate +T_{a}'s of 35 deg.C without raising their T_{b}'s (Chevillard-Hugot et al., +1980; Mugaas et al., in prep.), and they have a greater capacity for +evaporative cooling than _Potos flavus_ (Mugaas et al., in prep.). The +greater heat tolerance of these coatis is compatible with their diurnal +habits and widespread distribution in a variety of forest habitats in +both tropical and subtropical areas of the western hemisphere. + +_Bassariscus astutus._--In addition to living in Neotropical forests of +Mexico, _Bassariscus astutus_ also flourishes in hot arid climates, and +it has extended its range much farther north than _Nasua narica_ (Hall +and Kelson, 1959:881,892; Poglayen-Neuwall, 1975; Kaufmann, 1982). Its +T_{uc} is higher (35.5 deg.C; Table 7) than that of _Potos flavus_, but it +is comparable to those of _Nasua nasua_ and _Nasua narica_. Its capacity +for evaporative cooling is well developed; at 40 deg.C _Bassariscus astutus_ +is able to dissipate 100% of its resting metabolic heat via evaporative +water loss, and at 45 deg.C it is able to dissipate 172% (Chevalier, 1985). +In spite of its great capacity for evaporative cooling, this species is +nocturnal, a habit that, along with its low [.H]_{b}, should allow it +to keep thermoregulatory water requirements to a minimum. + +_Procyon lotor._--Our data suggested that T_{uc} for _Procyon lotor_ in +winter was comparable to that for _Bassariscus astutus_ (35 deg.C), and +that in summer it was even higher. When exposed to temperatures near +the upper end of its T_{n}, _Procyon lotor_ increased the gradient for +passive heat loss with a controlled rise in T_{b} (Figure 6). In summer +its capacity for passive heat loss was enhanced by the molt of its +heavy winter fur. _Procyon lotor_'s capacity for evaporative cooling +also appeared to be well developed, although our animals were not +heated to the point that evaporative cooling was fully expressed +(Figures 4, 5). However, _Procyon lotor_ is nocturnal, and this may +allow it to eliminate, or at least reduce, the need for evaporative +cooling, even in hot climates. Thus, _Procyon lotor_ appears to be +well equipped physiologically and behaviorally to cope with thermal +demands of hot environments in its distribution. + +_Procyon cancrivorus._--Unfortunately, data for the crab-eating +raccoon are not complete enough at high temperatures to include it in +this survey. + +SUMMARY.--This comparison demonstrates that capacity for evaporative +cooling, tolerance of an elevated T_{b} to enhance passive heat loss, +and behavioral avoidance of thermal stress are the primary methods used +by procyonids to thermoregulate at high temperatures. _Procyon lotor_ +and _Bassariscus astutus_, whose distributions extend into temperate +regions, have developed these abilities to a greater extent than other +procyonids. _Potos flavus_, whose distribution is confined to +lowland tropical forests, has the least ability in this regard. _Nasua +nasua_ and _Nasua narica_ appear to have thermoregulatory abilities +that are intermediate to those of _Bassariscus astutus_ and _Potos +flavus_. This suggests that ancestral procyonids may have had poor to +modest ability to thermoregulate at high temperatures, a condition that +would have limited their ability to leave the thermal stability +afforded by tropical forests. Dispersal into temperate climates, +therefore, required not only increased cold tolerance but also +selective enhancement of those mechanisms used in thermoregulation at +high temperatures. + + + TABLE 11.--Distribution by climate of selected procyonid species. + + -----------------------+--------------------------------------------- + | Mild[a] Cold[b] + Species | Tropics Subtropics temperate temperate + -----------------------+--------------------------------------------- + _Procyon lotor_ | + + + + + _Bassariscus astutus_ | + + + + _Nasua nasua_ | + + + _Nasua narica_ | + + + _Procyon cancrivorus_ | + + + _Potos flavus_ | + + -----------------------+--------------------------------------------- + + [a] Extends from the subtropics north to the northern limit of + _Bassariscus astutus_' distribution (Hall and Kelson, 1959:881), + which approximates the 10 deg.C isotherm for average annual + temperature in the United States (Kincer, 1941). + + [b] Extends northward from the 10 deg.C isotherm for average annual + temperature in the United States. + + + +COMPOSITE SCORES OF ADAPTIVE UNITS AND GEOGRAPHIC DISTRIBUTION + + +In Table 11, procyonid species are arranged in descending order with +respect to the number of major climates that are included in their +geographic distributions (Hall and Kelson, 1959:878-897; +Poglayen-Neuwall, 1975; Kortlucke and Ramirez-Pulido, 1982; Nowak and +Paradiso, 1983:977-985). Composite scores ranged from a high of 1.47 +for _Procyon lotor_ to a low of 0.39 for _Potos flavus_, whereas +_Nasua nasua_, _Nasua narica_, _Procyon