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+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. Breeding, feeding and other life habits of meadow mice. Jour.
+     Agric. Res., 27:523-536.
+
+BAKER, J. R., and R. M. RANSOM.
+
+     1932a. Factors affecting the breeding of the field mouse (_Microtus
+     agrestis_). Part I. Light. Proc. Roy. Soc. London, Series B,
+     110:313-322.
+
+     1932b. Factors affecting the breeding of the field mouse (_Microtus
+     agrestis_). Part II. Temperature. Proc. Roy. Soc. London, Series B,
+     112:39-46.
+
+BLACK, T. D.
+
+     1937. Mammals of Kansas. Kansas State Board Agric., 13th Biennial
+     Rep., 1935-36:116-217.
+
+BLAIR, W. F.
+
+     1939. Some observed effects of stream valley flooding on small
+     mammal populations in eastern Oklahoma. Jour. Mamm., 20:304-306.
+
+     1940. Home ranges and populations of the meadow vole in southern
+     Michigan. Jour. Wildlife Mgmt., 4:149-161.
+
+     1941. Techniques for the study of small mammal populations. Jour.
+     Mamm., 22:148-157.
+
+     1948. Population density, life span and mortality rates of small
+     mammals in the bluegrass meadow and bluegrass field associations of
+     southern Michigan. Amer. Midland Nat., 40:395-419.
+
+BODENHEIMER, F. S., and F. SULMAN.
+
+     1946. The estrous cycle of _Microtus guentheri_ D. and A. and its
+     ecological implications. Ecol., 27:255-256.
+
+BOLE, B. P., JR.
+
+     1939. The quadrat method of studying small mammal populations.
+     Cleveland Mus. Nat. Hist. Sci. Publ., 5:1-77.
+
+BROWN, H. L.
+
+     1946. Rodent activity in a mixed prairie near Hays, Kansas. Trans.
+     Kansas Acad. Sci., 48:448-458.
+
+BRUMWELL, M.
+
+     1951. An ecological survey of the Fort Leavenworth Military
+     Reservation. Amer. Midland Nat., 45:187-231.
+
+BURT, W. H.
+
+     1943. Territoriality and home range concepts as applied to mammals.
+     Jour. Mamm., 24:346-352.
+
+CALHOUN, J. B.
+
+     1945. Diel activity rhythms of the rodents _Microtus ochrogaster_
+     and _Sigmodon hispidus hispidus_. Ecol., 26:251-273.
+
+CHITTY, D., and D. A. KEMPSON.
+
+     1949. Prebaiting of small mammals and a new design of a live trap.
+     Ecol., 30:536-542.
+
+COCKRUM, E. L.
+
+     1947. Effectiveness of live traps vs. snap traps. Jour. Mamm.,
+     28:186.
+
+     1948. Distribution of the hispid cotton rat in Kansas. Trans.
+     Kansas Acad. Sci., 51:306-312.
+
+     1952. Mammals of Kansas. Univ. Kansas Mus. Nat. Hist. Publ.,
+     7:1-303.
+
+DAVIS, D. H. S.
+
+     1933. Rhythmic activity in the short-tailed vole, _Microtus_. Jour.
+     Animal Ecol., 2:232-238.
+
+DICE, L. R.
+
+     1922. Some factors affecting the distribution of the prairie vole,
+     forest deer mouse and prairie deer mouse. Ecol., 3:29-47.
+
+EADIE, W. R.
+
+     1944. The short-tailed shrew and field mouse predation. Jour.
+     Mamm., 25:359-362.
+
+     1948. Shrew-mouse predation during low mouse abundance. Jour.
+     Mamm., 29:35-37.
+
+     1952. Shrew predation and vole populations on a limited area. Jour.
+     Mamm., 33:185-189.
+
+     1953. Response of _Microtus_ to vegetative cover. Jour. Mamm.,
+     34:262-264.
+
+ELTON, C.
+
+     1949. Population interspersion. An essay in animal community
+     patterns. Jour. Ecol., 37:1-23.
+
+ERRINGTON, P. R.
+
+     1935. Food habits of midwestern foxes. Jour. Mamm., 16:192-200.
+
+     1936. What is the meaning of predation? Smithsonian Inst. Rep.,
+     1936:243-252.
+
+     1943. An analysis of mink predation upon muskrat in north central
+     United States. Agric. Exp. Sta., Iowa State Coll. Agric. Mech.
+     Arts, Res. Bull., 320:799-924.
+
+     1946. Predation and vertebrate populations. Quart. Rev. Biol.,
+     21:144-177.
+
+ERRINGTON, P. L., F. HAMERSTROM, and F. N. HAMERSTROM, JR.
+
+     1940. The great horned owl and its prey in north central United
+     States. Agric. Exp. Sta., Iowa State Coll. Agric. Mech. Arts, Res.
+     Bull., 277:759-831.
+
+FISHER, H. J.
+
+     1945. Notes on the voles in central Missouri. Jour. Mamm.,
+     26:435-437.
+
+FITCH, H. S.
+
+     1948. A study of coyote relationships on cattle range. Jour.
+     Wildlife Mgmt., 12:73-78.
+
+     1950. A new style live trap for small mammals. Jour. Mamm.,
+     31:364-365.
+
+     1952. The University of Kansas Natural History Reservation. Univ.
+     Kansas, Mus. Nat. Hist. Misc. Publ., 4:1-38.
+
+     1954. Seasonal acceptance of bait by small mammals. Jour. Mamm.,
+     35:39-47.
+
+FITCH, H. S., F. SWENSON, and D. F. TILLOTSON.
+
+     1946. Behavior and food habits of the red-tailed hawk. Condor,
+     48:205-237.
+
+GOODPASTOR, W. W., and D. F. HOFFMEISTER.
+
+     1952. Notes on the mammals of eastern Tennessee. Jour. Mamm.,
+     33:362-371.
+
+GUNDERSON, H. L.
+
+     1950. A study of some small mammal populations at Cedar Creek
+     Forest, Asoka County, Minnesota. Univ. Minnesota Mus. Nat. Hist.,
+     4:1-49.
+
+HALL, E. R.
+
+     1926. Changes during growth in the skull of the rodent
+     _Otospermophilus grammarus beecheyi_. Univ. California Publ. Zool.,
+     21:355-404.
+
+HALL, E. R., and E. L. COCKRUM.
+
+     1953. A synopsis of North American microtine rodents. Univ. Kansas,
+     Mus. Nat. Hist. Publ., 5:373-498.
+
+HAMILTON, W. J., JR.
+
+     1937a. Growth and life span of the field mouse. Amer. Nat.,
+     71:500-507.
+
+     1937b. The biology of microtine cycles. Jour. Agric. Res.,
+     54:779-790.
+
+     1937c. Activity and home range of the field mouse. Ecol.,
+     18:255-263.
+
+     1940. Life and habits of the field mouse. Sci. Monthly, 50:425-434.
+
+     1941. The reproduction of the field mouse, _Microtus
+     pennsylvanicus_. Cornell Univ. Agric. Exp. Sta. Mem., 237:1-23.
+
+     1949. The reproductive rates of some small mammals. Jour. Mamm.,
+     30:257-260.
+
+HATFIELD, D. M.
+
+     1935. A natural history of _Microtus californicus_. Jour. Mamm.,
+     16:261-271.
+
+HATT, R. T.
+
+     1930. The biology of the voles of New York. Roosevelt Wildlife
+     Bull., 5:513-623.
+
+HAYNE, D. M.
+
+     1949a. Two methods of estimating populations from trapping records.
+     Jour. Mamm., 30:399-411.
+
+     1949b. Calculation of the size of home range. Jour. Mamm., 30:1-18.
