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diff --git a/39396-8.txt b/39396-8.txt new file mode 100644 index 0000000..8b3317a --- /dev/null +++ b/39396-8.txt @@ -0,0 +1,3155 @@ +The Project Gutenberg EBook of A Population Study of the Prairie Vole +(Microtus ochrogaster) in Northeastern Kansas, by Edwin P. Martin + +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: A Population Study of the Prairie Vole (Microtus ochrogaster) in Northeastern Kansas + +Author: Edwin P. Martin + +Release Date: April 7, 2012 [EBook #39396] + +Language: English + +Character set encoding: ISO-8859-1 + +*** START OF THIS PROJECT GUTENBERG EBOOK POPULATION STUDY OF PRAIRIE VOLE *** + + + + +Produced by Chris Curnow, Paula Franzini, Joseph Cooper +and the Online Distributed Proofreading Team at +http://www.pgdp.net + + + + + + + + + + ================================================================== + UNIVERSITY OF KANSAS PUBLICATIONS + MUSEUM OF NATURAL HISTORY + + Volume 8, No. 6, pp. 361-416, 19 figures in text + ---------------------- April 2, 1956 ----------------------- + + + A Population Study + of the Prairie Vole (Microtus ochrogaster) + in Northeastern Kansas + + BY + + EDWIN P. MARTIN + + + UNIVERSITY OF KANSAS + LAWRENCE + 1956 + + + + + UNIVERSITY OF KANSAS PUBLICATIONS, MUSEUM OF NATURAL HISTORY + + Editors: E. Raymond Hall, Chairman, A. Byron Leonard, + Robert W. Wilson + + Volume 8, No. 6, pp. 361-416, 19 figures in text + Published April 2, 1956 + + + UNIVERSITY OF KANSAS + Lawrence, Kansas + + + PRINTED BY + FERD VOILAND, JR., STATE PRINTER + TOPEKA, KANSAS + 1956 + + 25-9225 + + + + +CONTENTS + + + PAGE + INTRODUCTION 363 + GENERAL METHODS 364 + HABITAT 366 + POPULATION STRUCTURE 373 + POPULATION DENSITY 376 + HOME RANGE 380 + LIFE HISTORY 383 + Reproduction 383 + Litter Size and Weight 386 + Size, Growth Rates and Life Spans 388 + Food Habits 397 + Runways and Nests 398 + Activity 400 + PREDATION 401 + MAMMALIAN ASSOCIATES 403 + SUMMARY AND CONCLUSIONS 408 + LITERATURE CITED 411 + + + + + A POPULATION STUDY + OF THE PRAIRIE VOLE (MICROTUS OCHROGASTER) + IN NORTHEASTERN KANSAS + + By + + Edwin P. Martin + + + + +INTRODUCTION + + +Perhaps the most important species of mammal in the grasslands of Kansas +and neighboring states is the prairie vole, _Microtus ochrogaster_ +(Wagner). Because of its abundance this vole exerts a profound influence +on the quantity and composition of the vegetation by feeding, trampling +and burrowing; also it is important in food chains which sustain many +other mammals, reptiles and birds. Although the closely related meadow +vole, _M. pennsylvanicus_, of the eastern United States, has been +studied both extensively and intensively, relatively little information +concerning _M. ochrogaster_ has been accumulated heretofore. + +I acknowledge my indebtedness to Dr. Henry S. Fitch, resident +investigator on the University of Kansas Natural History Reservation. In +addition to supplying guidance and encouragement in both the planning +and execution of the investigation, Dr. Fitch made available for study +the data from his extensive field work. Interest in and understanding of +ecology were stimulated by his teaching and his example. Special debts +are also acknowledged to Mr. John Poole for the use of his field notes +and to Professor E. Raymond Hall, Chairman of the Department of Zoology, +for several courtesies. Dr. R. L. McGregor of the Department of Botany +at the University of Kansas assisted with the identification of some of +the plants. Drawings of skulls were made by Victor Hogg. + +Of the numerous publications concerning _Microtus pennsylvanicus_, those +of Bailey (1924), Blair (1940; 1948) and Hamilton (1937a; 1937c; 1940; +1941) were especially useful in supplying background and suggesting +methods for the present study. Publications not concerned primarily with +voles, that were especially valuable to me in providing methods and +interpretations applicable to my study, were those of Blair (1941), +Hayne (1949a; 1949b), Mohr (1943; 1947), Stickel (1946; 1948) and +Summerhayes (1941). Faunal and ecological reports dealing with _M. +ochrogaster_ and containing useful information on habits and habitat +included those of Black (1937:200-202), Brumwell (1951:193-200; 213), +Dice (1922:46) and Johnson (1926). Lantz (1907) discussed the economic +relationships of _M. ochrogaster_; the section of his report concerning +the effects of voles on vegetation was especially useful to me. + +Fisher (1945) studied the voles of central Missouri and obtained +information concerning food habits and nesting behavior. Jameson (1947) +studied _M. ochrogaster_ on and near the campus of the University of +Kansas. His report is especially valuable in its treatment of the +ectoparasites of voles. In my investigation I have concentrated on those +aspects of the ecology of voles not treated at all by Fisher and +Jameson, or mentioned but not adequately explored by them. Also I have +attempted to obtain larger samples. + +The University of Kansas Natural History Reservation, where almost all +of the field work was done, is an area of 590 acres, comprising the +northeastern-most part of Douglas County, Kansas. Situated in the broad +ecotone between the deciduous forest and grassland, the reservation +provides a variety of habitat types (Fitch, 1952). Before 1948, much of +the area had been severely overgrazed and the original grassland +vegetation had been largely replaced by weeds. Since 1948 there has been +no grazing or cultivation. The grasses have partially recovered and, in +the summer of 1952, some grasses of the prairie climax were present even +on the parts of the Reservation which had been most heavily overgrazed. +Illustrative of the changes on the Reservation were those observed in +House Field by Henry S. Fitch (1953: _in litt._). He recalled that in +July, 1948, the field supported a closely grazed, grassy vegetation +providing insufficient cover for _Microtus_, with such coarse weeds as +_Vernonia_, _Verbena_ and _Solanum_ constituting a large part of the +plant cover. By 1950, the same area supported a lush stand of grass, +principally _Bromus inermis_, and supported many woody plants. Similar +changes occurred in the other study areas on the Reservation. Although +insufficient time has elapsed to permit analyses of successional +changes, it seems that trees and shrubs are gradually encroaching on the +grassland throughout the Reservation. + +The vole population has changed radically since the Reservation was +established. In September and October of 1948, when Fitch began his +field work, he maintained lines of traps totaling more than 1000 trap +nights near the future vole study plots without capturing a single vole. +In November and December, 1948, he caught several voles near a small +pond on the Reservation and found abundant sign in the same area. Late +in 1949 he began to capture voles over the rest of the Reservation, but +not until 1950 were voles present in sufficient numbers for convenient +study. + +I first visited the Reservation and searched there for sign of voles in +the summer of 1949. I found hardly any sign. In the area around the pond +mentioned above, however, several systems of runways were discovered. +This area had been protected from grazing for several years prior to the +reservation of the larger area. In House Field, where my main study plot +was to be established, there was no sign of voles. Slightly more than a +year later, in October, 1950, I began trapping and found _Microtus_ to +be abundant on House Field and present in smaller numbers throughout +grassland areas of the Reservation. + + + + +GENERAL METHODS + + +The present study was based chiefly on live-trapping as a means of +sampling a population of voles and tracing individual histories without +eliminating the animals. Live-trapping disturbs the biota less than +snap-trapping and gives a more reliable picture of the mammalian +community (Blair, 1948:396; Cockrum, 1947; Stickel, 1946:158; 1948:161). +The live-traps used were modeled after the trap described by Fitch +(1950). Other types of traps were tested from time to time but this +model proved superior in being easy to set, in not springing without a +catch, in protecting the captured animal and in permitting easy removal +of the animal from the trap. A wooden box was placed inside the metal +shelter attached to each trap and, in winter, cotton batting or woolen +scraps were placed inside the boxes for nesting material. With this +insulation against the cold, voles could survive the night unharmed and +could even deliver their litters successfully. In summer the nesting +material was removed but the wooden box was retained as insulation +against heat. + +Bait used in live-traps was a mixture of cracked corn, milo and wheat, +purchased at a local feed store. The importance of proper baiting, +especially in winter, has been emphasized by Howard (1951) and Llewellyn +(1950) who found an adequate supply of energy-laden food, such as corn, +necessary in winter to enable small rodents to maintain body temperature +during the hours of captivity. The rare instances of death of voles in +traps in winter were associated with wet nesting material, as these +animals can survive much lower temperatures when they are dry. Their +susceptibility to wet and cold was especially evident in rainy weather +in February and March. + +Preventing mortality in traps was more difficult in summer than in +winter. The traps were set in any available shade of tall grass or +weeds; or when such shade was inadequate, vegetation was pulled and +piled over the nest boxes. The traps usually were faced north so that +the attached number-ten cans, which served as shelters, cast shadows +over the hardware cloth runways during midday. Even these measures were +inadequate when the temperature reached 90°F. or above. Such high +temperatures rarely occurred early in the day, however, so that removal +of the animals from traps between eight and ten a. m. almost eliminated +mortality. Those individuals captured in the night were not yet harmed, +but it was already hot enough to reduce the activity of the voles and +prevent further captures until late afternoon. When it was necessary to +run trap lines earlier, the traps were closed in the morning and reset +in late afternoon. + +Reactions of small mammals to live-traps and the effects of prebaiting +were described by Chitty and Kempson (1949). In general, the results of +my trapping program fit their conclusions. Each of my trapping periods, +consisting of seven to ten consecutive days, showed a gradual increase +in the number of captures per day for the first three days, with a +tendency for the number of captures to level off during the remainder of +the period. Leaving the traps baited and locked open for a day or two +before a trapping period tended to increase the catch during the first +few days of the period without any corresponding increase during the +latter part of the period. Initial reluctance of the voles to enter the +traps decreased as the traps became familiar parts of their environment. + +At the beginning of the study the traps were set in a grid with +intervals of 20 feet. The interval was increased to 30 feet after three +months because a larger area could thus be covered and no loss in +trapping efficiency was apparent. The traps were set within a three foot +radius of the numbered stations, and were locked and left in position +between trapping periods. + +Each individual that was captured was weighed and sexed. The resulting +data were recorded in a field notebook together with the location of the +capture and other pertinent information. Newly captured voles were +marked by toe-clipping as described by Fitch (1952:32). Information was +transferred from the field notebook to a file which contained a separate +card for each individual trapped. + +In the course of the program of live-trapping, many marked voles were +recaptured one or more times. Most frequently captured among the females +were number 8 (33 captures in seven months) and number 73 (30 captures +in eight months). Among the males, number 37 (21 captures in six months) +and number 62 (21 captures in eight months) were most frequently taken. +The mean number of captures per individual was 3.6. For females, the +mean number of captures per individual was 3.8 and for males it was 3.4. +Females seemingly acquired the habit of entering traps more readily than +did males. No correlation between any seasonally variable factor and the +number of captures per individual was apparent. To a large degree, the +formation of trap habits by voles was an individual peculiarity. + +In order to study the extent of utilization of various habitats by +_Microtus_, a number of areas were sampled with Museum Special +snap-traps. These traps were set in linear series approximately 25 feet +apart. The number of traps used varied with the size of the area sampled +and ranged from 20 to 75. The lines were maintained for three nights. +The catch was assumed to indicate the relative abundance of _Microtus_ +and certain other small mammals but no attempt to estimate actual +population densities from snap-trapping data was made. In August, 1952, +when the live-trapping program was concluded, the study areas were +trapped out. The efficiency of the live-trapping procedure was +emphasized by the absence of unmarked individuals among the 45 voles +caught at that time. + +Further details of the methods and procedures used are described in the +appropriate sections which follow. + + + + +HABITAT + + +Although other species of the genus _Microtus_, especially _M. +pennsylvanicus_, have been studied intensively in regard to habitat +preference (Blair, 1940:149; 1948:404-405; Bole, 1939:69; Eadie, 1953; +Gunderson, 1950:32-37; Hamilton, 1940:425-426; Hatt, 1930:521-526; +Townsend, 1935:96-101) little has been reported concerning the habitat +preferences of _M. ochrogaster_. Black (1937:200) reported that, in +Kansas, _Microtus_ (mostly _M. ochrogaster_) preferred damp situations. +_M. ochrogaster_ was studied in western Kansas by Brown (1946:453) and +Wooster (1935:352; 1936:396) and found to be almost restricted to the +little-bluestem association of the mixed prairie (Albertson, 1937:522). +Brumwell (1951:213), in a survey of the Fort Leavenworth Military +Reservation, found that _M. ochrogaster_ preferred sedge and bluegrass +meadows but occurred also in a sedge-willow association. Dice (1922:46) +concluded that the presence of green herbage, roots or tubers for use as +a water source throughout the year was a necessity for _M. ochrogaster_. +Goodpastor and Hoffmeister (1952:370) found _M. ochrogaster_ to be +abundant in a damp meadow of a lake margin in Tennessee. In a study made +on and near the campus of the University of Kansas, within a few miles +of the area concerned in the present report, Jameson (1947:132) found +that voles used grassy areas in spring and summer, but that in the +autumn, when the grass began to dry, they moved to clumps of Japanese +honeysuckle (_Lonicera japonica_) and stayed among the shrubbery +throughout the winter. Johnson (1926:267, 270) found _M. ochrogaster_ +only in uncultivated areas where long grass furnished adequate cover. He +stated that the entire biotic association, rather than any single +factor, was the key to the distribution of the voles. None of these +reports described an intensive study of the habitat of voles, but the +data presented indicate that voles are characteristic of grassland and +that _M. ochrogaster_ can occupy drier areas than those used by _M. +pennsylvanicus_. Otherwise, the preferred habitats of the two species +seem to be much the same. + +In the investigation described here I attempted to evaluate various +types of habitats on the basis of their carrying capacity at different +stages of the annual cycle and in different years. The habitats were +studied and described in terms of yield, cover and species composition. +The areas upon which live-trapping was done were studied most +intensively. + +These two areas, herein designated as House Field and Quarry Field, were +both occupied by voles throughout the period of study. Population +density varied considerably, however (Fig. 5). Both of these areas were +dominated by _Bromus inermis_, and, in clipped samples taken in June, +1951, this grass constituted 67 per cent of the vegetation on House +Field and 54 per cent of the vegetation on Quarry Field. Estimates made +at other times in 1950, 1951 and 1952 always confirmed the dominance of +smooth brome and approximated the above percentages. Parts of House +Field had nearly pure stands of this grass. Those traps set in spots +where there was little vegetation other than the dominant grass caught +fewer voles than traps set in