cancrivorus_, and _Bassariscus +astutus_ had intermediate values ranging from 0.64 to 0.79 (Table 12). +Figure 8 demonstrates that there is a direct relationship between the +number of climates these species occupy and their composite scores. +Regression analysis (Y = 2.68.X + 0.24; where Y is number of climates, +and X is composite score) demonstrates a high degree of correlation +between these variables (R = 0.94) and indicates that 89% of the +variance in distribution can be explained by composite scores. The +various combinations of adaptations expressed by these species do, +therefore, play a role in delimiting their climatic (latitudinal) +distributions. + +_Procyon lotor's_ normalized scores were higher in all categories than +those of other procyonids. _Procyon lotor_, therefore, possesses those +traits that have allowed it to become the premier climate generalist +of the procyonid family. As an adaptive unit, these traits provide +_Procyon lotor_ with the physiological and behavioral flexibility +required to take full advantage of a wide range of climates and +habitats, and its distribution verifies that it has done so. Even so, +it is probably not fair to assume that this species represents a +perfect physiological match with climate over its entire distribution. +_Procyon lotor_ is, in many respects, still a forest-dwelling species, +and its ability to expand its distribution into other habitats such as +prairie and desert may well be due, in part, to its use of behavior to +take advantage of favorable microclimates in otherwise hostile +environments (Bartholomew, 1958, 1987). This feature of _Procyon +lotor's_ biology needs to be further examined. + + + TABLE 12.--Normalized and composite scores for selected procyonids. + (H_{br} = ratio of measured to predicted basal metabolism (Table 7), + C_{mwr} = ratio of measured to predicted minimum thermal conductance + (Table 7), D_{dr} = ratio of food categories actually utilized by + each species to total food categories eaten by all six species + (calculated from Table 9), r_{maxr} = ratio of calculated to + expected r_{max} (Table 10).) + + ----------------------+---------------------------------------------- + | Normalized scores + Species |---------------------------- Composite[a] + |H_{br}/C_{mwr} D_{dr} r_{maxr} score + ----------------------+---------------------------------------------- + _Procyon lotor_ | 0.95 0.95 2.52 1.47 + _Bassariscus astutus_ | 0.80 0.33 1.24 0.79 + _Nasua nasua_ | 0.48 0.33 1.11[b] 0.64 + _Nasua nasua_ | 0.48 0.33 1.11[b] 0.64 + _Nasua narica_ | 0.40 0.53 1.11 0.68 + _Procyon cancrivorus_ | 0.55 0.33 1.32 0.73 + _Potos flavus_ | 0.60 0.11 0.48 0.39 + ----------------------+---------------------------------------------- + + [a] Composite score = [(H_{br}/C_{mwr}) + D_{dr} + r_{maxr}]/3. + + [b] Value calculated for _Nasua narica_ (Table 10) and used with the + assumption that it must be similar to the value for _Nasua nasua_. + + +All five species with low [.H]_{b}'s have composite scores less than +1.0 (Table 12; Figure 8). Four of these five, _Nasua nasua_, _Nasua +narica_, _Procyon cancrivorus_, and _Potos flavus_, have +H_{br}/C_{mwr} ratios that are 0.6 or less, which indicates they are +the least cold-tolerant procyonids (McNab, 1966). These four species +also are confined to either tropic, or tropic and subtropic climates +(Table 11). This suggests that these species share a common +thermoregulatory adaptation that represents a specialization to these +climates. Attendant with this adaptation, however, is a high cost of +thermoregulation at temperatures below their T_{lc}, and this must be +an important factor in limiting their distributions to tropic and +subtropic climates. Differences in their distributions within these +climates, therefore, must hinge more on differences in their D_{dr} +and r_{maxr} values than on differences in their H_{br}/C_{mwr} +ratios. This is supported by the fact that _Potos flavus_, which has +the lowest D_{dr} and r_{maxr} values, is confined to a single +climate, whereas _Nasua nasua_, _Nasua narica_, and _Procyon +cancrivorus_ each possess larger D_{dr} and r_{maxr} values and are +found in two climates. Thus, _Potos flavus_, with its highly +specialized diet and low reproductive potential, is the most +ecologically specialized of these procyonids, and its distribution is +limited to the single climate that can provide its requirements. +_Nasua nasua_, _Nasua narica_, and _Procyon cancrivorus_ are less +specialized and thus show more ecological flexibility in their +distributions. + + [Illustration: FIGURE 8.