+
+     1950. 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. Martin
+
+*** END OF THIS PROJECT GUTENBERG EBOOK POPULATION STUDY OF PRAIRIE VOLE ***
+
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+
+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
+
+
+
+
+
+
+</pre>
+
+
+
+
+<hr class="fulldoubleweightblack" />
+<hr class="fullunderblack" />
+<p class="h3">
+<span class="smcap">University of Kansas Publications</span></p>
+<p class="h4"><span class="smcap">Museum of Natural History</span></p>
+<hr class="shortthinblack" />
+<p class="h3flat">Volume 8, No. 6, pp. 361-416, 19 figures in text</p>
+<p class="h3flat">April 2, 1956</p>
+<hr class="fullunderblack" />
+<p class="spacer">&nbsp;</p>
+
+<h1 id="booktitle">A Population Study
+of the Prairie Vole (Microtus ochrogaster)
+in Northeastern Kansas</h1>
+
+<p class="h4">BY</p>
+
+<p class="h4">EDWIN P. MARTIN</p>
+<p class="spacer">&nbsp;</p>
+<p class="h4"><span class="smcap">University of Kansas<br />
+Lawrence</span><br />
+1956
+</p>
+<p class="spacer">&nbsp;</p>
+
+
+<hr class="chap" />
+<p class="h4">
+<span class="smcap">University of Kansas Publications, Museum of Natural History</span><br />
+
+Editors: E. Raymond Hall, Chairman, A. Byron Leonard, Robert W. Wilson<br /><br />
+</p>
+
+<p class="h4">Volume 8, No. 6, pp. 361-416, 19 figures in text<br />
+Published April 2, 1956<br /> </p>
+<p class="spacer">&nbsp;</p>
+<p class="h4"><span class="smcap">University of Kansas</span><br />
+Lawrence, Kansas<br /></p>
+<p class="spacer">&nbsp;</p>
+<p class="h5">PRINTED BY<br />
+FERD VOILAND, JR., STATE PRINTER<br />
+TOPEKA, KANSAS<br />
+1956<br />
+<br />
+25-9225<br />
+</p>
+
+
+
+<hr class="chap" />
+
+
+<h2><a name="Contents" id="Contents">Contents</a></h2>
+<div class="center">
+    <table summary="contents" cellpadding="3" cellspacing="0" >
+        <tbody>
+            <tr>
+                <td align="left">&nbsp;</td>
+                <td align="left">&nbsp;</td>
+                <td align="right"><small>PAGE</small></td></tr>
+            <tr>
+                <td colspan="2" align="left">INTRODUCTION</td>
+                <td align="right"><a href="#introduction">363</a></td></tr>
+            <tr>
+                <td colspan="2" align="left">GENERAL METHODS</td>
+                <td align="right"><a href="#general_methods">364</a></td></tr>
+            <tr>
+                <td colspan="2" align="left">HABITAT</td>
+                <td align="right"><a href="#habitat">366</a></td></tr>
+            <tr>
+                <td colspan="2" align="left">POPULATION STRUCTURE</td>
+                <td align="right"><a href="#population_structure">373</a></td></tr>
+            <tr>
+                <td colspan="2" align="left">POPULATION DENSITY</td>
+                <td align="right"><a href="#population_density">376</a></td></tr>
+            <tr>
+                <td colspan="2" align="left">HOME RANGE</td>
+                <td align="right"><a href="#home_range">380</a></td></tr>
+            <tr>
+                <td colspan="2" align="left">LIFE HISTORY</td>
+                <td align="right"><a href="#life_history">383</a></td></tr>
+            <tr>
+                <td align="left">&nbsp;</td>
+                <td align="left">Reproduction</td>
+                <td align="right"><a href="#reproduction">383</a></td></tr>
+            <tr>
+                <td align="left">&nbsp;</td>
+                <td align="left">Litter Size and Weight</td>
+                <td align="right"><a href="#litter_size">386</a></td></tr>
+            <tr>
+                <td align="left">&nbsp;</td>
+                <td align="left">Size, Growth Rates and Life Spans</td>
+                <td align="right"><a href="#size_growth">388</a></td></tr>
+            <tr>
+                <td align="left">&nbsp;</td>
+                <td align="left">Food Habits</td>
+                <td align="right"><a href="#food_habits">397</a></td></tr>
+            <tr>
+                <td align="left">&nbsp;</td>
+                <td align="left">Runways and Nests</td>
+                <td align="right"><a href="#runways">398</a></td></tr>
+            <tr>
+                <td align="left">&nbsp;</td>
+                <td align="left">Activity</td>
+                <td align="right"><a href="#activity">400</a></td></tr>
+            <tr>
+                <td colspan="2" align="left">PREDATION</td>
+                <td align="right"><a href="#predation">401</a></td></tr>
+            <tr>
+                <td colspan="2" align="left">MAMMALIAN ASSOCIATES</td>
+                <td align="right"><a href="#mammalian_associates">403</a></td></tr>
+            <tr>
+                <td colspan="2" align="left">SUMMARY AND CONCLUSIONS</td>
+                <td align="right"><a href="#summary">408</a></td></tr>
+            <tr>
+                <td colspan="2" align="left">LITERATURE CITED</td>
+                <td align="right"><a href="#literature">411</a></td></tr>
+      </tbody>
+    </table>
+</div>
+
+
+<hr class="chap" />
+
+<p><span class="pagenum"><a name="Page_363" id="Page_363">[363]</a></span></p>
+
+<h2>
+A POPULATION STUDY<br />
+OF THE PRAIRIE VOLE (MICROTUS OCHROGASTER)<br />
+IN NORTHEASTERN KANSAS<br />
+</h2>
+
+<h4>By<br />
+
+Edwin P. Martin</h4>
+
+
+
+
+<h2><a name="introduction" id="introduction">INTRODUCTION</a></h2>
+
+
+<p>Perhaps the most important species of mammal in the grasslands of
+Kansas and neighboring states is the prairie vole, <i>Microtus
+ochrogaster</i> (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, <i>M. pennsylvanicus</i>, of the eastern
+United States, has been studied both extensively and intensively,
+relatively little information concerning <i>M. ochrogaster</i> has been
+accumulated heretofore.</p>
+
+<p>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.</p>
+
+<p>Of the numerous publications concerning <i>Microtus pennsylvanicus</i>,
+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 <i>M. ochrogaster</i> 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 <i>M. ochrogaster</i>; the section
+of his report concerning the effects of voles on vegetation was
+especially useful to me.</p>
+
+<p>Fisher (1945) studied the voles of central Missouri and obtained
+information concerning food habits and nesting behavior. Jameson
+(1947) studied <i>M. ochrogaster</i> 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
+<span class="pagenum"><a name="Page_364" id="Page_364">[364]</a></span>all by Fisher and Jameson, or mentioned but not adequately explored by
+them. Also I have attempted to obtain larger samples.</p>
+
+<p>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: <i>in
+litt.</i>). He recalled that in July, 1948, the field supported a closely
+grazed, grassy vegetation providing insufficient cover for <i>Microtus</i>,
+with such coarse weeds as <i>Vernonia</i>, <i>Verbena</i> and <i>Solanum</i>
+constituting a large part of the plant cover. By 1950, the same area
+supported a lush stand of grass, principally <i>Bromus inermis</i>, 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.</p>
+
+<p>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.</p>
+
+<p>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 <i>Microtus</i> to be abundant on House Field and
+present in smaller numbers throughout grassland areas of the
+Reservation.</p>
+
+
+
+<hr class="chap" />
+<h2><a name="general_methods" id="general_methods">GENERAL METHODS</a></h2>
+
+
+<p>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
+<span class="pagenum"><a name="Page_365" id="Page_365">[365]</a></span>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.</p>
+
+<p>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.</p>
+
+<p>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.</p>
+
+<p>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.</p>
+
+<p>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.</p>
+
+<p>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.</p>
+
+<p>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
+<span class="pagenum"><a name="Page_366" id="Page_366">[366]</a></span>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.</p>
+
+<p>In order to study the extent of utilization of various habitats by
+<i>Microtus</i>, 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
+<i>Microtus</i> 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.</p>
+
+<p>Further details of the methods and procedures used are described in
+the appropriate sections which follow.</p>
+
+
+<hr class="chap" />
+
+<h2><a name="habitat" id="habitat">HABITAT</a></h2>
+
+
+<p>Although other species of the genus <i>Microtus</i>, especially <i>M.
+pennsylvanicus</i>, 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 <i>M. ochrogaster</i>. Black (1937:200) reported that, in
+Kansas, <i>Microtus</i> (mostly <i>M. ochrogaster</i>) preferred damp
+situations. <i>M. ochrogaster</i> 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 <i>M. ochrogaster</i>
+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 <i>M. ochrogaster</i>. Goodpastor and
+Hoffmeister (1952:370) found <i>M. ochrogaster</i> 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
+(<i>Lonicera japonica</i>) and stayed among the shrubbery throughout the
+winter. Johnson (1926:267, 270) found <i>M. ochrogaster</i> 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 <i>M. ochrogaster</i> can occupy drier areas than those used by <i>M.
+pennsylvanicus</i>.
+<span class="pagenum"><a name="Page_367" id="Page_367">[367]</a></span> Otherwise, the preferred habitats of the two species
+seem to be much the same.</p>
+
+<p>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.</p>
+
+<p>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 (<a href="#img005">Fig. 5</a>). Both of these areas
+were dominated by <i>Bromus inermis</i>, 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. <i>Poa pratensis</i> 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
+<i>Poa</i> 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 <i>Bromus
+japonicus</i>. This grass occurred also in low densities over much of the
+study area among <i>B. inermis</i>. Other grasses present included <i>Triodia
+flava</i>, common in House Field, but with only spotty distribution in
+Quarry Field; <i>Elymus canadensis</i>, distributed over both areas in
+spotty fashion and almost always showing evidence of use by voles and
+other small mammals; <i>Aristida oligantha</i> and <i>Bouteloua
+curtipendula</i>, both more common on the higher and drier Quarry Field;
+<i>Panicum virgatum</i>, <i>Setaria</i> spp., especially on disturbed areas; and
+three bluestems, <i>Andropogon gerardi</i>, <i>A. virginicus</i> and <i>A.
+scoparius</i>. 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.</p>
+
+<p>On House Field the most common forbs were <i>Vernonia baldwini</i>,
+<i>Verbena stricta</i> and <i>Solanum carolinense</i>. On Quarry Field,
+<i>Solidago</i> spp. and <i>Asclepias</i> 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 (<i>Solanum carolinense</i>) 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 <i>Carex gravida</i>, observed frequently in House
+Field and less often in Quarry Field; <i>Amorpha canescens</i>, more common
+in Quarry Field; <i>Tradescantia bracteata</i>, <i>Capsella bursapastoris</i>,
+<i>Oxalis violacea</i>, <i>Euphorbia marginata</i>, <i>Convolvulus arvensis</i>,
+<i>Lithospermum arvense</i>, <i>Teucrium canadense</i>, <i>Physalis longifolia</i>,
+<i>Phytolacca americana</i>, <i>Plantago major</i>, <i>Ambrosia trifida</i>, <i>A.
+artemisiifolia</i>, <i>Helianthus annuus</i>, <i>Cirsium altissimum</i> and
+<i>Taraxacum erythrospermum</i>. Both areas were being invaded from one
+side by forest-edge vegetation; the woody plants noted included
+<span class="pagenum"><a name="Page_368" id="Page_368">[368]</a></span>
+<i>Prunus americana</i>, <i>Rubus argutus</i>, <i>Rosa setigera</i>, <i>Cornus
+drummondi</i>, <i>Symphoricarpus orbiculatus</i>, <i>Populus deltoides</i> and
+<i>Gleditsia triacanthos</i>.</p>
+
+<p>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
+<i>Bromus japonicus</i> along both roads. No mammal of any kind was taken
+in traps set where this grass was dominant.</p>
+
+<p>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 (<a href="#tab001">Table 1</a>). 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
+(<a href="#img005">Fig. 5</a>).