spots with a more varied cover. _Poa +pratensis_ formed an understory over most of the area studied, +especially on House Field, and attained local dominance in shaded spots +on both fields. The higher basal cover provided by the _Poa_ understory +seemed to support a vole population larger than those that occurred in +areas lacking the bluegrass. Disturbed situations, such as roadsides, +were characterized by the dominance of _Bromus japonicus_. This grass +occurred also in low densities over much of the study area among _B. +inermis_. Other grasses present included _Triodia flava_, common in +House Field, but with only spotty distribution in Quarry Field; _Elymus +canadensis_, distributed over both areas in spotty fashion and almost +always showing evidence of use by voles and other small mammals; +_Aristida oligantha_ and _Bouteloua curtipendula_, both more common on +the higher and drier Quarry Field; _Panicum virgatum_, _Setaria_ spp., +especially on disturbed areas; and three bluestems, _Andropogon +gerardi_, _A. virginicus_ and _A. scoparius_. The bluestems increased +noticeably during the study period (even though grasses in general were +being replaced by woody plants) and they furnished a preferred habitat +for voles because of their high yield of edible foliage and relatively +heavy debris which provided shelter. + +On House Field the most common forbs were _Vernonia baldwini_, _Verbena +stricta_ and _Solanum carolinense_. On Quarry Field, _Solidago_ spp. and +_Asclepias_ spp. were also abundant. All of them seemed to be used by +the voles for food during the early stages of growth, when they were +tender and succulent. The fruits of the horse nettle (_Solanum +carolinense_) were also eaten. The forbs themselves did not provide +cover dense enough to constitute good vole habitat. Mixed in a grass +dominated association they nevertheless raised the carrying capacity +above that of a pure stand of grass. Other forbs noted often enough to +be considered common on both House Field and Quarry Field included +_Carex gravida_, observed frequently in House Field and less often in +Quarry Field; _Amorpha canescens_, more common in Quarry Field; +_Tradescantia bracteata_, _Capsella bursapastoris_, _Oxalis violacea_, +_Euphorbia marginata_, _Convolvulus arvensis_, _Lithospermum arvense_, +_Teucrium canadense_, _Physalis longifolia_, _Phytolacca americana_, +_Plantago major_, _Ambrosia trifida_, _A. artemisiifolia_, _Helianthus +annuus_, _Cirsium altissimum_ and _Taraxacum erythrospermum_. Both areas +were being invaded from one side by forest-edge vegetation; the woody +plants noted included _Prunus americana_, _Rubus argutus_, _Rosa +setigera_, _Cornus drummondi_, _Symphoricarpus orbiculatus_, _Populus +deltoides_ and _Gleditsia triacanthos_. + +In House Field the herbaceous vegetation was much more lush than in +Quarry Field and woody plants and weeds were more abundant. A graveled +and heavily used road along one edge of House Field, leading to the +Reservation Headquarters, was a barrier which voles rarely crossed. A +little-used dirt road crossing the trapping plot in Quarry Field +constituted a less effective barrier. The disturbed areas bordering the +roads were likewise little used and tended to reinforce the effects of +the roads as barriers. There were almost pure stands of _Bromus +japonicus_ along both roads. No mammal of any kind was taken in traps +set where this grass was dominant. + +Because seasonal changes in vole density followed the curve for rate of +growth of the complex of grasses on the Reservation, and because years +in which there was a sparse growth of plants due to dry weather showed a +decrease in the density of voles, the relationships between productivity +of plants and vole population levels on the two study areas were +investigated. In both fields the composition of the plant cover was +similar, and the differences were chiefly quantitative. In June, 1951, +ten square-meter quadrats were clipped on each of the areas to be +studied. The clippings from each were dried in the sun and weighed. From +Quarry Field the mean yield amounted to 1513 ± 302 lbs. per acre; while +from House Field the yield was 2351 ± 190 lbs. per acre (Table 1). Using +experience gained in making these samples, I periodically estimated the +relative productivity of the two areas. House Field was from 1.5 to 3 +times as productive as Quarry Field during the growing seasons of 1951 +and 1952. Although House Field, being more productive, usually supported +a larger population of voles than Quarry Field the reverse was true at +the time of the clipping (Fig. 5). + + TABLE 1. RELATIONSHIP BETWEEN YIELD AND VARIOUS POPULATION DATA + + ====================================================================== + House Field Quarry Field + ---------------------------------------------------------------------- + Yield in June, 1951, lbs./acre 2351 ± 190 1513 ± 302 + _Microtus_, June, 1951, gms./acre 3867 5275 + Per cent immature _Microtus_, June, 1951 29.85 38.02 + Ratio _Microtus_, June/March 0.73 2.63 + _Sigmodon_, June, 1951, gms./acre 1376 746 + Per cent immature _Sigmodon_, June, 1951 35.72 44.44 + Ratio _Sigmodon_, June/March 1.40 2.25 + _Microtus-Sigmodon_, June, 1951, gms./acre 5243 6021 + _Microtus_ mean, gms./acre/month 2922 1831 + _Sigmodon_ mean, gms./acre/month 802 335 + _Sigmodon-Microtus_, gms./acre/month 3728 2166 + ---------------------------------------------------------------------- + +Although no explanation was discovered which accounted fully for the +seeming aberration, two sets of observations were made that may bear on +the problem. In June, 1951, the population of voles and cotton rats on +Quarry Field was increasing rapidly whereas in House Field that trend +was reversed. The trends were reflected by the percentages of immature +individuals in the two populations and by the ratios of the June, 1951, +densities to the March, 1951, densities (Table 1). Perhaps the density +curve was determined in part by factors inherent in the population and, +to that extent, was fluctuating independently of the environment +(Errington, 1946:153). + +The flood in 1951 reduced the population of voles and obscured the +normal seasonal trends. Although House Field produced a heavier crop of +vegetation, Quarry Field produced a larger crop of rodents, chiefly +_Microtus_ and _Sigmodon_. In House Field, however, the ratio of +_Sigmodon_ to _Microtus_ was notably higher. Presumably the cotton rats +competed with the voles and exerted a depressing effect on their +numbers. The intensity of the effect seemed to depend on the abundance +of both species. That this depressing effect involved more than direct +competition for plant food was suggested by the fact that in House +Field, with a heavy crop of vegetation and a seemingly high carrying +capacity for both herbivorous rodents, the biomass of voles, and of all +rodents combined, were lower than in Quarry Field which had less +vegetation and fewer cotton rats. The relationships between voles and +cotton rats are discussed further later in this report. + +When the centers of activity (Hayne, 1949b) of individual voles were +plotted it was seen that there was a shift in the places of high density +of voles on the trapping areas. This shift seemed to be related to the +advance of the forest edge with such woody plants as _Rhus_ and +_Symphoricarpos_ and young trees invading the area. These shifts were +clearly shown when the distribution of activity centers on both areas in +June, 1951, was compared with the distribution in June, 1952 (Fig. 1). +The shift was gradual and the more or less steady progress could be +observed by comparing the monthly trapping records. It was perhaps +significant that during the summers the centers of activity were less +concentrated than during the winter. The shift of voles away from the +woods was more nearly evident in winter when the voles were driven into +areas of denser ground cover, which provided better shelter. + +[Illustration: FIG. 1. Progressive encroachment of woody vegetation onto +study areas, and the accompanying shift of the centers of populations of +voles. Activity centers of individuals were calculated as described by +Hayne (1949b) and are indicated by dots. The cross-hatched areas show +places where the vegetation was influenced by the shade of woody +plants.] + +From 1948 to 1950 and again in 1952 and 1953 I trapped in various +habitat types in a mixed prairie near Hays, Kansas. Before the great +drought of the thirties, _Microtus ochrogaster_ was the most common +species of small mammal in that area. Since 1948, at least, it has been +taken only rarely and from a few habitats. No voles have been taken from +grazed sites. In a relict area, voles were trapped in a lowland +association dominated by big bluestem. Since 1948 only one vole has been +trapped in the more extensive hillside association characterized by a +mixture of big bluestem, little bluestem and side-oats grama. None was +taken in the upland parts of the relict area where buffalo grass and +blue grama dominated the association. + +In the pastured areas there are nine livestock exclosures established by +the Department of Botany of Ft. Hays Kansas State College. These +exclosures included many types of habitat found in the mixed prairie. +All of these exclosures were trapped and voles were taken in only two of +them. An exclosure situated near a pond, on low ground producing a +luxuriant growth of big bluestem and western wheat grass, has supported +voles in 1948, 1949, 1952 and 1953. An upland exclosure containing only +short grasses also supported a few voles in 1953. + +An examination of the nature of the various plant associations of the +mixed prairie indicates that yield of grasses, amount of debris and +basal cover may be critical factors in the distribution of voles. The +association to which the voles seemed to belong was the lowland +association. Hopkins _et al_ (1952:401; 409) reported the yield of +grasses from the lowland to be approximately twice as great as from the +hillside and upland in most years. Probably equally important to the +voles was the fact that debris accumulation in the lowland was +approximately five times as great as in the upland and approximately +2.5 times as great as on the hillside (Hopkins, unpublished data). The +unexpected presence of voles in the short grass exclosure was probably +due to two factors. In ungrazed short grass, basal cover may reach 90 +per cent (Albertson, 1937:545), thus providing excellent cover for +voles. Also, the ungrazed exclosure had greater yield and a thicker mat +of debris than the grazed short grass surrounding it and was thus a +relatively good habitat, although it did not compare favorably with the +lowland type. + +Samples of the populations of various areas, obtained by snap-trapping, +gave further information regarding the types of vegetation preferred by +voles. Voles were taken in all ungrazed and unmown grasslands trapped in +eastern Kansas, although some of the areas were not used at all seasons +of the year nor in years having a low population of _Microtus_. Reithro +Field, similar to Quarry Field in its general aspect, had a heavy +population of voles in the spring and summer of 1951, a time when voles +were generally abundant. On the same area the population of small +mammals was sampled in the summer of 1949 and, though occasional sign of +voles was seen, not one vole was trapped. Later trapping, in the spring +and summer of 1952, also failed to catch any voles and Fitch (1953, _in +litt._) caught none in several trapping attempts in 1953. These later +times were characterized by a general scarcity of voles. Reithro Field +was drier, with less dense vegetation, than the two main study areas and +had larger percentages of little bluestem (_Andropogon scoparius_) and +side-oats grama (_Bouteloua curtipendula_) and smaller percentages of +_Vernonia_, _Verbena_, _Solanum_ and _Solidago_. + +Various species of foxtail (_Setaria_) dominated most roadsides in the +vicinity of the Reservation. Voles almost always used these strips of +grass but never were abundant in them. Voles were taken near the margin +of a weedy field, fallow since 1948, but there was none in the middle of +the field. Most individuals were confined to the grassy areas around the +field and made only occasional forays away from the edge. The dam of a +small pond on the Reservation and low ground near the water were used by +_Microtus_ at all times. In the summer of 1949 no voles were taken +anywhere on the Reservation but their runways were more abundant around +the pond than in the other places examined. Of all the areas studied in +the summer of 1949, only the pond area had been protected from grazing +in previous years. _Polygonum coccineum_ was the most prominent plant in +the pond edge association. A few voles were trapped in large openings in +the woods, where a prairie vegetation remained and where voles seemingly +lived in nearly isolated groups. + +Voles were rarely taken in grazed or mown grassland or in fields of +alfalfa, stubble or row crops. The critical factor in these cases seemed +to be the absence of debris or other ground cover under which runways +and nests could be concealed satisfactorily. Woods, rocky outcroppings +and bare ground were not used regularly by voles. Fitch (1953, _in +litt._) has taken several _Microtus_ in reptile traps set along a rocky +ledge in woods but most of these voles were subadult males and seemed to +be transients. Fields in the early stages of succession also failed to +support a population of voles. Such areas on the Reservation were +characterized by giant ragweed, horse weed, thistles and other coarse +weeds. Basal cover was low and debris scanty. Not until an understory of +grasses was established did a population of voles appear on such areas. +The coarse weeds seemed to provide neither food nor cover adequate for +the needs of the voles. + +An analysis of trapping success at each station in House Field further +clarified habitat preferences. The tendency of voles to avoid woody +vegetation was again demonstrated. Not only was the population +concentrated on that part of the study plot farthest from the forest +edge but, as a general rule, voles tended to avoid single trees or +clumps of shrubby plants wherever these occurred on the area. As an +example, trap number 18 never caught more than one per cent of the +monthly catch and in many trapping periods caught nothing. This trap was +under a wild plum tree. Adjacent traps often were entered; the general +area was the most heavily populated part of the study plot. Only under +the plum tree was there a relatively unused portion. + +Traps number 29 and 30, in the shade of a large honey locust tree, also +caught but few voles. Trap number 30 was only six feet from the base of +the tree and caught but one vole throughout the study period. These two +traps caught more _Peromyscus leucopus_ than any other pair, however, +and both of them also caught pine voles (_M. pinetorum_). The area +shaded by this tree permitted an extension of parts of the forest edge +fauna into the grassland. + +In spite of the marked general tendency to avoid woody plants, some +voles made their runways around the roots of blackberry bushes, sumac +and wild plum trees. Some nests were found under larger roots, as if +placed there for protection. More vegetation was found under the woody +plants which the voles chose to use for shelter than under those which +they avoided. It seemed probable that the actual condition avoided by +voles was the bareness of the ground (a result of the shade cast by the +woody plants) rather than the woody plants themselves. + +Running diagonally across the eastern half of the trapping plot in House +Field there was a terracelike ridge of soil. On each side of this ridge +there was a slight depression. Observations of the study plot in the +growing season showed this strip to produce the most luxuriant +vegetation of any part of the plot. Clip-quadrat studies confirmed this +observation and showed the bluegrass understory to be especially heavy. +This strip included the areas trapped by traps numbered 4, 5, 17, 18, +22, 23 and 37. With the exception of trap number 18, discussed above, +these traps consistently made more captures than traps set in other +parts of the plot. In winter, these traps also caught more harvest mice +(_Reithrodontomys megalotis_) than any other comparable group of traps. + +Although the amount of growing tissue of plants probably is at least as +important to voles as the total amount of vegetation, some correlation +seemed to exist between the density of grassy vegetation and the density +of populations of voles. A mixed stand of grasses, with an obvious weedy +component, can support a larger population