--Relationship between number of climates + in which a species is found and its composite score. Symbols for + _Nasua nasua_ overlap at coordinates (0.64, 2). Solid line + represents linear regression of climates (Y) on composite scores + (X): Y = 2.68.X + 0.24; R = 0.94.] + +_Bassariscus astutus_, the other species with low [.H]_{b}, is found +in three climates, which indicates that it has greater ecological +flexibility than _Nasua nasua_, _Nasua narica_, or _Procyon +cancrivorus_. D_{dr} and r_{maxr} are comparable for these four +species (Table 12). This suggests that the greater ecological +flexibility of _Bassariscus astutus_ is derived largely from its +greater cold tolerance. _Bassariscus astutus_ has a more insulative +pelt than these other procyonids (C_{mwr} = 0.85; Table 7), so its +H_{br}/C_{mwr} ratio is higher (0.80; Table 12). This, and its greater +capacity for evaporative cooling (Chevalier, 1985), allows +_Bassariscus astutus_ to take advantage of a wider range of thermal +environments than these other species. However, even with its higher +H_{br}/C_{mwr} ratio, the composite score for _Bassariscus astutus_ +is not much different than those for _Nasua nasua_, _Nasua narica_, +and _Procyon cancrivorus_ (Table 12). Consequently, _Bassariscus +astutus_ is found in more climates than would be predicted for it on +the basis of its composite score (Figure 8). This suggests that either +the H_{br}/C_{mwr} ratio carries greater weight in determining +distribution than is reflected in this analysis, or as has been +described for some other species (Bartholomew, 1958, 1987), +_Bassariscus astutus_ may extend its distribution farther than +expected via use of its behavior. In either case, for procyonids with +low [.H]_{b}, _Bassariscus astutus_ represents the pinnacle of +adaptation for climate generalization. + + + +EVOLUTION OF METABOLIC ADAPTATIONS + + +_Evolution of Low Basal Metabolic Rate_ + +A radiation of frugivorous and omnivorous Procyoninae (Table 1) +occurred in the middle and late Miocene of North America. It included +origins of such terrestrial genera as _Cyonasua_, _Nasua_, and +_Procyon_ (Webb, 1985b). The earliest procyonid genus to find its way +to South America was _Cyonasua_, an omnivorous carnivore that +presumably split, along with its sister genus _Arctonasua_, from a +common North American ancestor (Baskin, 1982; Webb, 1985b). +_Cyonasua_, about the size of present-day raccoons, was adapted to a +wide range of habitats and was probably comparable to modern raccoons +with respect to the breadth of its feeding habits (Webb, 1985b; +Marshall, 1988). Because North American _Arctonasua_ was about the +same size as _Cyonasua_ (Webb, 1985b) and shared a number of +characters with it (Baskin, 1982), we speculate that it also may have +had similar habits and occupied similar climates and habitats. +_Bassariscus_, another member of Procyoninae, had an even earlier +origin in tropical North America (Webb, 1985b). The origin of the +small arboreal forms _Potos_ and _Bassaricyon_ (subfamily Potosinae) +is obscure but is thought to have occurred in the rainforests of +Central America (Webb, 1985b). What were the metabolic capabilities of +these early procyonids? We do not know, but for several million years, +from middle to late Miocene, procyonids lived in tropical and +subtropical forests of Central and North America (Webb, 1985b; +Marshall, 1988). Then, in the Pleistocene, several modern forms +crossed the Panamanian land bridge into similar habitats and climates +in South America; but none of them appear to have spread far enough +northward to have crossed the Bering land bridge. + +Several million years exposure to a tropical environment, with its +continuous high temperatures and modest range of thermal extremes, +would have favored selection of metabolic and thermoregulatory traits +that would minimize energy requirements: a lower than predicted basal +metabolic rate, a prolonged or continuous molt resulting in very +little annual change in minimum thermal conductance, and a modest +capacity for evaporative cooling. In addition, we would expect +selection to have favored a diverse diet, good reproductive potential, +and behavioral flexibility to utilize a variety of habitats within +these climates. Our analysis has shown that such characteristics are +the norm for extant members of this family living in tropical and +subtropical climates, and we speculate that these traits also were +common to early procyonids and served to restrict them to these +climates. Our speculation is supported by the fact that their known +fossil history from the Miocene is confined to geographic areas that +had tropical and subtropical climates. + +Later on, during Pleistocene glaciations, tropical and subtropical +forests shrank, savannas expanded, and temperate climate was pushed +toward equatorial regions. The opposite occurred during interglacial +periods (Raven and Axelrod, 1975; Webb, 1977, 1978; Marshall, 1988). +Consequently, mid-latitudes experienced alternating periods of +temperate and tropical, or at least subtropical, climate change. +Selection of characteristics that would have adapted a species with +low [.H]_{b} to temperate as well as tropic