+</p>
+<p>
+<br />
+</p>
+
+<p class="center"><span class="smcap"><a name="tab001" id="tab001"></a>Table 1. Relationship Between Yield and Various Population Data</span></p>
+<div class="center">
+    <table summary="Yield and population data" class="maintables" cellpadding="3" cellspacing="0" >
+        <tbody>
+            <tr>
+                <td class="tdtopleft">  </td>
+                <td class="tdtopright">  House Field</td>
+                <td class="tdtopright"> Quarry Field </td>
+            </tr>
+            <tr>
+                <td class="tdmainleft"> Yield in June, 1951, lbs./acre</td>
+                <td class="tdmainright">2351 ± 190</td>
+                <td class="tdmainright"> 1513 ± 302</td>
+            </tr>
+            <tr>
+                <td class="tdmainleft"> <i>Microtus</i>, June, 1951, gms./acre</td>
+                <td class="tdmainright">3867</td>
+                <td class="tdmainright"> 5275</td>
+            </tr>
+            <tr>
+                <td class="tdmainleft"> Per cent immature <i>Microtus</i>, June, 1951</td>
+                <td class="tdmainright">29.85</td>
+                <td class="tdmainright">38.02</td>
+            </tr>
+            <tr>
+                <td class="tdmainleft"> Ratio <i>Microtus</i>, June/March</td>
+                <td class="tdmainright">0.73</td>
+                <td class="tdmainright">2.63</td>
+            </tr>
+            <tr>
+                <td class="tdmainleft"> <i>Sigmodon</i>, June, 1951, gms./acre</td>
+                <td class="tdmainright">1376</td>
+                <td class="tdmainright">746</td>
+            </tr>
+            <tr>
+                <td class="tdmainleft"> Per cent immature <i>Sigmodon</i>, June, 1951</td>
+                <td class="tdmainright">35.72</td>
+                <td class="tdmainright">44.44</td>
+            </tr>
+            <tr>
+                <td class="tdmainleft"> Ratio <i>Sigmodon</i>, June/March</td>
+                <td class="tdmainright">1.40</td>
+                <td class="tdmainright">2.25</td>
+            </tr>
+            <tr>
+                <td class="tdmainleft"> <i>Microtus-Sigmodon</i>, June, 1951, gms./acre</td>
+                <td class="tdmainright">5243</td>
+                <td class="tdmainright">6021</td>
+            </tr>
+            <tr>
+                <td class="tdmainleft"> <i>Microtus</i> mean, gms./acre/month</td>
+                <td class="tdmainright">2922</td>
+                <td class="tdmainright">1831</td>
+            </tr>
+            <tr>
+                <td class="tdmainleft"> <i>Sigmodon</i> mean, gms./acre/month</td>
+                <td class="tdmainright">802</td>
+                <td class="tdmainright">335</td>
+            </tr>
+            <tr>
+                <td class="tdmainleft"> <i>Sigmodon-Microtus</i>, gms./acre/month</td>
+                <td class="tdmainright">3728</td>
+                <td class="tdmainright">2166</td>
+            </tr>
+
+        </tbody>
+    </table>
+<br />
+</div>
+
+<p>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 (<a href="#tab001">Table 1</a>). 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).</p>
+
+<p>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
+<i>Microtus</i> and <i>Sigmodon</i>. In House Field, however, the ratio of
+<i>Sigmodon</i> to <i>Microtus</i> 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.</p>
+
+<p>When the centers of activity (Hayne, 1949b) of individual voles were
+<span class="pagenum"><a name="Page_369" id="Page_369">[369]</a></span>
+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
+<i>Rhus</i> and <i>Symphoricarpos</i> 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 (<a href="#img001">Fig. 1</a>). 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.
+</p>
+
+<div class="figcenter">
+<a name="img001" id="img001"></a>
+<img src="images/fig01.png" width="316" height="600" alt="Progressive encroachment of woody vegetation" />
+<p class="caption" ><span class="smcap">Fig. 1.</span> 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.</p>
+<p class="link"><a href="images/fig01lg.png">View larger image</a></p>
+<br />
+</div>
+
+<p>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, <i>Microtus ochrogaster</i> 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.</p>
+
+<p>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.</p>
+
+<p>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 <i>et al</i> (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
+<span class="pagenum"><a name="Page_371" id="Page_371">[371]</a></span>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.</p>
+
+<p>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 <i>Microtus</i>. 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, <i>in litt.</i>) 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 (<i>Andropogon scoparius</i>) and side-oats grama
+(<i>Bouteloua curtipendula</i>) and smaller percentages of <i>Vernonia</i>,
+<i>Verbena</i>, <i>Solanum</i> and <i>Solidago</i>.</p>
+
+<p>Various species of foxtail (<i>Setaria</i>) 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 <i>Microtus</i> 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. <i>Polygonum coccineum</i>
+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.</p>
+
+<p>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, <i>in litt.</i>) has taken several <i>Microtus</i> 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
+<span class="pagenum"><a name="Page_372" id="Page_372">[372]</a></span>
+provide neither food nor cover adequate for the needs of the voles.</p>
+
+<p>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.</p>
+
+<p>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 <i>Peromyscus leucopus</i> than any other pair,
+however, and both of them also caught pine voles (<i>M. pinetorum</i>). The
+area shaded by this tree permitted an extension of parts of the forest
+edge fauna into the grassland.</p>
+
+<p>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.</p>
+
+<p>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 (<i>Reithrodontomys megalotis</i>) than
+any other comparable group of traps.</p>
+
+<p>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 <a name="homogeneous" id="homogeneous"></a><ins title="Original has homogenous">homogeneous</ins> 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.</p>
+
+<hr class="chap" />
+<h2>
+<a name="population_structure" id="population_structure">POPULATION STRUCTURE</a></h2>
+
+
+<p><span class="pagenum"><a name="Page_373" id="Page_373">[373]</a></span>
+During the period of study the percentage of males in most of my
+samples was less than 50 per cent (<a href="#img002">Fig. 2</a>). 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.</p>
+
+
+<div class="figcenter">
+<a name="img002" id="img002"></a>
+<img src="images/fig02.png" width="428" height="600" alt="Graphs of population structure" />
+<p class="caption" ><span class="smcap">Fig. 2.</span> 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.</p>
+<p class="link"><a href="images/fig02lg.png">View larger image</a></p>
+<br />
+</div>
+
+<p>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.</p>
+
+<p>Three age classes, juvenal, subadult and adult, were separated on the
+basis of condition of pelage. The percentage of adults in populations
+varied seasonally (<a href="#img002">Fig. 2</a>). 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 (<a href="#img002">Fig. 2</a>). 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
+<span class="pagenum"><a name="Page_374" id="Page_374">[374]</a></span>
+breeding females to the entire population, as discussed in this report
+under the heading of reproduction.</p>
+
+<p>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,
+<span class="pagenum"><a name="Page_375" id="Page_375">[375]</a></span>
+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.</p>
+
+<div class="figcenter"  >
+<a name="img003" id="img003"></a>
+<img src="images/fig03.png" width="514" height="600" alt="Percentages of individuals surviving" />
+<p class="caption" ><span class="smcap">Fig. 3.</span> 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.</p>
+<br />
+</div>
+
+<p>A study of the age groups in each month's population revealed a
+<span class="pagenum"><a name="Page_376" id="Page_376">[376]</a></span>
+differential survival based on the season of birth. Blair (1948:405)
+found that chances of survival in <i>Microtus pennsylvanicus</i> were
+approximately equal throughout the year. In the present populations of
+<i>M. ochrogaster</i>, however, voles born in October, November, December
+and January tended to live longer than those born in other months
+(<a href="#img003">Fig. 3</a>). 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.</p>
+
+<p>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. <a href="#img004">Fig. 4</a> 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) (<a href="#img004">Fig. 4</a>).
+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.</p>
+
+
+<div class="figcenter">
+<a name="img004" id="img004"></a>
+<img src="images/fig04.png" width="424" height="600" alt="Differential survival of voles" />
+<p class="caption" ><span class="smcap">Fig. 4.</span> 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.</p>
+<br />
+</div>
+
+
+<hr class="chap" />
+
+<h2><a name="population_density" id="population_density">POPULATION DENSITY</a></h2>
+
+
+<p>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 <i>Microtus</i> 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.</p>
+
+<p>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
+<span class="pagenum"><a name="Page_377" id="Page_377">[377]</a></span>
+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
+<span class="pagenum"><a name="Page_378" id="Page_378">[378]</a></span>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.</p>
+
+<p>Fluctuations of the populations were graphed in terms of individuals
+per acre (<a href="#img005">Fig. 5</a>). 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.</p>
+
+
+<div class="figcenter">
+<a name="img005" id="img005"></a>
+<img src="images/fig05.png" width="417" height="600" alt="Variations in density of voles" />
+<p class="caption" ><span class="smcap">Fig. 5.</span> 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.</p>
+<br />
+</div>
+
+<p>There are few records of density of <i>M. ochrogaster</i> 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 <i>M. o. haydeni</i> in a
+mixed prairie in west-central Kansas. High densities for <i>M.
+pennsylvanicus</i> 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).</p>
+
+<p>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 (<a href="#img008">Fig. 8</a>). 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.</p>
+
+<p>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 (<a href="#img005">Fig. 5</a>).