of voles than can either a +nearly pure stand of grass or the typical early seral stages dominated +by weeds. Probably the more or less continual supply of young plants +provided preferred food easily available to voles. A more homogeneous +vegetation would tend to pass through the young and tender stage as a +unit, thus causing a feast to be followed by a relative famine. + + + + +POPULATION STRUCTURE + + +During the period of study the percentage of males in most of my samples +was less than 50 per cent (Fig. 2). Only once, in June, 1952, did the +mean percentage of males in samples from three areas (House Field, +Quarry Field, Fitch traps) exceed that level and then it was only 50.1 +per cent. On several occasions, however, the percentage of males in a +sample from a single area was slightly above 50 per cent. The highest +percentage of males recorded was 56.69 per cent, in a sample taken from +the Quarry Field population in June, 1952. In the samples taken in +April, 1952, the mean percentage of males was 39.67 per cent, the lowest +mean recorded. The low point for one sample was 28.02 per cent in +August, 1952, from Quarry Field. The mean percentage of males in all +samples taken was 45.02 ± 2.72 per cent. Percentages observed would +occur in random samples taken from a population with 50 per cent males +less than one per cent of the time. Exactly 50 per cent of the young in +the 65 litters examined were classified as males but the sample was +small and the sexing of newborn individuals was difficult. + +[Illustration: FIG. 2. Graphs of population structure showing the +monthly changes in the mean percentages of juveniles, subadults, adults +and males in samples from the three study areas.] + +The extent to which sex ratios in samples were affected by trapping +procedure was not determined. A possibility considered was that the +greater wandering tendency of males (Blair, 1940:154; Hamilton, +1937c:261; Townsend, 1935:98) impaired the formation of trap habits +(Chitty and Kempson, 1949:536) on their part and thus unbalanced the sex +ratios of the samples. If this were the explanation, the apparent sex +ratio on larger areas would more nearly approximate the true ratio, and +the frequency of capture of females would exceed that of males. The +evidence is somewhat equivocal. In the populations described here the +mean number of captures per individual per month was 2.31 for females, +which was significantly greater (at the one per cent level) than the +2.20 captures per individual per month which was the mean number for +males. This difference supports the idea that differences in habits +between the sexes result in distorted sex ratios in samples obtained by +live-trapping. Mean percentages of males did not, however, differ +significantly between the House Field-Quarry Field samples and the +samples from the Fitch trapping area, nearly five times as large. + +Three age classes, juvenal, subadult and adult, were separated on the +basis of condition of pelage. The percentage of adults in populations +varied seasonally (Fig. 2). January, February and March were the months +when the adult fraction of the population was highest and October and +November were low points, with May and June showing percentages almost +as low. The only marked variation in this seasonal pattern occurred in +July and August, 1952, when the percentage of adults rose sharply. This +was due to a depression in the reproductive rate during the dry summer +of 1952, which is discussed later in this report. Juveniles made up only +a small fraction of the population from December through March and a +relatively large fraction in the October-November and May-June periods +(Fig. 2). Again, July and August of 1952 were exceptions to the pattern +as the percentages of juveniles in these months fell to midwinter +levels. As expected, the curve of the percentages of subadults in the +population followed that of the juveniles and preceded that of the +adults. The mean percentages for the thirty month period for which data +were available were: adults, 77.72 ± 4.48 per cent; subadults, 14.06 ± +3.14 per cent; and juveniles, 8.22 ± 2.62 per cent. Seasonal and yearly +changes in the population structure occurred, with notable variation in +the ratio of breeding females to the entire population, as discussed in +this report under the heading of reproduction. + +Since some of the juveniles did not move enough to be readily trapped, +the real percentage of juveniles in the population was probably far +greater than that shown by trapping data. I tried, therefore, to +estimate the number of juveniles on the study plot each month by +multiplying the number of lactating females by the mean litter size. As +expected, the results were consistently higher than the estimate based +on trapping data. The discrepancy was largest in April, May, June and +October. During the winter there was no important difference between the +two estimates. Even when the discrepancy was greatest, the estimated +weight of the juveniles missed by trapping was not large enough to +modify the picture of habitat utilization in any important way. I chose, +therefore, to count only those juveniles actually trapped. Although +probably consistently too low, such a figure seemed more reliable than +an estimate made on any other basis. + +[Illustration: FIG. 3. Percentages of individuals captured each month +surviving in subsequent months. The graph shows differential survival +according to time of birth. Individuals born in autumn seem to have a +longer life expectancy. The numbers on the lines refer to months of +first capture.] + +A study of the age groups in each month's population revealed a +differential survival based on the season of birth. Blair (1948:405) +found that chances of survival in _Microtus pennsylvanicus_ were +approximately equal throughout the year. In the present populations of +_M. ochrogaster_, however, voles born in October, November, December and +January tended to live longer than those born in other months (Fig. 3). +Presumably these animals, born in autumn and early winter, were more +vigorous than their older competitors and were therefore better able to +survive the shrinking habitat of winter. Their continued survival after +large numbers of younger voles had been added to the population probably +was permitted by the expanding habitat of spring and summer. The +percentage of the population surviving the winter of 1951-1952 was +approximately double the percentage surviving the winter of 1950-1951. +This difference seemed to be due to the smaller population entering the +winter of 1951-1952 rather than any major difference in the +environmental resistance. + +As a consequence of the differential survival, most of the breeding +population in the spring was made up of animals born the previous +October and November. Fig. 4 shows that in February, when the percentage +of breeding females ordinarily began to rise, 51.6 per cent of the +population was born in the previous October and November. Voles born in +these two months continued to form a large part of the population +through March (45.1 per cent), April (38.5 per cent), May (23.9 per +cent), June (18.7 per cent) and July (16.2 per cent) (Fig. 4). These +percentages suggest that the habitat conditions in October and November +were probably important in determining the population level for at least +the first half of the next year. + +[Illustration: FIG. 4. Differential survival of voles according to month +when first caught. Each column represents the percentage of the monthly +sample first caught in each of the preceding months. Those voles caught +first in October and November survived longer than those first caught in +other months. Relatively few individuals remained in the population as +long as one year.] + + + + +POPULATION DENSITY + + +Population densities were ascertained on the study areas by means of the +live-trapping program. Blair (1948:396) stated that almost all small +mammals old enough to leave the nest (except shrews and moles) are +captured by live-trapping. My experience, and that of other workers on +the Reservation, requires modification of such a statement. The distance +between traps is an important factor in determining the efficiency of +live-trapping. As mentioned earlier, when House Field and Quarry Field +were trapped out at the conclusion of the live-trapping program no +unmarked voles were taken. This showed that the 30 foot interval between +traps was short enough to cover the area as far as _Microtus_ was +concerned. The fact that unmarked adults were caught almost entirely in +marginal traps is additional evidence. On the other hand, the Fitch +traps were 50 feet apart and voles seemed to have lived within the grid +for several months before being captured. Fitch (1954:39) has shown that +some kinds of small mammals are missed in a live-trapping program +because of variation in bait acceptance, both seasonal and specific. + +A few individuals, missed in a trapping period, were captured again in +subsequent months. These voles were assumed to have been present during +the month in which they were not caught. The area actually trapped each +month was estimated by a modification of the method proposed by Stickel +(1946:153). The average maximum move was calculated each month and a +strip one half the average maximum move in width was added to each side +of the study area actually covered by traps. The study plots were +bounded in part by gravel roads and forest edge acting as barriers, and +for these parts no marginal strip was added. Trap lines on the opposite +sides of these roads rarely caught marked voles that had crossed in +either direction. It is perhaps advisable to say here that the size of +House Field and Quarry Field study plots (0.56 acres) was too small for +best results in estimating population levels (Blair, 1941:149). In the +computations of population levels the data for males and females were +combined, because no significant difference between the average maximum +move of the sexes was apparent. + +Fluctuations of the populations were graphed in terms of individuals per +acre (Fig. 5). The variation was great in the 30 month period for which +data were available, and was both chronological and topographical. The +lowest density recorded was 25.2 individuals per acre and the highest +density was 145.8 individuals per acre. The weight varied from a low of +847 grams per acre to a high of 5275 grams per acre. + +[Illustration: FIG. 5. Variations in density of voles from three +populations, as shown by live-trapping, and the mean density of these +populations. Juveniles are not represented in their true numbers since +many voles were caught first as subadults. The samples from the Fitch +trap line were incomplete due to the wide spacing of the traps.] + +There are few records of density of _M. ochrogaster_ in the literature. +Brumwell (1951:213) found nine individuals per acre in a prairie on the +Fort Leavenworth Military Reservation and Wooster (1939:515) reported +38.5 individuals per acre for _M. o. haydeni_ in a mixed prairie in +west-central Kansas. High densities for _M. pennsylvanicus_ reported in +the literature include 29.8 individuals per acre (Blair, 1948:404), 118 +individuals per acre (Bole, 1939:69), 160-230 individuals per acre +(Hamilton, 1937b:781) and 67 individuals per acre (Townsend, 1935:97). + +Because the study period included one period of unusually high rainfall +and one year of unusually low rainfall, the normal pattern of seasonal +variation of population density was obscured. An examination of the data +suggested, however, that the greatest densities were reached in October +and November with a second high point in the April-May-June period. +These high points generally followed the periods of high levels of +breeding activity (Fig. 8). The autumn rise in population may have been +due, in part, to the addition of spring and early summer litters to the +breeding population, but the rise occurred too late in the year to be +explained by that alone. Another factor may have been the spurt in +growth of grasses occurring in Kansas in early autumn, in September and +October. There was a seeming correlation between high rainfall with +rapid growth of grasses and reproductive activity, and, secondarily with +high population densities of voles. These relationships are discussed in +connection with reproduction. Lowest annual densities were found to +occur in January when there is but little breeding activity and when +rainfall is low and plant growth has ceased. + +Marked deviation from the usual seasonal trends accompanied flood and +drought. In the flood of July, 1951, although the study areas were not +inundated, the ground was saturated to the extent that every footprint +at once became a puddle. Immediately after the floods, on all three +areas studied, populations were found to have been drastically reduced. +The effect was most severe on the population of House Field, the lowest +area studied, and the recovery of the population there was much slower +than that of those on the other study areas (Fig. 5). Newborn voles were +killed by the saturated condition of the ground in which they lay. The +more precocious young of _Sigmodon hispidus_ survived wetting better. +They thus acquired an advantage in the competitive relationship between +cotton rats and voles. These relationships are discussed more fully in +the section on mammalian associates of _Microtus_. + +Adverse effects of heavy rainfall on populations of small mammals have +been reported by Blair (1939) and others. Goodpastor and Hoffmeister +(1952:370) reported that inundation sharply reduced populations of _M. +ochrogaster_ for a year after flooding but that the area was then +reoccupied by a large population of voles. Such a reoccupation may have +begun on the areas of this study in the spring of 1952 when the upward +trend of the population was abruptly reversed by drought. While cotton +rats were abundant their competition may have been an important factor +in depressing population levels of voles. The population of voles began +to rise only after the population of cotton rats had decreased (Fig. +19). + +In the unusually dry summer of 1952, there was a marked decline of +population levels beginning in June and continuing to August when my +field work was terminated. Dr. Fitch (1953, _in litt._) informed me that +the decline continued through the winter of 1952-53 and into the summer +of 1953, until daily catches of _Microtus_ on the Reservation were +reduced to 2-10 per cent of the number caught on the same trap lines in +the summer of 1951. The drought seemed to affect population levels by +inhibiting reproduction, as described elsewhere in this report. A +similar sensitivity to drought was reported by Wooster (1935:352) who +found _M. o. haydeni_ decreased more than any other species of small +mammal after the great drought of the thirties. + +No evidence of cycles in _M. ochrogaster_ was observed in this +investigation. All of the fluctuations noted were adequately explained +as resulting from the direct effects of weather or from its indirect +effect in determining the kinds and amounts of vegetation available as +food and shelter. + +The differences in densities supported by the various habitats were +discussed earlier in connection with the analysis of habitats. + + + + +HOME RANGE + + +Home ranges were calculated for individual voles according to the method +described by Blair (1940:149-150). The term, home range, is used as +defined by Burt (1943:350-351). Only those voles captured at least four +times were used for the home range studies. Individuals which included +the edge of the trap grid in their range were excluded unless a barrier +existed (see description of habitat) confining the seeming range to the +study area. + +The validity of home range calculations has been challenged (Hayne, +1950:39) and special methods of determining home range have been +advocated by a number of authors. The ranges calculated in this study +are assumed to approximate the actual areas used by individuals and are +considered useful for comparison with other ranges calculated by similar +methods, but no claim to exactness is intended. It is obvious, for +instance, that many plotted ranges contain so-called blank areas which, +at times, are not actually used by any vole (Elton, 1949:8; Mohr, +1943:553). Studies of the movements of mammals on a more detailed scale, +perhaps by live-traps set at shorter intervals and moved frequently, are +needed to increase our understanding of home range. + +In order to test the reliability of the range calculated, an examination +of the relationship between the size of the seeming range and the number +of captures was made. For the first three months, trapping