or subtropic climates +could have occurred in mid-latitudes at the temperate edge of these +tropical advances and retreats. Our analysis indicates that, for this +purpose, selection would have favored lower than predicted thermal +conductance, seasonal molt, increased capacity for evaporative +cooling, increased tolerance of elevated T_{b}, increased flexibility +of thermoregulatory behavior, food habits that provided for year-round +access to a high-quality diet in all three climates, and a higher than +predicted r_{max}. + +_Bassariscus astutus_ is the only species with low [.H]_{b} that has +all these characteristics, and it is the only one of them that has +added temperate climate to its distribution (Table 11). This suggests +that _Bassariscus astutus_ is a species that evolved away from the +norm for procyonids with low [.H]_{b}, toward characteristics that +allowed it to become more of a climate generalist. _Potos flavus_, +with its dietary specialization, low tolerance to high temperatures, +and arboreal mode of existence, has become a highly specialized +species totally dependent on tropical forests for its survival. As +such, it also represents a species that has evolved away from the +procyonid norm and portrays the extreme in climate specialization. +Olingos, _Bassaricyon gabbii_ (Table 1), may be similar to _Potos +flavus_ in this respect (see also Table 10). This suggests that of the +extant procyonids, _Nasua nasua_, _Nasua narica_, and _Procyon +cancrivorus_ have retained metabolic and behavioral characteristics +that are closest to those of their Miocene ancestors. + + +_Evolution of High Basal Metabolic Rate_ + +Between the time that _Cyonasua_ appeared and the Panamanian land +bridge was established in the upper Pliocene (4 to 5 million years +ago), northern climates continued their gradual cooling. This, along +with ongoing elevation of the continents and continuous modification +of their mountain ranges, served to shrink the tropical forest and +create pockets of climatic instability within it and on its edges +(Darlington, 1963:578-596; Marshall, 1988). In areas of instability, +selection would have favored traits that provided for a broader range +of thermal tolerance: higher [.H]_{b}, improved insulative quality of +pelt, a more sharply defined molt cycle, improved capacity for +evaporative cooling, greater D_{d}, and higher r_{max}. Consequently, +by the upper Pliocene, two metabolically distinct groups of procyonids +could have been established: those species with low [.H]_{b} living in +climatically stable forests and those with higher [.H]_{b} living in +unstable tropical, subtropical, and perhaps temperate climates. + +_Procyon lotor_ is the only extant procyonid with high [.H]_{b}. +_Procyon cancrivorus_ is its congeneric counterpart in Central and +South America (Table 1), and the two species are sympatric in Panama +and Costa Rica. However, in terms of its metabolism, thermal +conductance, molt, diversity of diet, r_{max}, and climatic +distribution, _Procyon cancrivorus_ shares more in common with other +procyonids than it does with _Procyon lotor_ (Tables 7, 11, 12; Figure +8). This suggests that metabolically _Procyon lotor_ portrays a +divergent line of this genus that arose as the result of a series of +mutations that gave rise to different metabolic characteristics. This +view is in keeping with a recent phylogenetic analysis of this family +that shows the genus _Procyon_ to be highly derived (Decker and +Wozencraft, 1991). Consequently, it would be instructive and would add +to our knowledge of the evolution of climatic adaptation to know more +about the genetic relatedness of these two species as well as their +historical relationship. + +Genus _Procyon_ appears in the fossil record (Hemphillian and Blancan +ages; Baskin, 1982) prior to Pleistocene glaciations. During the +Pleistocene, there were four different glacial advances and retreats +in a relatively short time period (the first appearing little more +than a million years ago; Darlington, 1963:578-596; Webb, 1985a; +Marshall, 1988). Glacial retreats created pulses of time during which +subtropic and temperate climates advanced toward the poles into areas +with large seasonal differences in light/dark cycles, whereas glacial +advances pushed these climates southward into areas having smaller +seasonal differences in light/dark cycles (Raven and Axelrod, 1975; +Webb, 1977, 1978; Marshall, 1988). Those members of the genus +_Procyon_ caught in these wide latitudinal fluctuations would have +experienced conditions favorable to continued selection for +characteristics conducive to physiologic adaptation to a wide range of +climatic conditions. _Procyon lotor_ is the only member of its genus +to have survived this selective process, and as we have seen, it does +possess traits that adapt it to a wide range of climatic conditions. +Primary among these is