+Newborn voles were killed by the saturated condition of the ground in
+which they lay. The more precocious young of <i>Sigmodon hispidus</i>
+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 <i>Microtus</i>.</p>
+
+<p>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
+<span class="pagenum"><a name="Page_380" id="Page_380">[380]</a></span>
+populations of <i>M. ochrogaster</i> 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 (<a href="#img019">Fig. 19</a>).</p>
+
+<p>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, <i>in litt.</i>) informed me
+that the decline continued through the winter of 1952-53 and into the
+summer of 1953, until daily catches of <i>Microtus</i> 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 <i>M. o. haydeni</i> decreased more than any other
+species of small mammal after the great drought of the thirties.</p>
+
+<p>No evidence of cycles in <i>M. ochrogaster</i> 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.</p>
+
+<p>The differences in densities supported by the various habitats were
+discussed earlier in connection with the analysis of habitats.</p>
+
+
+
+<hr class="chap" />
+<h2><a name="home_range" id="home_range">HOME RANGE</a></h2>
+
+
+<p>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.</p>
+
+<p>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.</p>
+
+<p>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
+<span class="pagenum"><a name="Page_381" id="Page_381">[381]</a></span>home range, but in my study the increased interval between traps was
+not accompanied by any change in the sizes of the calculated ranges.</p>
+
+<p>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. <a href="#tab002">Table 2</a> shows the progressive increase in sizes of the mean
+range of animals taken over periods of time from one month to ten
+months.</p>
+
+<p><br /></p>
+
+<p class="center"><span class="smcap"><a name="tab002" id="tab002"></a>Table 2. Relationship Between Home Range Size and Length of Time on
+the Study Area</span></p>
+<div class="center">
+
+
+    <table summary="Relationship Between Home Range Size ..." class="maintables" cellpadding="3" cellspacing="0" >
+        <tbody>
+            <tr>
+                <td class="tdtop2left"> No. months on area </td>
+                <td class="tdtop2right"> 1</td>
+                <td class="tdtop2right"> 2 </td>
+                <td class="tdtop2right"> 3</td>
+                <td class="tdtop2right"> 4 </td>
+                <td class="tdtop2right"> 5</td>
+                <td class="tdtop2right"> 6 </td>
+                <td class="tdtop2right"> 7</td>
+                <td class="tdtop2right"> 8 </td>
+                <td class="tdtop2right"> 9</td>
+                <td class="tdtop2right"> 10 </td>
+            </tr>
+            <tr>
+                <td class="tdmainleft"> Mean range in acres </td>
+                <td class="tdmainright"> .09</td>
+                <td class="tdmainright"> .09 </td>
+                <td class="tdmainright"> .10</td>
+                <td class="tdmainright"> .14 </td>
+                <td class="tdmainright"> .13</td>
+                <td class="tdmainright"> .17 </td>
+                <td class="tdmainright"> .22</td>
+                <td class="tdmainright"> .22 </td>
+                <td class="tdmainright"> .26</td>
+                <td class="tdmainright"> .24 </td>
+            </tr>
+
+        </tbody>
+    </table>
+<br />
+</div>
+
+
+<p>Nothing concerning the home range of <i>Microtus ochrogaster</i> was found
+in the literature. Several workers, including Blair (1940) and
+Hamilton (1937c), have studied the home range of <i>M. pennsylvanicus</i>.
+Blair (1940:153) reported a larger range for males than for females in
+all habitats and in all seasons represented in his sample. In <i>M.
+ochrogaster</i>, however, I found that the mean monthly range for both
+sexes was 0.09 of an acre. Blair (<i>loc. cit.</i>) reported no individuals
+with a range so small as that mean, but Hamilton (<i>op. cit.</i>: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 <i>M. pennsylvanicus</i>
+(Hamilton, <i>loc. cit.</i>), males tended to wander more than females and
+to shift their home range more often.</p>
+
+<p>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.</p>
+
+<p>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,
+<span class="pagenum"><a name="Page_382" id="Page_382">[382]</a></span>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.</p>
+
+<p>Many voles shifted their home range and a few did so abruptly. The
+large range of a female vole, described above and plotted in <a href="#img006">Fig. 6</a>,
+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 <a href="#img007">Fig. 7</a>.
+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.</p>
+
+<div class="figcenter"  >
+<a name="img006" id="img006"></a>
+<img src="images/fig06.png" width="550" height="235" alt="Map with cross-hatched areas showing the range
+of vole #20" />
+<p class="caption" ><span class="smcap">Fig. 6.</span> 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.</p>
+</div>
+
+<div class="figcenter"  >
+<a name="img007" id="img007"></a>
+<img src="images/fig07.png" width="550" height="245" alt="Map showing range of vole #52" />
+<p class="caption" ><span class="smcap">Fig. 7.</span> 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.</p>
+<br />
+</div>
+
+
+<p>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 <i>Microtus</i>, 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.</p>
+
+<p><span class="pagenum"><a name="Page_383" id="Page_383">[383]</a></span></p><p>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.</p>
+
+
+
+<hr class="chap" />
+<h2><a name="life_history" id="life_history">LIFE HISTORY</a></h2>
+
+
+<h3><a name="reproduction" id="reproduction">Reproduction</a></h3>
+
+<p>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 <i>Microtus</i> 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.</p>
+
+<p>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 (<a href="#img008">Fig. 8</a>).
+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 <i>Microtus</i> 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 (<a href="#img008">Fig. 8</a>).</p>
+
+<div class="figcenter"  >
+<a name="img008" id="img008"></a>
+<img src="images/fig08.png" width="550" height="490" alt="Variations in density and reproductive rate of
+voles" />
+<p class="caption" ><span class="smcap">Fig. 8.</span> 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.</p>
+<br />
+</div>
+
+
+<p>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 (<a href="#img009">Fig. 9</a>).
+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
+<span class="pagenum"><a name="Page_384" id="Page_384">[384]</a></span>
+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).</p>
+
+<p>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 (<a href="#img009">Fig. 9</a>).</p>
+
+<div class="figcenter">
+<a name="img009" id="img009"></a>
+<img src="images/fig09.png" width="550" height="396" alt=" Comparison between monthly rainfall and
+reproductive rate of voles in summer" />
+<p class="caption" ><span class="smcap">Fig. 9.</span> 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.</p>
+<br />
+</div>
+
+
+<p>Of the total captures 20.6 per cent involved more than one
+individual. When the distribution of these multiple captures was
+<span class="pagenum"><a name="Page_385" id="Page_385">[385]</a></span>
+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.</p>
+
+<p>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.</p>
+<p><span class="pagenum"><a name="Page_386" id="Page_386">[386]</a></span></p>
+<p>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 <i>Microtus</i> by several authors (Bailey, 1924:529; Hatfield,
+1935:264; Hamilton, <i>loc. cit.</i>; Leslie and Ransom, 1940:32).</p>
+
+<p>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. <i>M. ochrogaster</i> seems to be less prolific
+than <i>M. pennsylvanicus</i>. 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 <i>M. pennsylvanicus</i> by
+Bailey (1924:528) and Hamilton (1940:429; 1949:259) and probably
+occurs also in <i>M. ochrogaster</i>.</p>
+
+<p>The gestation period was approximately 21 days, the same as reported
+for <i>M. pennsylvanicus</i> (Bailey, <i>loc. cit.</i>; Hamilton, 1941:13) and
+<i>M. californicus</i> (Hatfield, 1935:264). A more precise study of the
+breeding habits of <i>M. ochrogaster</i> failed to materialize when the
+voles refused to breed in captivity. Fisher (1945:437) also reported
+that <i>M. ochrogaster</i> failed to breed in captivity although <i>M.
+pennsylvanicus</i> (Bailey, 1924) and <i>M. californicus</i> (Hatfield, 1935)
+reproduced readily in the laboratory.</p>
+
+
+<h3><a name="litter_size" id="litter_size">Litter Size and Weight</a></h3>
+
+<p>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 (<a href="#img010">Fig. 10</a>).
+Three litters contained but one individual and the largest litter
+contained six individuals. Other investigators have reported the
+number of young per litter in <i>M. ochrogaster</i> as three or four
+(Lantz, 1907:18) and 3.4 (1-7) (Jameson, 1947:146). <i>M.
+pennsylvanicus</i> 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, <i>M. agrestis</i>. Selle (1928:96)
+reported the average size of five litters of <i>M. californicus</i> 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
+<span class="pagenum"><a name="Page_387" id="Page_387">[387]</a></span>litters of <i>M. ochrogaster</i> 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.</p>
+
+<div class="figcenter"  >
+<a name="img010" id="img010"></a>
+<img src="images/fig10.png" width="500" height="416" alt=" Distribution of litter size among 65 litters
+of voles" />
+<p class="caption" ><span class="smcap">Fig. 10.</span>  Distribution of litter size among 65 litters
+of voles.</p>
+<br />
+</div>
+
+
+<p>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.</p>
+
+<p>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.</p>
+<p><span class="pagenum"><a name="Page_388" id="Page_388">[388]</a></span></p>
+<p>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 <i>M.
+ochrogaster</i> 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 <i>M. pennsylvanicus</i> and to the 2.7 grams (Selle,
+1928:97) and 2.8 grams (Hatfield, 1935:268) reported for <i>M.
+californicus</i>. No correlation between the weight of the individual
+newborn vole and the number of voles per litter was observed.</p>
+
+<p>Although the ratio of the average weight of newborn voles to the
+average weight of an adult female was approximately equal for <i>M.