on House +Field was done with a 20 foot grid and throughout the remainder of the +study a 30 foot grid was used. The effect of these different spacings on +the size of the seeming home range was also investigated. Hayne +(1950:38) found that an increase in the distance between traps caused an +increase in the size of the seeming home range, but in my study the +increased interval between traps was not accompanied by any change in +the sizes of the calculated ranges. + +The number of captures, above the minimum of four, did not seem to be a +factor in determining the size of the calculated monthly range. A +seeming relationship was observed between the number of times an +individual was trapped and the total area used during the entire time +the vole was trapped. Closer examination revealed that the most +important factor was the length of time over which the vole's captures +extended. Table 2 shows the progressive increase in sizes of the mean +range of animals taken over periods of time from one month to ten +months. + + TABLE 2. RELATIONSHIP BETWEEN HOME RANGE SIZE AND LENGTH OF TIME ON THE + STUDY AREA + + ====================================================================== + No. months on area 1 2 3 4 5 6 7 8 9 10 + Mean range in acres .09 .09 .10 .14 .13 .17 .22 .22 .26 .24 + ---------------------------------------------------------------------- + +Nothing concerning the home range of _Microtus ochrogaster_ was found in +the literature. Several workers, including Blair (1940) and Hamilton +(1937c), have studied the home range of _M. pennsylvanicus_. Blair +(1940:153) reported a larger range for males than for females in all +habitats and in all seasons represented in his sample. In _M. +ochrogaster_, however, I found that the mean monthly range for both +sexes was 0.09 of an acre. Blair (_loc. cit._) reported no individuals +with a range so small as that mean, but Hamilton (_op. cit._:261) +mentioned two voles with ranges of less than 1200 square feet. The mean +total range used by an individual during the entire time it was being +trapped showed a slight difference between the sexes. Males used an +average of 0.14 of an acre whereas females used an average of but 0.12 +of an acre. This suggested that, as in _M. pennsylvanicus_ (Hamilton, +_loc. cit._), males tended to wander more than females and to shift +their home range more often. + +The largest monthly range recorded was 0.28 of an acre used by a female +in March, 1951, and calculated on the basis of four captures. The +largest monthly range of a male was 0.25 of an acre for a vole caught +eight times in November, 1950. The smallest monthly range was 0.02 of an +acre; several individuals of both sexes were restricted to areas of this +size. Juveniles, not included in the home range study, were usually +restricted to 0.01 or, at most, 0.02 of an acre. Seasonal differences in +the sizes of home ranges were not significant. However, the voles caught +in the winter often enough to be used for home range studies were too +few for a thorough study of seasonal variation in the size of home +ranges. + +One female was captured 22 times in the seven-month period of October, +1950, to April, 1951. She used an area of 0.83 of an acre, but this +actually comprised two separate ranges. From October, 1950, through +December, 1950, she was taken 17 times within an area of 0.12 of an +acre; and from January, 1951, to April, 1951, she was taken five times +within an area of 0.15 of an acre. The largest area assumed to represent +one range of a female was 0.38 of an acre, recorded on the basis of six +captures in three months. The largest area encompassed by the record of +an individual male was 0.41 of an acre. He, too, shifted his range, +being taken five times on an area of 0.07 of an acre and twice, two +months later, on an area of 0.09 of an acre. Presumably, the remainder +of his calculated total range was used but little, or not at all. The +largest single range of a male was 0.36 of an acre, calculated on the +basis of 18 captures in seven months. The smallest total range for both +sexes was 0.02 of an acre. + +Many voles shifted their home range and a few did so abruptly. The large +range of a female vole, described above and plotted in Fig. 6, indicated +an abrupt shift from one home range to another. More common is a gradual +shift as indicated by the range of the male shown in Fig. 7. Large parts +of each monthly range of this vole overlapped the area used in other +months but his center of activity shifted from month to month. + +[Illustration: FIG. 6. Map with cross-hatched areas showing the range of +vole #20 (female). Dots show actual points of capture at permanent trap +stations 30 feet apart. Vertical lines mark area in which vole was taken +17 times in October and November, 1950. Horizontal lines mark area in +which vole was taken five times in March and April, 1951. This vole was +not captured in December and January.] + +[Illustration: FIG. 7. Map showing range of vole #52 (male) with seeming +shifts in its center of activity. Dots show actual points of capture at +permanent trap stations 30 feet apart. Solid line encloses points of six +captures in October and November, 1950. Broken line encloses points of +five captures in February and March, 1951. Dotted line encloses points +of nine captures in April, May and June, 1951.] + +That home ranges overlapped was demonstrated by frequent capture of two +or more individuals together in the same trap. No territoriality has +been reported in any species of _Microtus_, to my knowledge, and my +voles showed no objection to sharing their range. Voles taken from the +field into the laboratory lived together in pairs or larger groups +without much friction. + +Definable systems of runways and home ranges were not coextensive. +Runway systems tended to merge, as described later in this report, and +relationships between them and home range were not apparent. Home ranges +had no characteristic shape. + + + + +LIFE HISTORY + + +Reproduction + +Reproductive activity might have been measured in a number of ways. +Three indicators were tested: the percentage of females gravid or +lactating, the percentage of juveniles in the month following the +sampling period, and the percentage of females with a vaginal orifice in +the sampling period. The condition of vagina proved to be most useful. +Whether or not there is a vaginal cycle in _Microtus_ is uncertain. +Bodenheimer and Sulman (1946:255-256) found no evidence of such a cycle, +nor did I in my work with laboratory animals at Lawrence. How much the +artificial environment of the laboratory affected these findings is +unknown. The presence of an orifice seemed to indicate sexual activity +(Hamilton, 1941:9). The percentage of gravid females in the population +could not be determined accurately by a live-trapping study and was not +useful in this investigation. The percentage of juveniles trapped in the +month following the sampling period tended to follow the curve of the +percentage of adult females with a vaginal orifice. The ratio of trapped +juveniles to adults trapped was a poor indicator of reproductive +activity. Juveniles were caught in relatively small numbers because of +their restricted movements, and no way to determine prenatal and juvenal +mortality was available. + +Reproductive activity continues throughout the year. Within the +thirty-month period for which data were obtained, December and January +showed the lowest percentages of females with vaginal orifices (Fig. 8). +The other months all showed higher levels of reproductive activity with +a slight peak in the August-September-October period in both 1950 and +1951. In the species of _Microtus_ that are found in the United States, +such summer peaks of breeding seem to be the rule (Blair, 1940:151; +Gunderson, 1950:17; Hamilton, 1937b:785). Jameson (1947:147) worked in +the same county where my field study was made and found that the high +point of reproduction was in March, although his samples were too small +to be reliable. The peak of reproductive activity slightly preceded the +highest level of population density in each year (Fig. 8). + +[Illustration: FIG. 8. Variations in density and reproductive rate of +voles, with variation in monthly precipitation. Abnormally low rainfall +in 1952 caused a decrease in breeding activity and eventually in the +numbers of voles. The solid line indicates the number of voles per acre, +the broken line the percentage of females with a vaginal orifice and the +dotted line the inches of rainfall.] + +A marked reduction in the percentage of females having vaginal orifices +was observed in the unusually dry summer of 1952. The rate of +reproduction was found to be positively correlated with rainfall (Fig. +9). Correlation coefficients were higher in each case when the amount of +rainfall in the month preceding each sampling period was used instead of +that in the month of the sample. This suggested that the rainfall +exerted its influence indirectly through its effect on plant growth. +Bailey (1924:530) reported that a reduction in either the quantity or +quality of food had a depressing effect on reproduction. Drought, such +as occurred in 1952, would certainly have a depressing effect on both. +The critical factor seems to be the supply of new, actively growing +shoots available to the voles for food rather than the total amount of +vegetation. As far as could be determined from the small sample of males +examined, their fecundity was not affected by rainfall. Some decrease in +the percentage of males that were fecund was noted in the winter and was +reported also by Jameson (1947:145) but most of the males in any sample +were fecund. Thus any depression in the reproductive rate was due to +loss of fecundity by females. This was in agreement with reports in the +literature on the subject (Baker and Ransom, 1932a:320; 1932b:43). + +The correlation coefficient between rainfall and the percentage of adult +females with a vaginal orifice was 0.53. This was considered to be +surprisingly high in view of the expected effects on the breeding rate +of temperature, seasonal diet variations and whatever rhythms were +inherent in the voles. When only the summer months were considered the +correlation coefficient between rainfall and the percentage of adult +females with a vaginal orifice was 0.84. This indicated that, during the +season when breeding was at its height, rainfall was a factor in +determining the rate of reproduction and when rainfall was scarce, as in +the summer of 1952, it seemed to be a limiting factor (Fig. 9). + +[Illustration: FIG. 9. Comparison between monthly rainfall and +reproductive rate of voles in summer. The dry summer of 1952 caused a +notable decrease in reproductive activity. The correlation coefficient +between rainfall and the percentage of females with a vaginal orifice +was 0.84.] + +Of the total captures 20.6 per cent involved more than one individual. +When the distribution of these multiple captures was graphed for the +period of study, a high correlation between the percentage of captures +that were multiple and the percentage of females with a vaginal orifice +(r = 0.70) was found. An even higher correlation (r = 0.76) was observed +between the percentage of captures that were multiple and the population +density. The higher percentage of multiple captures may have been +largely a result of fewer available traps per individual on the area and +thus only indirectly related to the rate of reproduction. + +Of the multiple captures, 66 per cent involved both sexes. The +correlation coefficient between the percentage of captures involving +both sexes and the level of reproductive activity was 0.58. Among those +pairs of individuals caught together more than once, 61 per cent were +composed of both sexes. Among those pairs taken together three or more +times 76 per cent were male and female and among those pairs taken +together four or more times 80 per cent were male and female. When adult +voles stayed together any length of time their relationship usually +appeared to be connected with sex. Family groups were also noted, as +pairs were often trapped which seemed to be mother and offspring. A +lactating female would sometimes enter a trap even after it had been +sprung by a juvenile, presumably her offspring, or a juvenal vole would +enter a trap after its mother had been captured. Such family groups +persisted only until the young voles had been weaned. + +The youngest female known to be gravid was 26 days old and weighed 28 +grams. During summer most of the females were gravid before they were +six weeks old, although females born in October and after were often +more than 15 weeks old before they became gravid. The youngest male +known to be fecund was approximately six weeks old. Male fecundity was +determined as described by Jameson (1950). Difference in the age of +attainment of sexual maturity serves to reduce the mating of litter +mates (Hamilton, 1941:7) and has been noticed in various species of the +genus _Microtus_ by several authors (Bailey, 1924:529; Hatfield, +1935:264; Hamilton, _loc. cit._; Leslie and Ransom, 1940:32). + +For 35 females, each of which was caught at least once each month for +ten consecutive months or longer, the mean number of litters per year +was 4.07. Certain of the more productive members of the group produced +11 litters in 16 months. _M. ochrogaster_ seems to be less prolific than +_M. pennsylvanicus_. Bailey (1924:528) reported that one female meadow +vole delivered 17 litters in 12 months. Hamilton (1941:14) considered 17 +litters per year to be the maximum and stated that in years when the +vole population was low the females produced an average of five to six +litters per year. In "mouse years" the average rose to eight to ten +litters per year. During this study several females delivered two or +more litters in rapid succession. This was noted more frequently in +spring and early summer than in other parts of the year. Those females +which produced two or three litters in rapid succession in spring and +early summer often did not litter again until fall. Post-parous +copulation has been observed in _M. pennsylvanicus_ by Bailey (1924:528) +and Hamilton (1940:429; 1949:259) and probably occurs also in _M. +ochrogaster_. + +The gestation period was approximately 21 days, the same as reported for +_M. pennsylvanicus_ (Bailey, _loc. cit._; Hamilton, 1941:13) and _M. +californicus_ (Hatfield, 1935:264). A more precise study of the breeding +habits of _M. ochrogaster_ failed to materialize when the voles refused +to breed in captivity. Fisher (1945:437) also reported that _M. +ochrogaster_ failed to breed in captivity although _M. pennsylvanicus_ +(Bailey, 1924) and _M. californicus_ (Hatfield, 1935) reproduced readily +in the laboratory. + + +Litter Size and Weight + +In the course of this study 65 litters were observed. The mean number of +young per litter was 3.18 ± 0.24 and the median was three (Fig. 10). +Three litters contained but one individual and the largest litter +contained six individuals. Other investigators have reported the number +of young per litter in _M. ochrogaster_ as three or four (Lantz, +1907:18) and 3.4 (1-7) (Jameson, 1947:146). _M. pennsylvanicus_ seems to +have larger litters. Although Poiley (1949:317) found the mean size of +416 litters to be only 3.72 ± 0.18, both Bailey (1924:528) and Hamilton +(1941:15) found five to be the commonest number of young per litter in +that species. Leslie and Ransom (1940:29) reported the average number of +live births per litter to be 3.61 in the British vole, _M. agrestis_. +Selle (1928:96) reported the average size of five litters of _M. +californicus_ to be 4.8. Hatfield (1935:265), working with the same +species, found that litter size varied directly with the age of the +female producing the litter. He reported litters of young females as two +to four young per litter and of older females as five to seven young per +litter. In the litters of _M. ochrogaster_ that I examined, young +females did not have more than three young and usually had but two. +However, older females had litters of one, two and three often enough so +that no relationship, as described above, was indicated clearly. + +[Illustration: FIG. 10. Distribution of litter size among 65 litters of +voles.] + +No seasonal variation in litter size was noted. The mean size of the +litters in 1950, 2.68 ± 0.30, was significantly lower than that found in +1951 (3.76 ± 0.20) but neither differed significantly from the mean size +of litters in 1952 (3.35 ± 0.66). The lower mean size of litters was in +part coincidental with a high population level and the higher mean of +the two later years was in part coincidental with a low population +level. Since a sharp break in the