its higher [.H]_{b}, which provides it with +advantages not shared with other procyonids (see earlier discussion). +Three other adaptations also have had a profound influence on _Procyon +lotor_'s ability to generalize its use of climate: (1) the increased +insulative quality of its pelt coupled with its sharply defined molt +cycle, which allows for a large annual change in thermal conductance; +(2) its annual cycle of fat storage; and (3) a diverse high-quality +diet. The first two of these adaptations required evolution of +neuroendocrine pathways capable of responding to time-dependent +environmental cues such as changing day length, changing temperature, +etc. Such conditions would have been available as selective stimuli in +high-latitude forests and savannas of interglacial periods. _Procyon +lotor_'s elevated basal metabolic rate would have increased its +overall energy requirement, and it makes good intuitive sense, +therefore, that evolution during the Pleistocene also would have +favored selection of a diverse diet containing many items of high +nutritive value. + + + +SUMMARY + + +Our analysis has illustrated that within Procyonidae there are two +distinct modes of metabolic adaptation to climate. One is typified by +those species with low [.H]_{b}'s (_Bassariscus astutus_, _Nasua +nasua_, _Nasua narica_, _Procyon cancrivorus_, and _Potos flavus_), +and the other by _Procyon lotor_ with its higher [.H]_{b}. Those with +low [.H]_{b}'s have more restricted geographic distributions, and, +with the exception of _Bassariscus astutus_, they are all confined to +tropical and subtropical areas. The fossil history of this family +indicates that it had its origins in tropical forests of North and +Central America. This indicates that those procyonids whose +distributions are still primarily restricted to tropical forests share +many of the metabolic adaptations characteristic of their ancestors. +We speculate, therefore, that ancestral procyonids had a lower than +predicted [.H]_{b}, a pelt with modest to poor insulative quality, +good thermogenic ability but poor heat tolerance, modest to poor +capacity for evaporative cooling, no well-defined molt cycle, no +cyclic period of fattening, nocturnal habits, and a modestly diverse +diet of high-enough quality to provide for an average reproductive +potential. Although this pedigree contributed to the success of this +family in tropical and subtropical forests, it limited the ability of +its members to expand their distributions into cooler, less stable +climates. Viewed in this perspective, _Procyon lotor_'s high basal +metabolic rate, extraordinarily diverse diet, well-defined cyclic +changes in fat content and thermal conductance, high level of heat +tolerance, high capacity for evaporative cooling, and high +reproductive potential all stand out in sharp contrast to the +condition described for other procyonids. This suggests that the North +American raccoon represents culmination of a divergent evolutionary +event that has given this species the ability to break out of the old +procyonid mold and carry the family into new habitats and climates. + + + + +APPENDIX: LIST OF SYMBOLS + + + a potential age of females first producing young + + b potential annual birth rate of female young + + C_{a} conductance of air + + C_{d} conductance of den walls + + C_{m} minimum thermal conductance + + C_{md} minimum dry thermal conductance + + C_{mw} minimum wet thermal conductance + + C_{mwr} ratio of measured to predicted minimum wet thermal + conductance + + C_{t} total conductance + + D_{d} diversity of diet + + D_{dr} ratio of food categories actually used by a species to + the total number of food categories taken by all species + tested + + [.E] evaporative water loss + + E_{c} ratio of evaporative heat lost to metabolic heat produced + + [.E]_{eq} oxygen equivalent for heat lost by evaporation + + [.H]_{b} basal metabolic rate + + [.H]_{r} lowest resting metabolic rate at each temperature + + H_{br} ratio of measured to predicted basal metabolic rate + + m mass of animal + + m_{w} mass of water + + n potential age of females producing their final young + + r_{max} intrinsic rate of natural increase + + r_{maxe} expected intrinsic rate of natural increase + + r_{maxr} ratio of calculated to expected intrinsic rate of natural + increase + + RQ respiratory quotient + + T_{a} chamber air temperature + + T_{b} body temperature + + T_{lc} lower critical temperature + + T_{n} thermoneutral zone + + T_{uc} upper critical temperature + + t time + + [.V]_{a} rate of air flow through U-tubes + + [.V]_{e} rate of air flow into metabolism chamber + + [alpha] active phase of the daily cycle + + [gamma] heat equivalent of oxygen + + [lambda] heat of vaporization of water + + [rho] rest phase of the daily cycle + + + + +LITERATURE CITED + + + Aschoff, Juergen + + 1981. 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