+pennsylvanicus</i> and <i>M. ochrogaster</i>, 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.</p>
+
+
+<h3><a name="size_growth" id="size_growth">Size, Growth Rates and Life Spans</a></h3>
+
+<p>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.</p>
+
+<div class="figcenter"  >
+<a name="img011" id="img011"></a>
+<img src="images/fig11.png" width="550" height="310" alt=" Relationship between rainfall and the mean
+weight" />
+<p class="caption" ><span class="smcap">Fig. 11.</span> Relationship between rainfall and the
+<a name="weighterror" id="weighterror"></a>
+<ins title="the bottommost y-axis label in the scale of gms. is probably an error: 45 should be 35">mean weight of adult males</ins>
+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.</p>
+<br />
+</div>
+
+
+<p>During the unusually dry summer of 1952, a notable reduction in the
+mean weight of adults was recorded (<a href="#img011">Fig. 11</a>). 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
+<span class="pagenum"><a name="Page_389" id="Page_389">[389]</a></span>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.</p>
+
+<p>No data on the growth rate of <i>M. ochrogaster</i> 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 <i>M. pennsylvanicus</i> (Hamilton, 1941:12) and for
+<i>M. californicus</i> (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 (<a href="#img012">Fig. 12</a>).</p>
+
+<div class="figcenter"  >
+<a name="img012" id="img012"></a>
+<img src="images/fig12.png" width="550" height="385" alt="Growth rates of two voles" />
+<p class="caption" ><span class="smcap">Fig. 12.</span> 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.</p>
+<br />
+</div>
+
+
+
+<p>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
+(<i>op. cit.</i>: 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
+<span class="pagenum"><a name="Page_390" id="Page_390">[390]</a></span>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.</p>
+
+<p>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 <i>Microtus montanus</i>, and Hall
+(1926) studied the changes due to growth in skulls of <i>Otospermophilus
+grammarus beecheyi</i>. 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.</p>
+
+<p>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. <a href="#tab003">Table 3</a> 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.</p>
+
+<p><br /></p>
+<p class="center"><span class="smcap"><a name="tab003" id="tab003"></a>Table 3. Distribution Among Age Groups of 21 Voles Used in the Study
+of Variation Due to Age</span></p>
+<div class="center">
+
+    <table summary="Distribution Among Age Groups of 21 Voles" class="maintables" cellpadding="3" cellspacing="0" >
+        <tbody>
+            <tr>
+                <td class="tdtopleft"> Age in months</td>
+                <td class="tdtop"> 1<small><sup>1</sup>&frasl;<sub>2</sub></small></td>
+                <td class="tdtop"> 2 </td>
+                <td class="tdtop"> 2<small><sup>1</sup>&frasl;<sub>2</sub></small></td>
+                <td class="tdtop"> 3 </td>
+                <td class="tdtop"> 3<small><sup>1</sup>&frasl;<sub>2</sub></small> </td>
+                <td class="tdtop"> 4 </td>
+                <td class="tdtop"> 4<small><sup>1</sup>&frasl;<sub>2</sub></small> </td>
+                <td class="tdtop"> 6 </td>
+                <td class="tdtop"> 12</td>
+            </tr>
+            <tr>
+                <td class="tdmainleft"> No. of individuals </td>
+                <td class="tdmain"> 1</td>
+                <td class="tdmain"> 4 </td>
+                <td class="tdmain"> 5</td>
+                <td class="tdmain"> 1 </td>
+                <td class="tdmain"> 3</td>
+                <td class="tdmain"> 2 </td>
+                <td class="tdmain"> 3</td>
+                <td class="tdmain"> 1 </td>
+                <td class="tdmain"> 1</td>
+            </tr>
+
+        </tbody>
+    </table>
+<br />
+</div>
+
+
+<p>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. <a href="#tab004">Table 4</a>
+summarizes these data.</p>
+
+<p><br /></p>
+<p class="center"><span class="smcap"><a name="tab004" id="tab004"></a>Table 4. Mean Sizes and Ages of Voles Molting from Subadult to Adult
+Pelage</span></p>
+<div class="center">
+    <table summary="Mean Sizes and Ages of Voles Molting" class="maintables" cellpadding="3" cellspacing="0" >
+        <tbody>
+            <tr>
+                <td class="tdtopleft">  </td>
+                <td class="tdtopleft"> Weight</td>
+                <td class="tdtopleft"> Body length minus tail </td>
+                <td class="tdtopleft"> Condylo-basilar length</td>
+                <td class="tdtopleft"> Age </td>
+            </tr>
+            <tr>
+                <td class="tdmainleft"> Six males </td>
+                <td class="tdmainleft"> 32.67 gms.</td>
+                <td class="tdmainleft">  106.16 mm.</td>
+                <td class="tdmainleft"> 23.78 mm.</td>
+                <td class="tdmainleft">9.67 wks.</td>
+            </tr>
+            <tr>
+                <td class="tdmainleft">   </td>
+                <td class="tdmainleft"> (30-36)</td>
+                <td class="tdmainleft">  (96-116)</td>
+                <td class="tdmainleft"> (23.2-24.4)</td>
+                <td class="tdmainleft">(8-12)</td>
+            </tr>
+            <tr>
+                <td class="tdmainleft"> Four females </td>
+                <td class="tdmainleft"> 29.0 gms.</td>
+                <td class="tdmainleft"> 100.25 mm.</td>
+                <td class="tdmainleft"> 23.45 mm. </td>
+                <td class="tdmainleft">  10.5 wks.</td>
+            </tr>
+            <tr>
+                <td class="tdmainleft">   </td>
+                <td class="tdmainleft"> (28-30)</td>
+                <td class="tdmainleft">  (98-102) </td>
+                <td class="tdmainleft"> (23.5-23.8)</td>
+                <td class="tdmainleft">(8-12)</td>
+            </tr>
+            <tr>
+                <td class="tdmainleft"> Ten voles</td>
+                <td class="tdmainleft">  31.2 gms.</td>
+                <td class="tdmainleft">  103.8 mm.</td>
+                <td class="tdmainleft">  23.73 mm.</td>
+                <td class="tdmainleft">  10.0 wks.</td>
+            </tr>
+            <tr>
+                <td class="tdmainleft">   </td>
+                <td class="tdmainleft"> (28-36)</td>
+                <td class="tdmainleft">  (96-116) </td>
+                <td class="tdmainleft"> (23.2-24.4)</td>
+                <td class="tdmainleft">(8-12)</td>
+            </tr>
+        </tbody>
+    </table>
+<br />
+</div>
+
+
+<p>The mean age of the ten voles molting was ten weeks (8-12). Six males
+<span class="pagenum"><a name="Page_396" id="Page_396">[396]</a></span>
+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).</p>
+
+<p>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.</p>
+
+<p>When Howell (<i>op. cit.</i>:1014) studied skulls of <i>Microtus montanus</i> 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 (<a href="#img013">Fig. 13</a>). No separation of sexes was made because the sample did not permit
+it. In <a href="#img013">Fig. 13</a> 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.</p>
+
+<div class="figcenter">
+<a name="img013" id="img013"></a>
+<img src="images/fig13.png" width="446" height="600" alt=" Graphs of the condylobasilar lengths..." />
+<p class="caption" ><span class="smcap">Fig. 13.</span> 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.</p>
+<br />
+</div>
+
+
+<p>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.</p>
+
+<p>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. <a href="#img014">Figs 14</a> and <a href="#img015">15</a> are drawings of skulls of
+voles of known age. The most obvious change, related to aging, evident
+in the dorsal view of the skulls (<a href="#img014">Fig. 14</a>) 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.</p>
+
+
+<div class="figcenter">
+<a name="img014" id="img014"></a>
+<img src="images/fig14.png" width="600" height="494" alt="Dorsal views of skulls of voles of known age" />
+<p class="caption" ><span class="smcap">Fig. 14.</span> Dorsal views of skulls of voles of known age.
+(Ages 1<small><sup>1</sup>&frasl;<sub>2</sub></small>, 2<small><sup>1</sup>&frasl;<sub>2</sub></small>, 3, 3<small><sup>1</sup>&frasl;<sub>2</sub></small>, 4, 4<small><sup>1</sup>&frasl;<sub>2</sub></small>, 6 and 12 months). All &times; 3. </p>
+<p class="link"><a href="images/fig14lg.png">View larger image</a></p>
+</div>
+
+<div class="figcenter">
+<a name="img015" id="img015"></a>
+<img src="images/fig15.png" width="600" height="492" alt="Palatal views of skulls of voles of known age" />
+<p class="caption" ><span class="smcap">Fig. 15.</span> Palatal views of skulls of voles of known age.
+(Ages 1<small><sup>1</sup>&frasl;<sub>2</sub></small>, 2<small><sup>1</sup>&frasl;<sub>2</sub></small>, 3, 3<small><sup>1</sup>&frasl;<sub>2</sub></small>, 4, 4<small><sup>1</sup>&frasl;<sub>2</sub></small>, 6 and 12 months). All &times; 3. </p>
+<p class="link"><a href="images/fig15lg.png">View larger image</a></p>
+<br />
+</div>
+
+
+
+<p>From a palatal view (<a href="#img015">Fig. 15</a>) the skulls of voles also showed age
+<span class="pagenum"><a name="Page_397" id="Page_397">[397]</a></span>
+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.</p>
+
+<p>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<small><sup>1</sup>&frasl;<sub>2</sub></small> months of age, and progressively more
+marked in the older voles.</p>
+
+
+<h3><a name="food_habits" id="food_habits">Food Habits</a></h3>
+
+<p>The prairie vole, like other members of the genus <i>Microtus</i>, 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.</p>
+
+<p>In all piles of cut plants that were examined, <i>Bromus inermis</i> was
+the most common grass, and <i>Poa pratensis</i> was the grass second in
+abundance. These were, by far, the most common grasses present on the
+areas studied; in most places, <i>B. inermis</i> was dominant. Other
+grasses present on the areas were occasionally found in the piles of
+cuttings. Jameson (1947:133-136) found no utilization of <i>B. inermis</i>
+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.</p>
+
+<p>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 <i>Triodia flava</i>,
+<i>Bromus inermis</i> and <i>Poa pratensis</i> were offered to the voles. At
+<span class="pagenum"><a name="Page_398" id="Page_398">[398]</a></span>
+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.</p>
+
+<p>Winter foods consisted of stored hay and fruits and of underground
+plant parts. <i>Bromus inermis</i> 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 <i>Solanum
+carolinense</i> 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,
+<i>in litt.</i>) often observed girdling of honey locust and crab apple
+(<i>Pyrus ioensis</i>) 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.</p>
+
+
+<h3><a name="runways" id="runways">Runways and Nests</a></h3>
+
+<p>Perhaps the most characteristic sign of the presence of <i>Microtus
+ochrogaster</i> 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.</p>
+
+<p>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.</p>
+
+<p>A runway system usually consisted of a long, crooked runway and
+several branches. Two typical systems are illustrated in <a href="#img016">Fig. 16</a>. 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
+<span class="pagenum"><a name="Page_399" id="Page_399">[399]</a></span>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.</p>
+
+
+<div class="figcenter"  >
+<a name="img016" id="img016"></a>
+<img src="images/fig16.png" width="444" height="600" alt="Maps of runway systems" />
+<p class="caption" ><span class="smcap">Fig. 16.</span> 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.</p>
+<br />
+</div>
+
+<p><span class="pagenum"><a name="Page_400" id="Page_400">[400]</a></span></p><p>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
+(<i>loc. cit.</i>) 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.</p>
+
+<p>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.</p>
+
+
+<h3><a name="activity" id="activity">Activity</a></h3>
+
+<p>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
+<i>Microtus</i> by various methods. Calhoun (1945:256) reported <i>M.