curve for population density occurred +after the flood in July, 1951, the litters were arranged in pre-flood +and post-flood categories for study. Pre-flood litters averaged 3.07 ± +0.28 young per litter whereas post-flood litters averaged 3.34 ± 0.48. +This difference was not significant. Increase in litter size, if it had +actually occurred, might have been a response to the increasing food +supply and lower population density after the flood. + +A difference in the mean number of young per litter was noted for those +litters delivered in traps as compared with those delivered in captivity +and the numbers of embryos examined in the uterus. The mean number of +embryos per female was higher than the mean number of young per litter +delivered in captivity and the mean number of young per litter delivered +in traps was lower than in those delivered in captivity. The differences +were not statistically significant. In some instances females that +delivered young voles in traps may have delivered others prior to +entering the trap or the mother or her trapmates may have eaten some of +the newborn voles before they were discovered. + +The mean weight of 16 newborn (less than one day old) individuals was +2.8 ± 0.36 grams. No other data on the weight of newborn _M. +ochrogaster_ were found in the literature but this mean was close to the +3.0 grams (Bailey, 1924:530) and 2.07 grams (Hamilton, 1937a:504; +1941:10) reported for _M. pennsylvanicus_ and to the 2.7 grams (Selle, +1928:97) and 2.8 grams (Hatfield, 1935:268) reported for _M. +californicus_. No correlation between the weight of the individual +newborn vole and the number of voles per litter was observed. + +Although the ratio of the average weight of newborn voles to the average +weight of an adult female was approximately equal for _M. +pennsylvanicus_ and _M. ochrogaster_, the ratio of the weight of a +litter to the average weight of an adult female was larger in the +eastern meadow vole because the mean litter size was larger. Perhaps +this is related to the more productive habitat in which the eastern +meadow vole is ordinarily found. + + +Size, Growth Rates and Life Spans + +The mean weight of adult voles during the period of study was 43.78 +grams. The females averaged slightly heavier than the males but the +overlapping of weights was so extensive that sexual difference in weight +could not be affirmed. The difference observed was less in December and +January when gravid females were rare, suggesting that the difference +was due, at least in part, to pregnancy. Jameson (1947:128) found, for a +sample of 50 voles, a mean weight of 44 grams and a range of 38 to 58 +grams. The range in the adult voles I studied was much greater, from 25 +to 73 grams. In part, this increase in the range of adult weights was +due to a much larger sample. + +[Illustration: FIG. 11. Relationship between rainfall and the mean +weight of adult males in summer. The abnormally low rainfall in the +summer of 1952 was accompanied by a decrease in mean weight. The solid +line represents mean weight and the broken line rainfall. The +correlation coefficient between the two was 0.68.] + +During the unusually dry summer of 1952, a notable reduction in the mean +weight of adults was recorded (Fig. 11). The correlation coefficient +between the mean weight of adults and the amount of rainfall for the +summer months was 0.68. It seems reasonable to attribute the drop in +mean weight to an alteration of plant growth due to decreased rainfall. +Some of the reduction in mean weight was due to the loss of weight in +older individuals but most of it was due to the failure of voles born in +the spring to continue growing. + +No data on the growth rate of _M. ochrogaster_ were found in the +literature. According to the somewhat scanty data from my study, secured +from observations of individuals born in the laboratory, young voles +gained approximately 0.6 of a gram per day for the first ten days, +approximately one gram per day up to an age of one month, and +approximately 0.5 of a gram per day from an age of one month until +growth ceases. This growth rate was especially variable after the voles +reached an age of thirty days. The growth rate approximates those +described for _M. pennsylvanicus_ (Hamilton, 1941:12) and for _M. +californicus_ (Hatfield, 1935:269; Selle, 1928:97). Although the data +were inadequate for a definite statement, I gained the impression that +there was no difference between the sexes in growth rate. In general, +young voles grow most rapidly in the April-May-June period and least +rapidly in mid-winter. Several voles, born in late autumn, stopped +growing while still far short of adult size and lived through the winter +without gaining weight, then gained as much as 30 per cent after spring +arrived (Fig. 12). + +[Illustration: FIG. 12. Growth rates of two voles selected to show +typical growth pattern of voles born late in the year. Growth nearly +stops in winter and is resumed in spring.] + +The recorded life spans of most voles studied were less than one year. +No accurate mean life span could be determined. Leslie and Ransom +(1940:46), Hamilton (1937a:506) and Fisher (1945:436) also found that +most voles lived less than one year. Leslie and Ransom (_op. cit._: 47) +reported a mean life span of 237.59 ± 10.884 days in voles of a +laboratory population. In the present study one female was trapped 624 +days after first being captured; another female was trapped 617 days +after first being captured; and a male was trapped 611 days after first +being captured. The two females were subadults when first captured. The +male was already an adult when first captured; consequently its life +span must have exceeded 650 days. No evidence of any decrease in vigor +or fertility was observed to accompany old age. + +Of the 45 marked voles snap-trapped in August of 1952, 21 had been +captured first as juveniles. The ages of these voles could be estimated +within a few days, and the series presented a unique opportunity for +studying individual and age variation. Only individuals weighing less +than 18 grams when first captured were used, and their ages were +estimated according to the growth rate described above. Howell (1924) +reported an analysis of individual and age variation in a series of +specimens of _Microtus montanus_, and Hall (1926) studied the changes +due to growth in skulls of _Otospermophilus grammarus beecheyi_. The +series of specimens described here differs from those of Hall and +Howell, and from any other collection known to me, in the fact that the +specimens are of approximately known age and drawn from a wild +population. + +Unfortunately, this sample was small, and the distribution of the +specimens among age groups left much to be desired. No specimens less +than one and one-half months old were taken and only a few individuals +older than four and one-half months. Table 3 shows the age distribution. +The small size of the sample and the absence of juveniles were due, +partly, to the unusually dry weather in the summer of 1952. The +reduction in the rate of reproduction, caused by drought (as described +elsewhere in this paper), reduced the populations and the percentage of +juveniles to low levels. + + TABLE 3. DISTRIBUTION AMONG AGE GROUPS OF 21 VOLES USED IN THE STUDY OF + VARIATION DUE TO AGE + + ====================================================================== + Age in months 1-1/2 2 2-1/2 3 3-1/2 4 4-1/2 6 12 + ---------------------------------------------------------------------- + No. of individuals 1 4 5 1 3 2 3 1 1 + ---------------------------------------------------------------------- + +In the series of voles studied, ten individuals were in the process of +molting from subadult to adult pelage. Jameson (1947:131) reported the +molt to occur between eight and 12 weeks of age and selected 38 grams as +the lower limit of weight of adults. I also found all voles molting to +be between eight and 12 weeks old but found none so large as 38 grams +without full adult pelage. This may have been, in part, due to the dry +weather delaying or inhibiting growth. Because of the small size of the +sample and the influence of the unusual weather conditions, no +conclusions concerning normal molting were drawn from the data described +below. They are presented only as a description of a small sample drawn +from a single population at one time. Table 4 summarizes these data. + + TABLE 4. MEAN SIZES AND AGES OF VOLES MOLTING FROM SUBADULT TO ADULT + PELAGE + + ===================================================================== + Body length Condylo-basilar + Weight minus tail length Age + --------------------------------------------------------------------- + Six males 32.67 gms. 106.16 mm. 23.78 mm. 9.67 wks. + (30-36) (96-116) (23.2-24.4) (8-12) + Four females 29.0 gms. 100.25 mm. 23.45 mm. 10.5 wks. + (28-30) (98-102) (23.5-23.8) (8-12) + Ten voles 31.2 gms. 103.8 mm. 23.73 mm. 10.0 wks. + (28-36) (96-116) (23.2-24.4) (8-12) + --------------------------------------------------------------------- + +The mean age of the ten voles molting was ten weeks (8-12). Six males +averaged 9.67 weeks, almost a week younger than four females, who +averaged 10.5 weeks. The difference in age at time of molting between +the sexes was not significant. Differences between the sexes in other +characteristics to be described also lacked significance. Mean weights +at the time of molting were: males, 32.67 gms. (30-36); females, 29.0 +gms. (28-30); and all individuals, 31.2 gms. (28-36). Because a piece of +the tail of each vole had been removed in marking, the total length of +the voles could not be determined. Body length, excluding tail, was +used. Howell (1924:986) found this measurement subject to less +individual variation than total length and thought body length was +probably a better indicator of age. Mean body length at the time of +molting was 103.8 mm. (96-116). Males averaged longer than females and +were also more variable. The mean body length of males was 106.16 mm. +(96-116) and that of females was 100.25 mm. (98-102). + +Of the subadults showing no signs of molting, none was above the mean +age of molting. Twenty-five per cent of them were longer and heavier +than the mean length and weight of those that were molting. Of the 20 +adults in the series, one was below the mean weight of molting and one +was shorter than the mean length of molting. + +When Howell (_op. cit._:1014) studied skulls of _Microtus montanus_ he +found that the condylobasilar length was the most satisfactory means for +arranging his series of specimens according to their age. When the +skulls of my series were arranged according to their age (as determined +from trapping records) the graph of the condylobasilar lengths showed a +clear, though not perfect, relationship to age (Fig. 13). No separation +of sexes was made because the sample did not permit it. In Fig. 13 +graphs of weight, as determined in the field, and of length (excluding +tail) also were included because they are the most easily measured +characters of live voles. The graphs indicate individual variation in +these characters which limits their usefulness in determining age. + +[Illustration: FIG. 13. Graphs of the condylobasilar lengths, body +lengths and weights of a series of voles of known age. Within each age +group, the youngest vole is on the left in the graphs.] + +When other cranial measurements, and ratios of pairs of measurements, +were plotted in the same order, individual variation obscured some of +the variation due to age and the curves resembled those of weight and +length of body rather than that of condylobasilar length. When the +cranial measurements were averaged for the age groups the curves showed +a relationship to age but the relationship of mean measurements is of +little use in determining the age of individual specimens. The data +described above indicated that a study of the relationship of the +condylobasilar length and age in a large sample might provide useful +information. + +Anyone who has examined mammalian skulls knows of many other characters +which vary with age but which are difficult to measure and describe with +precision. Figs 14 and 15 are drawings of skulls of voles of known age. +The most obvious change, related to aging, evident in the dorsal view of +the skulls (Fig. 14) is the increasing prominence and closer +approximation of the temporal ridges in older specimens. The lambdoidal +ridge is also more prominent in older voles, and their skulls have a +generally rougher and more angular appearance. The individual variation +evident in these ridges is probably due to variations in the development +of the muscles operating the jaws (Howell, 1924:1003). There is an +increased flattening of the roof of the skull of older voles. + +[Illustration: 1-1/2 months 2-1/2 months 3 months 3-1/2 months + +4 months 4-1/2 months 6 months 12 months + +All × 3. + +FIG. 14. Dorsal views of skulls of voles of known age.] + + +[Illustration: 1-1/2 months 2-1/2 months 3 months 3-1/2 months + +4 months 4-1/2 months 6 months 12 months + +All × 3. + +FIG. 15. Palatal views of skulls of voles of known age.] + +From a palatal view (Fig. 15) the skulls of voles also showed age +variation which was apparent but not easily correlated with precise age. +The median ridge on the basioccipital bone increases in prominence in +older voles. The shape of the posterior margin of the palatine bones +changes from a V-shape to a U-shape. On the skull of the oldest (12 +months) vole the pterygoid processes are firmly fused to the bullae, a +condition not found in any of the other specimens. The anterior spine of +the palatine approaches the posterior projection of the premaxillae more +closely as age increases and, in the oldest vole is firmly attached and +forms a complete partition separating the incisive foramina. + +Tooth wear during the life of a vole causes a considerable variation in +the enamel patterns, especially of the third upper molar. Howell +(1924:1012) considered such variation to be independent of age, but +Hinton (1926:103) related the changes to age and interpreted them as a +recapitulation of the evolution of microtine molars. In my series, an +indentation on the medial margin of the posterior loop of the third +upper molar seemed to be related to age. This indentation was absent in +the youngest vole (one and one-half months), absent or indefinite in +those voles less than 3-1/2 months of age, and progressively more marked +in the older voles. + + +Food Habits + +The prairie vole, like other members of the genus _Microtus_, feeds +mostly on growing grass in spring and summer. Piles of cuttings in the +runways are characteristic sign of the presence of voles. The voles cut +successive sections from the bases of grasses until the young and tender +growing tips are within reach. The quantity of grass destroyed is +greater than that actually eaten, a fact which will have to be +considered in any attempt to evaluate the effects of voles upon a range. + +In all piles of cut plants that were examined, _Bromus inermis_ was the +most common grass, and _Poa pratensis_ was the grass second in +abundance. These were, by far, the most common grasses present on the +areas studied; in most places, _B. inermis_ was dominant. Other grasses +present on the areas were occasionally found in the piles of cuttings. +Jameson (1947:133-136) found no utilization of _B. inermis_ by voles but +that grass was present in a relative abundance of only one per cent in +the areas studied by him. The voles that he studied ate alfalfa in large +amounts and alfalfa was, perhaps, the most common plant on the +particular areas where his voles were caught. Seemingly, the diet of +voles is determined mostly by the species composition of the habitat. + +Other summer foods included pokeberries, blackberries and a few forbs +and insects. Forbs most commonly found in the piles of cuttings were the +leaves of the giant ragweed (younger plants only) and dandelion. Insect +remains were found in the stomachs of voles killed in summer and +occurred most frequently in those killed in August and September. At no +time did insects seem to be a major part of the diet but they were +present in most vole stomachs examined in late summer. Laboratory +experiments with summer foods gave inconclusive results but suggested +that the voles chose grasses on the basis of their growth stage rather +than according to their species. Young and tender grasses were chosen, +regardless of species, when various combinations of _Triodia flava_, +_Bromus inermis_ and _Poa pratensis_ were offered to the voles. At +times the voles showed a marked preference for dandelion greens, perhaps +because of their high moisture content; the voles' water needs were +satisfied mostly by eating such succulent vegetation. + +Winter foods consisted of stored hay and fruits and of underground plant +parts. _Bromus inermis_ made up nearly all of the hay and was stored in +lengths of up to ten inches in underground chambers specially +constructed for storage. Underground parts of plants were reached by +tunnelling and were an especially important part of the voles' diet in +January and February. The fruit of _Solanum carolinense_ was eaten +throughout the winter and one underground chamber, opened in February, +1952, was packed full of these seemingly unsavory fruits. Fisher +(1945:436), in Missouri, found this fruit to be an important part of the +winter diet of voles. An occasional pod of the honey locust tree was +found partly eaten in a runway. Fitch (1953, _in litt._) often observed +girdling of honey locust and crab apple (_Pyrus ioensis_) root crowns on +the Reservation but I saw no evidence of bark eating, perhaps because my +study plots were mostly grassland. On two occasions when two voles were +in the same trap one of them was eaten. In both traps, all of the bait +had been eaten and the captured voles probably were approaching +starvation. Because the trapping procedure offered abundant opportunity +for cannibalism, the low frequency of its occurrence suggested that it +was not an important factor in satisfying food requirements under normal +conditions. + + +Runways and Nests + +Perhaps the most characteristic sign of the presence of _Microtus +ochrogaster_ were their surface runways and underground tunnels. Only +rarely was a vole observed to expose itself to full view. When a trapped +vole was released it immediately dove out of sight into a runway. Once +in a runway, the vole showed no further evidence of alarm and was +usually in no hurry to get away. The runways seemed to provide a sense +of security and the voles were familiar with their range only through +runway travel. The urge to seek a runway immediately when exposed has +obvious survival value. + +Surface runways were usually under a mat of debris. In areas where +debris was scanty or lacking, runways were usually absent. Jameson +(1947:136) reported that in alfalfa and clover fields the voles did not +make runways as they did in grassland, even in fields where trapping +records showed voles to be abundant. Typical surface runways are +approximately 50 mm. wide, only slightly cut into the ground and bare of +vegetation while in use. Usually they could be distinguished from the +runways of the pine vole, which were cut more deeply into the ground, +and those of the cotton rat which were wider and not so well cleared of +vegetation. Some runways ended in surface chambers and some of these +were lined with grass. Their size varied from a diameter of 90 mm. to +250 mm. and they seemed to be used primarily for resting places. + +A runway system usually consisted of a long, crooked runway and several +branches. Two typical systems are illustrated in Fig. 16. The runway +systems often were not clearly limited; they merged with other systems +more or less completely. One map showed a runway system extending across +140 square meters and including 12 underground burrows. All of these +runways seemed to be part of a single runway system but the system +probably was used by more than one vole or family group of voles. +Sixteen of the 22 maps that were made extended across areas between 50 +and 90 square meters. One map, mentioned above, was larger and the +remaining five smaller. The smallest extended across only 20 square +meters. Of course, the area encompassed by a set of runways changed +almost daily, as the voles extended some runways, added some and +abandoned others in the course of their daily travels. + +[Illustration: FIG. 16. Maps of runway systems of the prairie vole. The +runways follow an irregular course and are frequently changed. The solid +lines represent surface runways and the dotted lines underground +passages.] + +Each runway system contained underground nests. These were in chambers +from 70 mm. to 200 mm. below the surface and were up to 200 mm. in +diameter. Most systems that were mapped had from two to six of these +burrows. Most of these were lined with dried grass and seemed to be used +for delivering and nursing litters. Each burrow was connected to a +surface runway by a tunnel. Often the tunnel was short and the hole +opened almost directly into the burrow from the surface runway. Others +had tunnels several meters long. Jameson (1947:137) reported every +burrow to have two connections with the surface. In the present study, +however, I found three arrangements in approximately equal frequency of +occurrence: (1) one hole to one tunnel leading to a burrow; (2) two +holes to two short tunnels which joined a long tunnel leading to a +burrow; and (3) two separate tunnels from the surface to a burrow. The +size, depth and number of underground burrows in the systems that I +studied varied and so did those reported in the literature. Jameson +(_loc. cit._) found burrows in eastern Kansas as deep as 18 inches, far +deeper than any found in my study. Fisher (1945:435) reported none +deeper than five inches in central Missouri. The soil data in my study, +as well as in the two reports cited immediately above, were not adequate +to permit conclusions, but the type and condition of the soil probably +determine the extent of burrowing by the voles of any given locality. + +The number of voles using a runway system at one time was difficult to +ascertain. In one system, however, four adult individuals were trapped +in a ten day period. In August, 1952, at the conclusion of the +live-trapping program, a runway system was mapped which had included two +trapping stations. In the preceding ten days, four adult voles (three +males and one female) had been taken in both traps. During that time, +therefore, the runway system was shared by at least four voles. The +voles used an area that was considerably larger than that encompassed by +any one runway system, a fact obvious when the sizes of home ranges as +computed from trapping data were compared with the sizes of the runway +systems mapped. A runway system seemed not to be a complete unit, but +was only a part of the network of runways used by a single individual. + + +Activity + +Although no special investigation of activity was made, some conclusions +concerning it were apparent in the data gathered. There have been a few +laboratory studies of the activity pattern of _Microtus_ by various +methods. Calhoun (1945:256) reported _M. ochrogaster_ to be mainly +nocturnal with activity reaching a peak between dark and midnight and +again just before dawn. Davis (1933:235), working with _M. agrestis_, +and Hatfield (1935:263), working with _M. californicus_, both found +voles to be more nocturnal than diurnal. In a field study of _M. +pennsylvanicus_, Hatt (1930:534) found the species to be chiefly +nocturnal, although some activity was reported throughout the day. +Hamilton (1937c:256-259), however, reported the same species to be more +active in the daytime. Agreement on the activity patterns of these +species of _Microtus_ has not yet been attained. + +From occasional changes in the time of tending a trap line, and from +running lines of traps at night a few times in the summer of 1951, I +gained the impression that these voles were primarily diurnal. +Relatively few of them were caught in the hours of darkness. In summer, +however, their activity was mostly limited to the periods between dawn +and approximately eight o'clock and between sunset and dark. In colder +weather, there was increased activity on sunny days. + + + + +PREDATION + + +Although voles were a common item of prey for many species of predators +on the Reservation, no marked effect on the density of the population of +this vole could be attributed to predation pressure. Only when densities +reached a point that caused many voles to expose themselves abnormally +could they be heavily preyed upon. Their normally secretive habits, +keeping them more or less out of sight, suggest that they are an +especially obvious illustration of the concept that predation is an +expression of population vulnerability, rising to high levels only when +a population is ecologically insecure, rather than a major factor +regulating population levels (Errington, 1935; 1936; 1943; Errington _et +al_, 1940). + +Scats from predatory mammals and reptiles and pellets from raptorial +birds were examined. Most of these materials were collected by Dr. Henry +S. Fitch, who kindly granted permission to use them. The results of the +study of the scats and pellets are summarized in Table 5. Remains of +voles were identified in 28 per cent of the scats of the copperhead +snake (_Ancistrodon contortix_) examined. Copperheads were moderately +common on the Reservation (Fitch, 1952:24) and were probably important +as predators on voles in some habitats. Uhler _et al_ (1939:611), in +Virginia, reported voles to be the most important prey item for +copperheads. A vole was taken from the stomach of a rattlesnake +(_Crotalus horridus_) found dead on a county road adjoining the +Reservation. Rattlesnakes were present in small numbers on the +Reservation but were usually found along rocky ledges rather than in +areas where voles were common (Fitch, _loc. cit._). The rattlesnakes +probably were less important as predators on voles than on other small +mammals more common in the usual habitat of these snakes. The blue racer +(_Coluber constrictor_) was common in grassland situations on the +Reservation (Fitch, 1952:24) and twice was observed in the role of a +predator on voles; one small blue racer entered a live-trap in pursuit +of a vole and another blue racer was observed holding a captured vole in +its mouth. The blue racer seems well adapted to hunt voles and probably +preys on them extensively. The pilot black snake (_Elaphe obsoleta_) has +been reported as a predator on _M. ochrogaster_ in the neighboring state +of Missouri (Korschgen, 1952:60) and was moderately common on the +Reservation (Fitch, _loc. cit._). _M. pennsylvanicus_, with habits +similar to those of _M. ochrogaster_, has been reported as a prey for +all of the above snakes (Uhler, _et al_, 1939). + + TABLE 5. FREQUENCY OF REMAINS OF VOLES IN SCATS AND PELLETS + + ========================================================================= + No. of scats or No. containing + Predator pellets examined remains of voles Percentage + ------------------------------------------------------------------------- + Copperhead 25 7 28 + Red-tailed hawk 25 3 12 + Long-eared owl 25 18 72 + Great horned owl 32 6 19 + Crow 25 4 16 + Coyote 25 3 12 + ------------------------------------------------------------------------- + +The red-tailed hawk (_Buteo jamaicensis_), the long-eared owl (_Asio +otus_), the great horned owl (_Bubo virginianus_) and the crow (_Corvus +brachyrhynchos_) fed on _Microtus_. All four birds were fairly common +permanent residents on the Reservation (Fitch, 1952:25). The low density +and the strict territoriality of the red-tailed hawk (Fitch, _et al_, +1946:207) prevented it from exerting any important influence on the +population of voles, even though individual red-tailed hawks ate many +voles. Predation by the long-eared owl was especially heavy; remains of +voles were identified in 72 per cent of its pellets examined. Korschgen +(1952:39) found remains of voles in 70 per cent of 704 pellets of the +long-eared owl. The reason for the heavy diet of _Microtus_ seems to be +that both the owl and the vole are especially active at dusk. A group of +long-eared owls, living near the edge of Quarry Field, probably exerted +an influence on the density of the local population of voles because of +the high ratio of predator to prey animals. The crows ate some, and +perhaps most, of their voles after the animals had died from other +causes. Other birds, mostly raptors, occurring in northeastern Kansas +and reported to prey on voles include the sharp-shinned hawk (_Accipiter +striatus_), Cooper's hawk (_A. cooperi_), red-shouldered hawk (_Buteo +lineatus_), broad-winged hawk (_B. platypterus_), American rough-legged +hawk (_B. lagopus_), ferruginous rough-legged hawk (_B. regalis_), marsh +hawk (_Circus cyaneus_), barn owl (_Tyto alba_), screech owl (_Otus +asio_), barred owl (_Strix varia_) and shrike (_Lanius excubitor_) +(Korschgen, 1952:26; 28; 34; 35; 37; McAtee, 1935:9-27; Wooster, +1936:396). + +Coyotes, house cats and raccoons were identified as predators on voles +in the study areas. Remains of voles were present in 12 per cent of the +scats of the coyote (_Canis latrans_) examined. In Missouri, Korschgen +(1952:40-43) reported remains of voles in slightly more than 20 per cent +of the coyote stomachs that he examined. Fitch (1948:74), Hatt +(1930:559) and others have reported other species of _Microtus_ as eaten +by the coyote. Although coyotes were rarely seen on the Reservation, +coyote sign was abundant (Fitch, 1952:29) and coyotes probably ate large +numbers of voles. House cats (_Felis domesticus_), seemingly feral, were +observed to tour the trap lines on several occasions and were noted by +Fitch (_loc. cit._) as important predators on small vertebrates. Four +cats were killed in the course of the study and remains of voles were +found in the stomachs of all of them. On several occasions, raccoon +tracks were noted following the trap line when the traps had been +overturned and broken open, suggesting that raccoons are not averse to +eating voles although no further evidence of predation on voles by +raccoons was obtained. Fitch (_loc. cit._) reported raccoons (_Procyon +lotor_) to be moderately common on the Reservation. Reports of predation +by raccoons on voles are numerous (Hatt, 1930:554; Lantz, 1907:41). The +opossum (_Didelphis marsupialis_), common on the Reservation, +occasionally eats voles (Sandidge, 1953:99-101). Other mammals which are +probably important predators on voles on the Reservation, though no +specific information is available, are the striped skunk (_Mephitis +mephitis_), spotted skunk (_Spilogale putorius_), weasel (_Mustela +frenata_) and the red fox (_Vulpes fulva_). Eadie (1944; 1948; 1952), +Shapiro (1950:360) and others have reported that the short-tailed shrew +(_Blarina brevicauda_) was an important predator on _Microtus_. Shrews +were present on the Reservation but were not trapped often enough to +permit study. + +The variety of vertebrates preying on voles suggests that they occupy a +position of importance in many food chains. Errington (1935:199) and +McAtee (1935:4) refer to voles as staple items of prey for all classes +of predatory vertebrates. An attempt to evaluate prey species was made +by Wooster (1939). He proposed a formula which involved multiplying the +density of a species, its mean individual weight, the fraction of the +day it was active and the fraction of the year it was active to give a +numerical index of prey value. Although his methods of determining +population densities would now be considered questionable, the purpose +of his investigation merits further consideration. He reported _M. +ochrogaster_ to be second only to the jack-rabbit (_Lepus californicus_) +as a prey species in west-central Kansas. + + + + +MAMMALIAN ASSOCIATES + + +In the course of live-trapping operations several species of small +mammals other than _Microtus ochrogaster_ were taken in the traps. Also, +from time to time, direct observations of certain mammals were made and +various types of sign of larger mammals were noted. These records gave a +picture of the mammalian community of which the voles were a part. The +three associated species which were most commonly trapped were _Sigmodon +hispidus_, _Reithrodontomys megalotis_ and _Peromyscus leucopus_. These +three species have been commonly found associated with _Microtus_ in +this part of the country (Fisher, 1945:435; Jameson, 1947:137). + +The Texas cotton rat, _Sigmodon hispidus_, was the most commonly trapped +associate of the voles between November, 1950, and February, 1952. +Although a greater number of individuals of the harvest mouse were taken +in a few months, the cotton rat had a greater ecological importance +because of its larger size (Figs. 17, 18, 19). The cotton rat was an +especially noteworthy member of the community for two reasons. It has +arrived in northern Kansas only recently and its progressive range +extension northward and westward has attracted the attention of many +mammalogists (Bailey, 1902:107; Cockrum, 1948; 1952:183-187; Rinker, +1942b). Secondly, _Sigmodon_ has long been considered to be almost the +ecological equivalent of _Microtus_ and to replace the vole in the +southern United States (Calhoun, 1945:251; Svihla, 1929:353). Since the +two species are now