+ochrogaster</i> to be mainly nocturnal with activity reaching a peak
+between dark and midnight and again just before dawn. Davis
+(1933:235), working with <i>M. agrestis</i>, and Hatfield (1935:263),
+working with <i>M. californicus</i>, both found voles to be more nocturnal
+than diurnal. In a field study of <i>M. pennsylvanicus</i>, 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 <i>Microtus</i> has not yet
+been attained.</p>
+
+<p>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
+<span class="pagenum"><a name="Page_401" id="Page_401">[401]</a></span>
+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.</p>
+
+
+
+<hr class="chap" />
+<h2><a name="predation" id="predation">PREDATION</a></h2>
+
+
+<p>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 <i>et al</i>, 1940).</p>
+
+<p>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 <a href="#tab005">Table 5</a>.
+Remains of voles were identified in 28 per cent of the scats of the
+copperhead snake (<i>Ancistrodon contortix</i>) examined. Copperheads were
+moderately common on the Reservation (Fitch, 1952:24) and were
+probably important as predators on voles in some habitats. Uhler <i>et
+al</i> (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 (<i>Crotalus horridus</i>) 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, <i>loc. cit.</i>). 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 (<i>Coluber constrictor</i>) 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 (<i>Elaphe obsoleta</i>) has been reported as a predator
+on <i>M. ochrogaster</i> in the neighboring state of Missouri (Korschgen,
+1952:60) and was moderately common on the Reservation (Fitch, <i>loc.
+cit.</i>). <i>M. pennsylvanicus</i>, with habits similar to those of <i>M.
+ochrogaster</i>, has been reported as a prey for all of the above snakes
+(Uhler, <i>et al</i>, 1939).</p>
+<p><br /></p>
+<p class="center"><span class="smcap"><a name="tab005" id="tab005"></a>Table 5. Frequency of Remains of Voles in Scats and Pellets
+</span></p>
+<div class="center">
+    <table summary="Frequency of Remains of Voles in Scats and Pellets" class="maintables" cellpadding="3" cellspacing="0" >
+        <tbody>
+            <tr>
+                <td class="tdtopleft"> Predator </td>
+                <td class="tdtop"> No. of scats or pellets examined </td>
+                <td class="tdtop">  No. containing remains of voles </td>
+                <td class="tdtop"> Percentage</td>
+            </tr>
+            <tr>
+                <td class="tdmainleft"> Copperhead </td>
+                <td class="tdmain">  25</td>
+                <td class="tdmain"> 7 </td>
+                <td class="tdmain"> 28 </td>
+            </tr>
+            <tr>
+                <td class="tdmainleft"> Red-tailed hawk  </td>
+                <td class="tdmain">  25</td>
+                <td class="tdmain"> 3 </td>
+                <td class="tdmain"> 12 </td>
+            </tr>
+            <tr>
+                <td class="tdmainleft"> Long-eared owl </td>
+                <td class="tdmain">  25</td>
+                <td class="tdmain"> 18 </td>
+                <td class="tdmain"> 72 </td>
+            </tr>
+            <tr>
+                <td class="tdmainleft"> Great horned owl </td>
+                <td class="tdmain">  32</td>
+                <td class="tdmain"> 6 </td>
+                <td class="tdmain"> 19 </td>
+            </tr>
+            <tr>
+                <td class="tdmainleft"> Crow </td>
+                <td class="tdmain">  25</td>
+                <td class="tdmain"> 4 </td>
+                <td class="tdmain"> 16 </td>
+            </tr>
+            <tr>
+                <td class="tdmainleft"> Coyote </td>
+                <td class="tdmain">  25</td>
+                <td class="tdmain"> 3 </td>
+                <td class="tdmain"> 12 </td>
+            </tr>
+        </tbody>
+    </table>
+<br />
+</div>
+
+<p>The red-tailed hawk (<i>Buteo jamaicensis</i>), the long-eared owl (<i>Asio
+otus</i>), <span class="pagenum"><a name="Page_402" id="Page_402">[402]</a></span>
+the great horned owl (<i>Bubo virginianus</i>) and the crow
+(<i>Corvus brachyrhynchos</i>) fed on <i>Microtus</i>. 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, <i>et al</i>, 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 <i>Microtus</i> 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 (<i>Accipiter striatus</i>), Cooper's
+hawk (<i>A. cooperi</i>), red-shouldered hawk (<i>Buteo lineatus</i>),
+broad-winged hawk (<i>B. platypterus</i>), American rough-legged hawk (<i>B.
+lagopus</i>), ferruginous rough-legged hawk (<i>B. regalis</i>), marsh hawk
+(<i>Circus cyaneus</i>), barn owl (<i>Tyto alba</i>), screech owl (<i>Otus asio</i>),
+barred owl (<i>Strix varia</i>) and shrike (<i>Lanius excubitor</i>) (Korschgen,
+1952:26; 28; 34; 35; 37; McAtee, 1935:9-27; Wooster, 1936:396).</p>
+
+<p>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 (<i>Canis latrans</i>) 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 <i>Microtus</i>
+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 (<i>Felis domesticus</i>),
+seemingly feral, were observed to tour the trap lines on several
+occasions and were noted by Fitch (<i>loc. cit.</i>) 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 (<i>loc. cit.</i>)
+reported raccoons (<i>Procyon lotor</i>) to be moderately common on the
+Reservation. Reports of predation by raccoons on voles are numerous
+(Hatt, 1930:554; Lantz, 1907:41). The opossum (<i>Didelphis
+marsupialis</i>), 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 (<i>Mephitis mephitis</i>), spotted
+skunk (<i>Spilogale putorius</i>), weasel (<i>Mustela frenata</i>) and the red
+fox (<i>Vulpes fulva</i>). Eadie (1944; 1948; 1952), Shapiro (1950:360) and
+others have reported that the short-tailed shrew (<i>Blarina
+brevicauda</i>) was an important predator on <i>Microtus</i>. Shrews were
+present on the Reservation but were not trapped often enough to permit
+study.</p>
+
+<p>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
+<span class="pagenum"><a name="Page_403" id="Page_403">[403]</a></span>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 <i>M. ochrogaster</i> to be second only to the jack-rabbit (<i>Lepus
+californicus</i>) as a prey species in west-central Kansas.</p>
+
+
+
+<hr class="chap" />
+<h2><a name="mammalian_associates" id="mammalian_associates">MAMMALIAN ASSOCIATES</a></h2>
+
+
+<p>In the course of live-trapping operations several species of small
+mammals other than <i>Microtus ochrogaster</i> 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 <i>Sigmodon hispidus</i>, <i>Reithrodontomys megalotis</i> and
+<i>Peromyscus leucopus</i>. These three species have been commonly found
+associated with <i>Microtus</i> in this part of the country (Fisher,
+1945:435; Jameson, 1947:137).</p>
+
+<p>The Texas cotton rat, <i>Sigmodon hispidus</i>, 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 (<a href="#img017">Figs 17</a>, <a href="#img018">18</a>, <a href="#img019">19</a>). 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, <i>Sigmodon</i> has long been
+considered to be almost the ecological equivalent of <i>Microtus</i> 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.</p>
+
+<div class="figcenter"  >
+<a name="img017" id="img017"></a>
+<img src="images/fig17.png" width="525" height="615" alt=" Variations in density and mass of three common
+rodents on House Field" />
+<p class="caption" ><span class="smcap">Fig. 17.</span> 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 <i>Microtus</i>, the broken line <i>Sigmodon</i>, and the dotted line
+<i>Reithrodontomys</i>.</p>
+</div>
+
+<div class="figcenter"  >
+<a name="img018" id="img018"></a>
+<img src="images/fig18.png" width="550" height="635" alt=" Variations in density and biomass of three common
+rodents on House Field" />
+<p class="caption" ><span class="smcap">Fig. 18.</span>  Variations in density and biomass of three
+common rodents on Quarry Field. For key, see legend of <a href="#img017">Fig. 17</a>.</p>
+</div>
+
+<div class="figcenter"  >
+<a name="img019" id="img019"></a>
+<img src="images/fig19.png" width="418" height="600" alt=" Variations in density and biomass of three common
+rodents on House Field" />
+<p class="caption" ><span class="smcap">Fig. 19.</span>  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 <i>Microtus</i>, the broken line <i>Sigmodon</i>, and the dotted line
+<i>Reithrodontomys</i>.</p>
+<br />
+</div>
+
+
+
+<p>Both this study and the literature (Black, 1937:197; Calhoun, <i>loc.