found together over large parts of Kansas their +relationships in the state need careful study. + +[Illustration: FIG. 17. Variations in density and mass of three common +rodents on House Field. The upper graph shows the sum of the biomass of +the three rodents. In the two lower graphs the solid line represents +_Microtus_, the broken line _Sigmodon_, and the dotted line +_Reithrodontomys_.] + +[Illustration: FIG. 18. Variations in density and biomass of three +common rodents on Quarry Field. For key, see legend of Fig. 17.] + +[Illustration: FIG. 19. Changing biomass ratios of three common rodents +on House Field and Quarry Field. In late 1951 and early 1952 the cotton +rats attained relatively high levels and seemingly caused compensatory +decreases in the numbers of voles. The solid line represents _Microtus_, +the broken line _Sigmodon_, and the dotted line _Reithrodontomys_.] + +Both this study and the literature (Black, 1937:197; Calhoun, _loc. +cit._; Meyer and Meyer, 1944:108; Phillips, 1936:678; Rinker, 1942a:377; +Strecker, 1929:216-218; Svihla, 1929:352-353) showed that, in general, +the habitat needs of _Microtus_ and _Sigmodon_ were similar. Studies on +the Natural History Reservation, both in connection with my problem and +otherwise, suggested, however, that _Sigmodon_ occurred in only the more +productive habitat types used by voles, where the vegetation was +relatively high and rank. On the Reservation the cotton rat was found +mostly in the lower meadows; they were more moist and had a more +luxuriant vegetation than the higher fields. Although a few cotton rats +were taken in Quarry Field and still fewer in Reithro Field, the +population of those hilltop areas did not approach, at any time, the +levels reached on House Field, which produced a more luxuriant cover. +Only when the levels of population were exceptionally high did the +cotton rats spread into less productive habitats. At all times, there +were areas on the Reservation used by _Microtus_ which could not support +a population of _Sigmodon_. + +The cotton rats reacted differently to the floods of July, 1951, than +did the voles. Although the population of the cotton rat decreased +slightly immediately after the wet period, this decrease was +insignificant when compared with the drop in population level of other +species of small mammals on the same area. During the autumn of 1951 and +until March, 1952, the cotton rat became the most important mammal on +the House Field study area in terms of grams per acre (Fig. 17), +although the number of cotton rats per acre never matched the density of +the voles. A similar, though less pronounced, trend was observed on the +Quarry Field study area (Fig. 18). One factor in the success of the +cotton rat at this time seemed to be the greater resistance to wetting +shown by very young individuals. Few adults (of any species) marked +before the heavy rains of July, 1951, were trapped in September, 1951, +when trapping was resumed after a lapse of one month. Several subadults +and some juvenal cotton rats did survive, however, and provided a +breeding population from which the area was repopulated. Cotton rats are +born fully furred and able to move well, and are often weaned at ten +days (Meyer and Meyer, 1944:123-124). Voles, on the other hand, are born +naked and helpless and are often not weaned for three weeks. It seems, +therefore, that extremely wet soil would harm the voles more than it +would the cotton rats. + +Several instances of cotton rats eating voles, caught in the same +live-trap, were noted. There is reason to believe that young voles, +unable to leave the nest, are subject to predation by cotton rats. This +would accentuate any competitive advantage gained otherwise by the +cotton rats. + +The population of _Sigmodon_ retained its high level, relative to +_Microtus_, until February, 1952. In March only one individual was +captured and after that none was trapped until August, 1952, when a +single subadult male was captured. Early in March, 1952, before the +trapping period for the month had begun, the area suffered three +successive days of unusually low temperature, with snow, which lay more +than six inches deep in places. As suggested by Cockrum (1952:185), such +conditions proved detrimental to the cotton rats and, at least to the +end of the study period in August, 1952, the population of cotton rats +had failed to recover. Perhaps the extremely dry weather which followed +the heavy winter mortality delayed the recovery of the population. + +These limited data seem to indicate competition between _Sigmodon_ and +_Microtus_ in Kansas. Extremely wet conditions seem to give _Sigmodon_ a +competitive advantage whereas _Microtus_ is better able to survive dry +summers and severe winters. However, these relationships need further +clarification by an intensive study of the life history of _Sigmodon_ in +Kansas (especially the more arid western part), including its coactions +with the communities it has invaded successfully recently. + +The harvest mouse (_Reithrodontomys megalotis_) also was a common +inhabitant of the study plots, but this small rodent seemed not to be a +serious competitor of the voles, as its food consists almost entirely of +seeds (Cockrum, _op. cit._:165) not usually used by voles. In this +study, at least, no conflict over space was apparent. Harvest mice +frequently were taken in the runways of voles and even in the same trap +with voles. Reithro Field, the part of the Reservation having the +heaviest population of the harvest mouse, differed from the habitats +that were better for voles in being higher, drier and less densely +covered with vegetation. However, during the summer of 1951 when the +voles were most abundant, Reithro Field supported a large population of +voles. Estimates of population of the harvest mouse were of doubtful +validity in summer because it was readily trapped only in winter and +early spring. Many individuals marked in late spring were not trapped +again until late autumn although presumably they remained on the area. +This seasonal variation in trapping success seemed to be a matter of +acceptance and refusal of bait (Fitch, 1954:45). + +The presence of the wood mouse (_Peromyscus leucopus_) on the study +plots indicated an overlapping of habitats. Both House and Quarry Fields +were on the ecotone between forest and meadow and a mixture of mammals +from both types of habitat occurred. No sign of the homes of the wood +mouse was found on the study plots, and on the larger trap line, +operated by Fitch, wood mice were captured only near the edge of the +woods. + +Only six deer mice (_Peromyscus maniculatus_) were taken on the study +plots. This small number probably provided an inaccurate index of the +association of the deer mouse and the prairie vole, because samples from +snap-traps and the data of other workers on the Reservation showed a +more common occurrence of the two species together. The deer mice seemed +to prefer a sparser vegetation and did not approach so closely to the +forest edge as did the voles. It may have been, in part, the presence of +_P. leucopus_ in the ecotonal region which made it unsuitable for _P. +maniculatus_. + +Other mammals noted on the study areas were the following: _Didelphis +marsupialis_, _Blarina brevicauda_, _Scalopus aquaticus_, _Canis +familiaris_, _Canis latrans_, _Procyon lotor_, _Felis domesticus_, +_Sylvilagus floridanus_, _Microtus pinetorum_, _Mus musculus_ and _Zapus +hudsonius_. + + + + +SUMMARY AND CONCLUSIONS + + +In the 23-month period from October, 1950, to August, 1952, the ecology +of the prairie vole, _Microtus ochrogaster_, was investigated on the +Natural History Reservation of the University of Kansas. In all, 817 +voles were captured 2941 times in 13,880 "live-trap days." For some +aspects of this study, Dr. Henry S. Fitch, resident investigator on the +Reservation, permitted the use of his trapping records. He had captured +1416 voles 5098 times. The total number of live voles used in the study +was thus 2233, and they were captured 8039 times. In addition to the +voles, I caught 96 cotton rats, 108 harvest mice, 29 wood mice, 2 pine +voles and 6 deer mice in live traps. When Fitch's records were used, the +live-trapping data covered a thirty-month period and general field data +were available from July, 1949, to August, 1952. + +Hall and Cockrum (1953:406) stated that probably all microtine rodents +fluctuate markedly in numbers. Certainly the populations I studied did +so, but the fluctuations were not regularly recurring for _M. +ochrogaster_ as they seem to be for some species of the genus in more +northern life zones. The changes in the density of populations described +in this paper can be explained without recourse to cycles of long +time-span and literature dealing specifically with _M. ochrogaster_ +makes no references to such cycles. There is, however, an annual cycle +of abundance: greatest density of population occurs in autumn, and the +least density in January. + +This annual pattern is often, perhaps usually, obscured because of the +extreme sensitivity of voles to a variety of changes in their +environment. These changes are reflected as variations in reproductive +success. In this study, some of these changes were accentuated by the +great range in annual precipitation. Annual rainfall was approximately +average in 1950 (36.32 inches, 0.92 inches above normal), notably high +in 1951 (50.68 inches, 15.28 inches above normal) and notably low in +1952 (23.80 inches, 11.60 inches below normal). + +Among the types of environmental modification to which the populations +of voles reacted were plant succession, an increase in competition with +_Sigmodon_, abnormal rainfall and concentration of predators. In the +overgrazed disclimax existing in 1948 when the study areas were +reserved, no voles were found because cover was insufficient. After the +area was protected a succession of good growing years hastened the +recovery of the grasses and the populations of voles reached high +levels. In areas where the vegetation approached the climax community, +the densities of voles decreased from the levels supported by the +immediately preceding seral stages. The higher carrying capacity of +these earlier seral stages was probably due to the greater variety of +herbaceous vegetation which tended to maintain a more constant supply of +young and growing parts of plants which were the preferred food of +voles. Later in the period of study the succession from grasses to woody +plants on parts of the study areas also affected the population of +voles. Not only did the voles withdraw from the advancing edge of the +forest, but their density decreased in the meadows as the number of +shrubs and other woody plants increased. These influences of the +succession of plants on the population density of voles were exerted +through changes in cover and in the quality, as well as the quantity, of +the food supply. + +Whenever voles were in competition with cotton rats, there was a +depression in the population levels of voles. Primarily, the competition +between the two species is the result of an extensive coincidence of +food habits, but competition for space, cover and nesting material is +also present. There was one direct coaction between these two species +observed. Cotton rats, at least occasionally, ate voles, especially +young individuals. In extremely wet weather, as in the summer of 1951, +the high survival rate of newborn cotton rats resulted in an increase in +their detrimental effect on the population of voles. However, cotton +rats proved to be less well adapted to severe cold or drought than were +voles. + +Heavy rainfall reduced the densities of populations of voles by killing +a large percentage of juveniles. During the summer of 1951 the +competition of cotton rats further depressed the population level of the +voles, but the relative importance of competition with cotton rats and +superabundant moisture in effecting the observed reduction in population +density is difficult to judge. Perhaps most of the decrease in +population which followed the heavy rains was due to competition rather +than to weather. Subnormal rainfall, as in 1952, reduced the population +of voles by inhibiting reproduction. Presumably because of an altered +food supply, reproduction almost ceased during the drought. Utilization +of the habitat was further reduced in the summer of 1952 because the +voles did not grow so large as they otherwise did. + +Predation, as a general rule, does not significantly affect densities of +populations, but large numbers of predators concentrating on small areas +may rapidly reduce the numbers of prey animals. In the course of my +study, such a situation occurred but once, when a group of long-eared +owls roosted in the woods adjacent to Quarry Field. The population of +voles in that area was probably reduced somewhat as a result of +predation by owls. + +Population trends in either direction may be reversed suddenly by +changes in the factors discussed above. In the fall of 1951, a downward +trend in the density of the voles was evident. At this time, populations +of cotton rats were increasing rapidly and competition between cotton +rats and voles was intensified. In February, 1952, the population of +cotton rats was decimated suddenly by a short period of unusually cold +weather. The voles were suddenly freed from the stress of competition +and the population immediately began to rise. The upward trend began +prior to the annual spring increase and was subsequently reinforced by +it. In the last part of May, 1952, the upward trend of the population +was reversed, as the drought became severe, and the density of the +population decreased rapidly. This drop was too sudden and too extreme +to be only the normal summer slump. The relatively rapid response of +voles to a heavy rain after a dry period, first by increased breeding +and later by increases in density, is one more example of abrupt changes +in population trends caused by altered environmental conditions. + +In the population changes that I observed, no evident "die-off" of +adults accompanied even the most drastic reductions in population +density. The causative factor directly influences the population either +by inhibiting reproduction or by increasing infant and prenatal +mortality. The net reduction is due to an inadequate replacement of +those voles lost by normal attrition. + +Most voles, under natural conditions, live less than one year. Those +individuals born in the autumn live longer, as a group, than those born +at any other time. Since the heaviest mortality is in young voles, +adults which become established in an area may live more than 18 months +and, if they are females, may produce more than a dozen litters. No +decrease in vigor and fertility was found to accompany aging. A +relationship between the condylobasilar length of the skull and the age +of a vole was discovered and, with further study, may yield a method of +aging voles more accurately than has been possible heretofore. Other +characteristics, varying with age, were described. The most reliable +indicator of age seemed to be the prominence of the temporal ridges. + +Runway systems and burrows are used by groups of voles rather than by +individuals. Most of the activity of voles is confined to these runways +and an exposed individual is seldom seen. A home range may include +several runway systems, and the ranges of individuals overlap +extensively. Both home ranges and patterns of runway systems change +constantly. Runways seem to be primarily feeding trails, and are +extended or abandoned as the voles change their feeding habits. Groups +of adult voles using a system of runways seem to have no special +relationship. Juveniles tend to stay near their mothers, but as they +mature, they shift their ranges and are replaced by other individuals. +Males wander more than females, and shift their ranges more often. No +intolerance of other voles exists and, in laboratory cages, groups of +voles lived together peaceably from the time they are placed together. +Crowding does not seem to be harmful directly, therefore, and high +densities will develop if food and cover resources permit. + +As a prey item, the prairie vole proved to be an important part of the +biota of the Reservation. It was eaten frequently by almost all of the +larger vertebrate predators on the Reservation and was, seemingly, the +most important food item of the long-eared owl. The ability of the +prairie vole to maintain high levels of population over relatively broad +areas enhances its value as a prey species. + + + + +LITERATURE CITED + + +ALBERTSON, F. W. + + 1937. Ecology of a mixed prairie in west-central Kansas. Ecol. + Monog., 7:481-547. + +BAILEY, V. + + 1902. Synopsis of the North American species of _Sigmodon_. Proc. + Biol. Soc. Washington, 15:101-116. + + 1924. 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Apparent home range of _Microtus_ in relation to distance + between traps. Jour. Mamm., 31:26-39. + +HINTON, M. A. C. + + 1926. Monograph of the voles and lemmings (Microtinae) living and + extinct. British Museum of Nat. Hist., London, xvi + 488 pp. 15 + pls. + +HOPKINS, H. H., F. W. ALBERTSON, and D. A. RIEGEL. + + 1952. Ecology of grassland utilization in a mixed prairie. Trans. + Kansas Acad. Sci., 55:395-418. + +HOWARD, W. E. + + 1951. Relation between low temperature and available food to + survival of small rodents. Jour. Mamm., 32:300-312. + +HOWELL, A. B. + + 1924. Individual and age variation in _Microtus montanus yosemite_. + Jour. Agric. Res., 28:977-1015. + +JAMESON, E. W. + + 1947. Natural history of the prairie vole. Univ. Kansas, Mus. Nat. + Hist. Publ., 1:125-151. + + 1950. Determining fecundity in male small mammals. Jour. Mamm., + 31:433-436. + +JOHNSON, M. S. + + 1926. Activity and distribution of certain wild mice in relation to + the biotic community. Jour. Mamm., 7:245-277. + +KORSCHGEN, L. J. + + 1952. A general summary of the food of Missouri predatory and game + animals. Conserv. Comm., Div. Fish and Game, State of Missouri. + July, 1952. 61 pp. + +LANTZ, D. E. + + 1907. An economic survey of the field mice (genus _Microtus_). USDA + Biol. Surv. Bull, 31:1-64. + +LESLIE, P. H., and R. M. RANSOM. + + 1940. The mortality, fertility and rate of natural increase of the + vole (_Microtus agrestis_) as observed in the laboratory. Jour. + Animal Ecol., 9:27-52. + +LLEWELLYN, L. M. + + 1950. Reduction of mortality in live-trapping mice. Jour. Wildlife + Mgmt., 14:84-85. + +MCATEE, W. L. + + 1935. Food habits of common hawks. USDA Circ., 370:1-36. + +MEYER, B. J., and R. K. MEYER. + + 1944. Growth and reproduction of the cotton rat, _Sigmodon hispidus + hispidus_, under laboratory conditions. Jour. Mamm., 25:107-129. + +MOHR, C. O. + + 1943. A comparison of North American small mammal censuses. Amer. + Midland Nat., 29:545-587. + + 1947. Table of equivalent populations of North American small + mammals. Amer. Midland Nat., 37:223-249. + +PHILLIPS, P. + + 1936. The distribution of rodents in overgrazed and normal + grassland in central Oklahoma. Ecol., 17:673-679. + +POILEY, S. M. + + 1949. Raising captive meadow voles (_Microtus p. pennsylvanicus_). + Jour. Mamm., 30:317. + +RINKER, G. C. + + 1942a. Litter records of some mammals of Meade County, Kansas. + Trans. Kansas Acad. Sci., 45:376-378. + + 1942b. An extension of the range of the Texas cotton rat in Kansas. + Jour. Mamm., 23:439. + +SANDIDGE, L. L. + + 1953. Food and dens of the opossum (_Didelphis virginiana_) in + northeastern Kansas. Trans. Kansas Acad. Sci., 56:97-106. + +SELLE, R. M. + + 1928. _Microtus californicus_ in captivity. Jour. Mamm., 9:93-98. + +SHAPIRO, J. + + 1950. Notes on the population dynamics of _Microtus_ and _Blarina_ + with a record of albinism in _Blarina_. Jour. Wildlife Mgmt, + 14:359-360. + +STICKEL, L. F. + + 1946. Experimental analysis of methods of measuring small mammal + populations. Jour. Wildlife Mgmt., 10:150-159. + + 1948. The trap line as a measure of small mammal populations. Jour. + Wildlife Mgmt., 12:153-161. + +STRECKER, J. K. + + 1929. Notes on the Texas cotton and Atwater wood rats. Jour. Mamm., + 10:216-220. + +SUMMERHAYES, V. S. + + 1941. The effects of voles (_Microtus agrestis_) on vegetation. + Jour. Ecol., 29:14-48. + +SVIHLA, A. + + 1929. Life history notes on _Sigmodon hispidus hispidus_. Jour. + Mamm., 10:352-353. + +TOWNSEND, M. T. + + 1935. Studies on some small mammals of central New York. Roosevelt + Wildlife Annals, 4:1-120. + +UHLER, F. M., C. COTTAM, and T. E. CLARKE. + + 1939. Food of the snakes of George Washington National Forest, + Virginia. Trans. 4th N. A. Wildlife Conf., 605-622. + +WOOSTER, L. D. + + 1935. Notes on the effects of drought on animal populations in + western Kansas. Trans. Kansas Acad. Sci., 38:351-352. + + 1936. The contents of owl pellets as indicators of habitat + preferences of small mammals. Trans. Kansas Acad. Sci., 39:395-397. + + 1939. An ecological evaluation of predatees on a mixed prairie area + in western Kansas. Trans. Kansas Acad. Sci., 42:515-517. + + + _Transmitted May 19, 1955._ + + + + +UNIVERSITY OF KANSAS PUBLICATIONS, MUSEUM OF NATURAL HISTORY + + +Institutional libraries interested in publications exchange may obtain +this series by addressing the Exchange Librarian, University of Kansas +Library, Lawrence, Kansas. Copies for individuals, persons working in a +particular field of study, may be obtained by addressing instead the +Museum of Natural History, University of Kansas, Lawrence, Kansas. There +is no provision for sale of this series by the University Library which +meets institutional requests, or by the Museum of Natural History which +meets the requests of individuals. However, when individuals request +copies from the Museum, 25 cents should be included, for each separate +number that is 100 pages or more in length, for the purpose of defraying +the costs of wrapping and mailing. + +* An asterisk designates those numbers of which the Museum's supply (not +the Library's supply) is exhausted. Numbers published to date, in this +series, are as follows: + +Vol. 1. + + Nos. 1-26 and index. Pp. 1-638, 1946-1950. + + Index. Pp. 605-638. + +*Vol. 2. + + (Complete) Mammals of Washington. By Walter W. Dalquest. Pp. 1-444, + 140 figures in text. April 9, 1948. + +Vol. 3. + + *1. The avifauna of Micronesia, its origin, evolution, and + distribution. By Rollin H. Baker. Pp. 1-359, 16 figures in text. + June 12, 1951. + + *2. A quantitative study of the nocturnal migration of birds. By + George H. Lowery, Jr. Pp. 361-472, 47 figures in text. June 29, + 1951. + + 3. Phylogeny of the waxwings and allied birds. By M. Dale Arvey. + Pp. 473-530, 49 figures in text, 13 tables. October 10, 1951. + + 4. Birds from the state of Veracruz, Mexico. By George H. Lowery, + Jr., and Walter W. Dalquest. Pp. 531-649, 7 figures in text, 2 + tables. October 10, 1951. + + Index. Pp. 651-681. + +*Vol. 4. + + (Complete) American weasels. By E. Raymond Hall. Pp. 1-466, 41 + plates, 31 figures in text. December 27, 1951. + +Vol. 5. + + 1. Preliminary survey of a Paleocene faunule from the Angels Peak + area, New Mexico. By Robert W. Wilson. Pp. 1-11, 1 figure in text. + February 24, 1951. + + 2. Two new moles (Genus Scalopus) from Mexico and Texas. By Rollin + H. Baker. Pp. 17-24. February 28, 1951. + + 3. Two new pocket gophers from Wyoming and Colorado. By E. Raymond + Hall and H. Gordon Montague. Pp. 25-32. February 28, 1951. + + 4. Mammals obtained by Dr. Curt von Wedel from the barrier beach of + Tamaulipas, Mexico. By E. Raymond Hall. Pp. 33-47, 1 figure in + text. October 1, 1951. + + 5. Comments on the taxonomy and geographic distribution of some + North American rabbits. By E. Raymond Hall and Keith R. Kelson. Pp. + 49-58. October 1, 1951. + + 6. Two new subspecies of Thomomys bottae from New Mexico and + Colorado. By Keith R. Kelson. Pp. 59-71, 1 figure in text. October + 1, 1951. + + 7. A new subspecies of Microtus montanus from Montana and comments + on Microtus canicaudus Miller. By E. Raymond Hall and Keith R. + Kelson. Pp. 73-79. October 1, 1951. + + 8. A new pocket gopher (Genus Thomomys) from eastern Colorado. By + E. Raymond Hall. Pp. 81-85. October 1, 1951. + + 9. Mammals taken along the Alaskan Highway. By Rollin H. Baker. Pp. + 87-117, 1 figure in text. November 28, 1951. + + *10. A synopsis of the North American Lagomorpha. By E. Raymond + Hall. Pp. 119-202, 68 figures in text. December 15, 1951. + + 11. A new pocket mouse (Genus Perognathus) from Kansas. By E. + Lendell Cockrum. Pp. 203-206. December 15, 1951. + + 12. Mammals from Tamaulipas, Mexico. By Rollin H. Baker. Pp. + 207-218. December 15, 1951. + + 13. A new pocket gopher (Genus Thomomys) from Wyoming and Colorado. + By E. Raymond Hall. Pp. 219-222. December 15, 1951. + + 14. A new name for the Mexican red bat. By E. Raymond Hall. Pp. + 223-226. December 15, 1951. + + 15. Taxonomic notes on Mexican bats of the Genus Rhogeëssa. By E. + Raymond Hall. Pp. 227-232. April 10, 1952. + + 16. Comments on the taxonomy and geographic distribution of some + North American woodrats (Genus Neotoma). By Keith R. Kelson. Pp. + 233-242. April 10, 1952. + + 17. The subspecies of the Mexican red-bellied squirrel, Sciurus + aureogaster. By Keith R. Kelson. Pp. 243-250, 1 figure in text. + April 10, 1952. + + 18. Geographic range of Peromyscus melanophrys, with description of + new subspecies. By Rollin H. Baker. Pp. 251-258, 1 figure in text. + May 10, 1952. + + 19. A new chipmunk (Genus Eutamias) from the Black Hills. By John + A. White. Pp. 259-262. April 10, 1952. + + 20. A new piñon mouse (Peromyscus truei) from Durango, Mexico. By + Robert B. Finley, Jr. Pp. 263-267. May 23, 1952. + + 21. An annotated checklist of Nebraskan bats. By Olin L. Webb and + J. Knox Jones, Jr. Pp. 269-279. May 31, 1952. + + 22. Geographic variation in red-backed mice (Genus Clethrionomys) + of the southern Rocky Mountain region. By E. Lendell Cockrum and + Kenneth L. Fitch. Pp. 281-292, 1 figure in text. November 15, 1952. + + 23. Comments on the taxonomy and geographic distribution of North + American microtines. By E. Raymond Hall and E. Lendell Cockrum. Pp. + 293-312. November 17, 1952. + + 24. The subspecific status of two Central American sloths. By E. + Raymond Hall and Keith R. Kelson. Pp. 313-317. November 21, 1952. + + 25. Comments on the taxonomy and geographic distribution of some + North American marsupials, insectivores, and carnivores. By E. + Raymond Hall and Keith R. Kelson. Pp. 319-341. December 5, 1952. + + 26. Comments on the taxonomy and geographic distribution of some + North American rodents. By E. Raymond Hall and Keith R. Kelson. Pp. + 343-371. December 15, 1952. + + 27. A synopsis of the North American microtine rodents. By E. + Raymond Hall and E. Lendell Cockrum. Pp. 373-498, 149 figures in + text. January 15, 1953. + + 28. The pocket gophers (Genus Thomomys) of Coahuila, Mexico. By + Rollin H. Baker. Pp. 499-514, 1 figure in text. June 1, 1953. + + 29. Geographic distribution of the pocket mouse, Perognathus + fasciatus. By J. Knox Jones, Jr. Pp. 515-526, 7 figures in text. + August 1, 1953. + + 30. A new subspecies of wood rat (Neotoma mexicana) from Colorado. + By Robert B. Finley, Jr. Pp. 527-534, 2 figures in text. August 15, + 1953. + + 31. Four new pocket gophers of the genus Cratogeomys from Jalisco, + Mexico. By Robert J. Russell. Pp. 535-542. October 15, 1953. + + 32. Genera and subgenera of chipmunks. By John A. White. Pp. + 543-561, 12 figures in text. December 1, 1953. + + 33. Taxonomy of the chipmunks, Eutamias quadrivittatus and Eutamias + umbrinus. By John A. White. Pp. 563-582, 6 figures in text. + December 1, 1953. + + 34. Geographic distribution and taxonomy of the chipmunks of + Wyoming. By John A. White. Pp. 584-610, 3 figures in text. December + 1, 1953. + + 35. The baculum of the chipmunks of western North America. By John + A. White. Pp. 611-631, 19 figures in text. December 1, 1953. + + 36. Pleistocene Soricidae from San Josecito Cave, Nuevo Leon, + Mexico. By James S. Findley. Pp. 633-639. December 1, 1953. + + 37. Seventeen species of bats recorded from Barro Colorado Island, + Panama Canal Zone. By E. Raymond Hall and William B. Jackson. Pp. + 641-646. December 1, 1953. + + Index. Pp. 647-676. + +*Vol. 6. + + (Complete) Mammals of Utah, _taxonomy and distribution_. By Stephen + D. Durrant. Pp. 1-549, 91 figures in text, 30 tables. August 10, + 1952. + +Vol. 7. + + *1. Mammals of Kansas. By E. Lendell Cockrum. Pp. 1-303, 73 figures + in text, 37 tables. August 25, 1952. + + 2. Ecology of the opossum on a natural area in northeastern Kansas. + By Henry S. Fitch and Lewis L. Sandidge. Pp. 305-338, 5 figures in + text. August 24, 1953. + + 3. The silky pocket mice (Perognathus flavus) of Mexico. By Rollin + H. Baker. Pp. 339-347, 1 figure in text. February 15, 1954. + + 4. North American jumping mice (Genus Zapus). By Philip H. + Krutzsch. Pp. 349-472, 47 figures in text, 4 tables. April 21, + 1954. + + 5. Mammals from Southeastern Alaska. By Rollin H. Baker and James + S. Findley. Pp. 473-477. April 21, 1954. + + 6. Distribution of some Nebraskan Mammals. By J. Knox Jones, Jr. + Pp. 479-487. April 21, 1954. + + 7. Subspeciation in the montane meadow mouse, Microtus montanus, in + Wyoming and Colorado. By Sydney Anderson. Pp. 489-506, 2 figures in + text. July 23, 1954. + + 8. A new subspecies of bat (Myotis velifer) from southeastern + California and Arizona. By Terry A. Vaughn. Pp. 507-512. July 23, + 1954. + + 9. Mammals of the San Gabriel mountains of California. By Terry A. + Vaughn. Pp. 513-582, 1 figure in text, 12 tables. November 15, + 1954. + + 10. A new bat (Genus Pipistrellus) from northeastern Mexico. By + Rollin H. Baker. Pp. 583-586. November 15, 1954. + + 11. A new subspecies of pocket mouse from Kansas. By E. Raymond + Hall. Pp. 587-590. November 15, 1954. + + 12. Geographic variation in the pocket gopher, Cratogeomys + castanops, in Coahuila, Mexico. By Robert J. Russell and Rollin H. + Baker. Pp. 591-608. March 15, 1955. + + 13. A new cottontail (Sylvilagus floridanus) from northeastern + Mexico. By Rollin H. Baker. Pp. 609-612. April 8, 1955. + + 14. Taxonomy and distribution of some American shrews. By James S. + Findley. Pp. 613-618. June 10, 1955. + + 15. Distribution and systematic position of the pigmy woodrat, + Neotoma goldmani. By Dennis G. Rainey and Rollin H. Baker. Pp. + 619-624, 2 figs. in text. June 10, 1955. + + Index. Pp. 625-651. + +Vol. 8. + + 1. Life history and ecology of the five-lined skink, Eumeces + fasciatus. By Henry S. Fitch. Pp. 1-156, 2 pls., 26 figs. in text, + 17 tables. September 1, 1954. + + 2. Myology and serology of the Avian Family Fringillidae, a + taxonomic study. By William B. Stallcup. Pp. 157-211, 23 figures in + text, 4 tables. November 15, 1954. + + 3. An ecological study of the collared lizard (Crotaphytus + collaris). By Henry S. Fitch. Pp. 213-274, 10 figures in text. + February 10, 1956. + + 4. A field study of the Kansas ant-eating frog, Gastrophryne + olivacea. By Henry S. Fitch. Pp. 275-306, 9 figures in text. + February 10, 1956. + + 5. Check-list of the birds of Kansas. By Harrison B. Tordoff. Pp. + 307-359, 1 figure in text. March 10, 1956. + + 6. A population study of the prairie vole (Microtus ochrogaster) in + Northeastern Kansas. By Edwin P. Martin. Pp. 361-416, 19 figures in + text. April 2, 1956. + + More numbers will appear in volume 8. + +Vol. 9. + + 1. Speciation of the wandering shrew. By James S. Findley. Pp. + 1-68, 18 figures in text. December 10, 1955. + + 2. Additional records and extensions of ranges of mammals from + Utah. By Stephen D. Durrant, M. Raymond Lee, and Richard M. Hansen. + Pp. 69-80. December 10, 1955. + + 3. A new long-eared myotis (Myotis evotis) from northeastern + Mexico. By Rollin H. Baker and Howard J. Stains. Pp. 81-84. + December 10, 1955. + + More numbers will appear in volume 9. + + + + + * * * * * + + + + +Transcriber's note: + +A Table of Contents has been added to this ebook for the reader's +convenience. + +Some words in this text are found in both hyphenated and non-hyphenated +form (for instance: Condylo-basilar/condylobasilar, +mid-winter/midwinter). These variations match the text of the original +document. A few obvious punctuation errors have been repaired. Spelling +has been retained as it appears in the original publication, except as +follows: + + p. 372, in "A more homogeneous vegetation would tend to pass" homogenous + has been changed to homogeneous. + + p. 415, "1953. Foods, and dens of the opossum ..." has been changed to + "1953. Food and dens of the opossum ..." + +In Fig. 11 the bottommost y-axis label in the scale of gms. is probably +an error: 45 should be 35. + +Some illustrations have been moved from their original locations to +paragraph breaks, so as to be nearer to their corresponding text, and +for ease of document navigation. References to scale in illustration +captions are those of the original publication, and therefore do not +correspond to the scale of the images in the HTML version of this ebook. + +The list of University of Kansas Publications from the front of the +original document has been joined to its mate at the end of this text. + +Because the cover of the original document contained text exactly +duplicated on the title page, this cover information has been omitted. + + + + + +End of the Project Gutenberg EBook of A Population Study of the Prairie Vole +(Microtus ochrogaster) in Northeastern Kansas, by Edwin P. 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