+cit.</i>; 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 <i>Microtus</i> and <i>Sigmodon</i> were
+similar. Studies on the Natural History Reservation, both in
+connection with my problem and otherwise, suggested, however, that
+<i>Sigmodon</i> 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 <i>Microtus</i> which could not support a population of
+<i>Sigmodon</i>.</p>
+
+<p>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
+<span class="pagenum"><a name="Page_404" id="Page_404">[404]</a></span>
+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 (<a href="#img017">Fig. 17</a>),
+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 (<a href="#img018">Fig. 18</a>). 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,
+<span class="pagenum"><a name="Page_405" id="Page_405">[405]</a></span>
+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.</p>
+
+<p>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.</p>
+
+<p>The population of <i>Sigmodon</i> retained its high level, relative to
+<i>Microtus</i>, 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
+<span class="pagenum"><a name="Page_407" id="Page_407">[407]</a></span>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.</p>
+
+<p>These limited data seem to indicate competition between <i>Sigmodon</i> and
+<i>Microtus</i> in Kansas. Extremely wet conditions seem to give <i>Sigmodon</i>
+a competitive advantage whereas <i>Microtus</i> 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
+<i>Sigmodon</i> in Kansas (especially the more arid western part),
+including its coactions with the communities it has invaded
+successfully recently.</p>
+
+<p>The harvest mouse (<i>Reithrodontomys megalotis</i>) 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, <i>op. cit.</i>: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).</p>
+
+<p>The presence of the wood mouse (<i>Peromyscus leucopus</i>) 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.</p>
+
+<p>Only six deer mice (<i>Peromyscus maniculatus</i>) 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 <i>P. leucopus</i> in the ecotonal region which made
+it unsuitable for <i>P. maniculatus</i>.</p>
+
+<p>Other mammals noted on the study areas were the following: <i>Didelphis
+marsupialis</i>, <i>Blarina brevicauda</i>, <i>Scalopus aquaticus</i>, <i>Canis
+familiaris</i>, <i>Canis latrans</i>, <i>Procyon lotor</i>, <i>Felis domesticus</i>,
+<i>Sylvilagus floridanus</i>, <i>Microtus pinetorum</i>, <i>Mus musculus</i> and
+<i>Zapus hudsonius</i>.</p>
+
+
+
+<hr class="chap" />
+<p><span class="pagenum"><a name="Page_408" id="Page_408">[408]</a></span></p>
+<h2>
+<a name="summary" id="summary">SUMMARY AND CONCLUSIONS</a></h2>
+
+
+<p>In the 23-month period from October, 1950, to August, 1952, the
+ecology of the prairie vole, <i>Microtus ochrogaster</i>, 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.</p>
+
+<p>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 <i>M.
+ochrogaster</i> 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 <i>M.
+ochrogaster</i> 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.</p>
+
+<p>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).</p>
+
+<p>Among the types of environmental modification to which the
+populations of voles reacted were plant succession, an increase in
+competition with <i>Sigmodon</i>, 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
+<span class="pagenum"><a name="Page_409" id="Page_409">[409]</a></span>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.</p>
+
+<p>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.</p>
+
+<p>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.</p>
+
+<p>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
+<span class="pagenum"><a name="Page_410" id="Page_410">[410]</a></span>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.</p>
+
+<p>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.</p>
+
+<p>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.</p>
+
+<p>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.</p>
+
+<p>Runway systems and burrows are used by groups of voles rather than by
+individuals. Most of the activity of voles is confined to these
+<span class="pagenum"><a name="Page_411" id="Page_411">[411]</a></span>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.</p>
+
+<p>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.</p>
+
+
+
+<hr class="chap" />
+<h2><a name="literature" id="literature">LITERATURE CITED</a></h2>
+
+
+<p><span class="smcap">Albertson, F. W.</span></p>
+
+<p class="indnt">1937. Ecology of a mixed prairie in west-central Kansas. Ecol.
+Monog., 7:481-547.</p>
+
+<p><span class="smcap">Bailey, V.</span></p>
+
+<p class="indnt">1902. Synopsis of the North American species of <i>Sigmodon</i>.
+Proc. Biol. Soc. Washington, 15:101-116.</p>
+
+<p class="indnt">1924. Breeding, feeding and other life habits of meadow mice.
+Jour. Agric. Res., 27:523-536.</p>
+
+<p><span class="smcap">Baker, J. R.</span>, and <span class="smcap">R. M. Ransom</span>.</p>
+
+<p class="indnt">1932a. Factors affecting the breeding of the field mouse
+(<i>Microtus agrestis</i>). Part I. Light. Proc. Roy. Soc. London,
+Series B, 110:313-322.</p>
+
+<p class="indnt">1932b. Factors affecting the breeding of the field mouse
+(<i>Microtus agrestis</i>). Part II. Temperature. Proc. Roy. Soc.
+London, Series B, 112:39-46.</p>
+
+<p><span class="smcap">Black, T. D.</span></p>
+
+<p class="indnt">1937. Mammals of Kansas. Kansas State Board Agric., 13th
+Biennial Rep., 1935-36:116-217.</p>
+
+<p><span class="smcap">Blair, W. F.</span></p>
+
+<p class="indnt">1939. Some observed effects of stream valley flooding on small
+mammal populations in eastern Oklahoma. Jour. Mamm.,
+20:304-306.</p>
+
+<p class="indnt">1940. Home ranges and populations of the meadow vole in
+southern Michigan. Jour. Wildlife Mgmt., 4:149-161.</p>
+<p><span class="pagenum"><a name="Page_412" id="Page_412">[412]</a></span></p>
+<p class="indnt">1941. Techniques for the study of small mammal populations.
+Jour. Mamm., 22:148-157.</p>
+
+<p class="indnt">1948. Population density, life span and mortality rates of
+small mammals in the bluegrass meadow and bluegrass field
+associations of southern Michigan. Amer. Midland Nat.,
+40:395-419.</p>
+
+<p><span class="smcap">Bodenheimer, F. S.</span>, and <span class="smcap">F. Sulman</span>.</p>
+
+<p class="indnt">1946. The estrous cycle of <i>Microtus guentheri</i> D. and A. and
+its ecological implications. Ecol., 27:255-256.</p>
+
+<p><span class="smcap">Bole, B. P., Jr.</span></p>
+
+<p class="indnt">1939. The quadrat method of studying small mammal populations.
+Cleveland Mus. Nat. Hist. Sci. Publ., 5:1-77.</p>
+
+<p><span class="smcap">Brown, H. L.</span></p>
+
+<p class="indnt">1946. Rodent activity in a mixed prairie near Hays, Kansas.
+Trans. Kansas Acad. Sci., 48:448-458.</p>
+
+<p><span class="smcap">Brumwell, M.</span></p>
+
+<p class="indnt">1951. An ecological survey of the Fort Leavenworth Military
+Reservation. Amer. Midland Nat., 45:187-231.</p>
+
+<p><span class="smcap">Burt, W. H.</span></p>
+
+<p class="indnt">1943. Territoriality and home range concepts as applied to
+mammals. Jour. Mamm., 24:346-352.</p>
+
+<p><span class="smcap">Calhoun, J. B.</span></p>
+
+<p class="indnt">1945. Diel activity rhythms of the rodents <i>Microtus
+ochrogaster</i> and <i>Sigmodon hispidus hispidus</i>. Ecol.,
+26:251-273.</p>
+
+<p><span class="smcap">Chitty, D.</span>, and <span class="smcap">D. A. Kempson</span>.</p>
+
+<p class="indnt">1949. Prebaiting of small mammals and a new design of a live
+trap. Ecol., 30:536-542.</p>
+
+<p><span class="smcap">Cockrum, E. L.</span></p>
+
+<p class="indnt">1947. Effectiveness of live traps vs. snap traps. Jour. Mamm.,
+28:186.</p>
+
+<p class="indnt">1948. Distribution of the hispid cotton rat in Kansas. Trans.
+Kansas Acad. Sci., 51:306-312.</p>
+
+<p class="indnt">1952. Mammals of Kansas. Univ. Kansas Mus. Nat. Hist. Publ.,
+7:1-303.</p>
+
+<p><span class="smcap">Davis, D. H. S.</span></p>
+
+<p class="indnt">1933. Rhythmic activity in the short-tailed vole, <i>Microtus</i>.
+Jour. Animal Ecol., 2:232-238.</p>
+
+<p><span class="smcap">Dice, L. R.</span></p>
+
+<p class="indnt">1922. Some factors affecting the distribution of the prairie
+vole, forest deer mouse and prairie deer mouse. Ecol.,
+3:29-47.</p>
+
+<p><span class="smcap">Eadie, W. R.</span></p>
+
+<p class="indnt">1944. The short-tailed shrew and field mouse predation. Jour.
+Mamm., 25:359-362.</p>
+
+<p class="indnt">1948. Shrew-mouse predation during low mouse abundance. Jour.
+Mamm., 29:35-37.</p>
+
+<p class="indnt">1952. Shrew predation and vole populations on a limited area.
+Jour. Mamm., 33:185-189.</p>
+
+<p class="indnt">1953. Response of <i>Microtus</i> to vegetative cover. Jour. Mamm.,
+34:262-264.</p>
+
+<p><span class="smcap">Elton, C.</span></p>
+
+<p class="indnt">1949. Population interspersion. An essay in animal community
+patterns. Jour. Ecol., 37:1-23.</p>
+<p><span class="pagenum"><a name="Page_413" id="Page_413">[413]</a></span></p>
+<p><span class="smcap">Errington, P. R.</span></p>
+
+<p class="indnt">1935. Food habits of midwestern foxes. Jour. Mamm.,
+16:192-200.</p>
+
+<p class="indnt">1936. What is the meaning of predation? Smithsonian Inst.
+Rep., 1936:243-252.</p>
+
+<p class="indnt">1943. An analysis of mink predation upon muskrat in north
+central United States. Agric. Exp. Sta., Iowa State Coll.
+Agric. Mech. Arts, Res. Bull., 320:799-924.</p>
+
+<p class="indnt">1946. Predation and vertebrate populations. Quart. Rev. Biol.,
+21:144-177.</p>
+
+<p><span class="smcap">Errington, P. L.</span>, <span class="smcap">F. Hamerstrom</span>, and <span class="smcap">F. N. Hamerstrom, Jr.</span></p>
+
+<p class="indnt">1940. The great horned owl and its prey in north central
+United States. Agric. Exp. Sta., Iowa State Coll. Agric. Mech.
+Arts, Res. Bull., 277:759-831.</p>
+
+<p><span class="smcap">Fisher, H. J.</span></p>
+
+<p class="indnt">1945. Notes on the voles in central Missouri. Jour. Mamm.,
+26:435-437.</p>
+
+<p><span class="smcap">Fitch, H. S.</span></p>
+
+<p class="indnt">1948. A study of coyote relationships on cattle range. Jour.
+Wildlife Mgmt., 12:73-78.</p>
+
+<p class="indnt">1950. A new style live trap for small mammals. Jour. Mamm.,
+31:364-365.</p>
+
+<p class="indnt">1952. The University of Kansas Natural History Reservation.
+Univ. Kansas, Mus. Nat. Hist. Misc. Publ., 4:1-38.</p>
+
+<p class="indnt">1954. Seasonal acceptance of bait by small mammals. Jour.
+Mamm., 35:39-47.</p>
+
+<p><span class="smcap">Fitch, H. S.</span>, <span class="smcap">F. Swenson</span>, and <span class="smcap">D. F. Tillotson</span>.</p>
+
+<p class="indnt">1946. Behavior and food habits of the red-tailed hawk. Condor,
+48:205-237.</p>
+
+<p><span class="smcap">Goodpastor, W. W.</span>, and <span class="smcap">D. F. Hoffmeister</span>.</p>
+
+<p class="indnt">1952. Notes on the mammals of eastern Tennessee. Jour. Mamm.,
+33:362-371.</p>
+
+<p><span class="smcap">Gunderson, H. L.</span></p>
+
+<p class="indnt">1950. A study of some small mammal populations at Cedar Creek
+Forest, Asoka County, Minnesota. Univ. Minnesota Mus. Nat.
+Hist., 4:1-49.</p>
+
+<p><span class="smcap">Hall, E. R.</span></p>
+
+<p class="indnt">1926. Changes during growth in the skull of the rodent
+<i>Otospermophilus grammarus beecheyi</i>. Univ. California Publ.
+Zool., 21:355-404.</p>
+
+<p><span class="smcap">Hall, E. R.</span>, and <span class="smcap">E. L. Cockrum</span>.</p>
+
+<p class="indnt">1953. A synopsis of North American microtine rodents. Univ.
+Kansas, Mus. Nat. Hist. Publ., 5:373-498.</p>
+
+<p><span class="smcap">Hamilton, W. J., Jr.</span></p>
+
+<p class="indnt">1937a. Growth and life span of the field mouse. Amer. Nat.,
+71:500-507.</p>
+
+<p class="indnt">1937b. The biology of microtine cycles. Jour. Agric. Res.,
+54:779-790.</p>
+
+<p class="indnt">1937c. Activity and home range of the field mouse. Ecol.,
+18:255-263.</p>
+
+<p class="indnt">1940. Life and habits of the field mouse. Sci. Monthly,
+50:425-434.</p>
+
+<p class="indnt">1941. The reproduction of the field mouse, <i>Microtus
+pennsylvanicus</i>. Cornell Univ. Agric. Exp. Sta. Mem.,
+237:1-23.</p>
+
+<p class="indnt">1949. The reproductive rates of some small mammals. Jour.
+Mamm., 30:257-260.</p>
+
+<p><span class="smcap">Hatfield, D. M.</span></p>
+
+<p class="indnt">1935. A natural history of <i>Microtus californicus</i>. Jour.
+Mamm., 16:261-271.</p>
+<p><span class="pagenum"><a name="Page_414" id="Page_414">[414]</a></span></p>
+<p><span class="smcap">Hatt, R. T.</span></p>
+
+<p class="indnt">1930. The biology of the voles of New York. Roosevelt Wildlife
+Bull., 5:513-623.</p>
+
+<p><span class="smcap">Hayne, D. M.</span></p>
+
+<p class="indnt">1949a. Two methods of estimating populations from trapping
+records. Jour. Mamm., 30:399-411.</p>
+
+<p class="indnt">1949b. Calculation of the size of home range. Jour. Mamm.,
+30:1-18.</p>
+
+<p class="indnt">1950. Apparent home range of <i>Microtus</i> in relation to
+distance between traps. Jour. Mamm., 31:26-39.</p>
+
+<p><span class="smcap">Hinton, M. A. C.</span></p>
+
+<p class="indnt">1926. Monograph of the voles and lemmings (Microtinae) living
+and extinct. British Museum of Nat. Hist., London, xvi + 488
+pp. 15 pls.</p>
+
+<p><span class="smcap">Hopkins, H. H.</span>, <span class="smcap">F. W. Albertson</span>, and <span class="smcap">D. A. Riegel</span>.</p>
+
+<p class="indnt">1952. Ecology of grassland utilization in a mixed prairie.
+Trans. Kansas Acad. Sci., 55:395-418.</p>
+
+<p><span class="smcap">Howard, W. E.</span></p>
+
+<p class="indnt">1951. Relation between low temperature and available food to
+survival of small rodents. Jour. Mamm., 32:300-312.</p>
+
+<p><span class="smcap">Howell, A. B.</span></p>
+
+<p class="indnt">1924. Individual and age variation in <i>Microtus montanus
+yosemite</i>. Jour. Agric. Res., 28:977-1015.</p>
+
+<p><span class="smcap">Jameson, E. W.</span></p>
+
+<p class="indnt">1947. Natural history of the prairie vole. Univ. Kansas, Mus.
+Nat. Hist. Publ., 1:125-151.</p>
+
+<p class="indnt">1950. Determining fecundity in male small mammals. Jour.
+Mamm., 31:433-436.</p>
+
+<p><span class="smcap">Johnson, M. S.</span></p>
+
+<p class="indnt">1926. Activity and distribution of certain wild mice in
+relation to the biotic community. Jour. Mamm., 7:245-277.</p>
+
+<p><span class="smcap">Korschgen, L. J.</span></p>
+
+<p class="indnt">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.</p>
+
+<p><span class="smcap">Lantz, D. E.</span></p>
+
+<p class="indnt">1907. An economic survey of the field mice (genus <i>Microtus</i>).
+USDA Biol. Surv. Bull, 31:1-64.</p>
+
+<p><span class="smcap">Leslie, P. H.</span>, and <span class="smcap">R. M. Ransom</span>.</p>
+
+<p class="indnt">1940. The mortality, fertility and rate of natural increase of
+the vole (<i>Microtus agrestis</i>) as observed in the laboratory.
+Jour. Animal Ecol., 9:27-52.</p>
+
+<p><span class="smcap">Llewellyn, L. M.</span></p>
+
+<p class="indnt">1950. Reduction of mortality in live-trapping mice. Jour.
+Wildlife Mgmt., 14:84-85.</p>
+
+<p><span class="smcap">McAtee, W. L.</span></p>
+
+<p class="indnt">1935. Food habits of common hawks. USDA Circ., 370:1-36.</p>
+
+<p><span class="smcap">Meyer, B. J.</span>, and <span class="smcap">R. K. Meyer</span>.</p>
+
+<p class="indnt">1944. Growth and reproduction of the cotton rat, <i>Sigmodon
+hispidus hispidus</i>, under laboratory conditions. Jour. Mamm.,
+25:107-129.</p>
+<p><span class="pagenum"><a name="Page_415" id="Page_415">[415]</a></span></p>
+<p><span class="smcap">Mohr, C. O.</span></p>
+
+<p class="indnt">1943. A comparison of North American small mammal censuses.
+Amer. Midland Nat., 29:545-587.</p>
+
+<p class="indnt">1947. Table of equivalent populations of North American small
+mammals. Amer. Midland Nat., 37:223-249.</p>
+
+<p><span class="smcap">Phillips, P.</span></p>
+
+<p class="indnt">1936. The distribution of rodents in overgrazed and normal
+grassland in central Oklahoma. Ecol., 17:673-679.</p>
+
+<p><span class="smcap">Poiley, S. M.</span></p>
+
+<p class="indnt">1949. Raising captive meadow voles (<i>Microtus p.
+pennsylvanicus</i>). Jour. Mamm., 30:317.</p>
+
+<p><span class="smcap">Rinker, G. C.</span></p>
+
+<p class="indnt">1942a. Litter records of some mammals of Meade County, Kansas.
+Trans. Kansas Acad. Sci., 45:376-378.</p>
+
+<p class="indnt">1942b. An extension of the range of the Texas cotton rat in
+Kansas. Jour. Mamm., 23:439.</p>
+
+<p><span class="smcap">Sandidge, L. L.</span></p>
+
+<p class="indnt">1953. <a name="fooddens" id="fooddens"></a><ins title="Original has Foods, and dens">Food and dens</ins> of the opossum (<i>Didelphis virginiana</i>)
+in northeastern Kansas. Trans. Kansas Acad. Sci., 56:97-106.</p>
+
+<p><span class="smcap">Selle, R. M.</span></p>
+
+<p class="indnt">1928. <i>Microtus californicus</i> in captivity. Jour. Mamm.,
+9:93-98.</p>
+
+<p><span class="smcap">Shapiro, J.</span></p>
+
+<p class="indnt">1950. Notes on the population dynamics of <i>Microtus</i> and
+<i>Blarina</i> with a record of albinism in <i>Blarina</i>. Jour.
+Wildlife Mgmt, 14:359-360.</p>
+
+<p><span class="smcap">Stickel, L. F.</span></p>
+
+<p class="indnt">1946. Experimental analysis of methods of measuring small
+mammal populations. Jour. Wildlife Mgmt., 10:150-159.</p>
+
+<p class="indnt">1948. The trap line as a measure of small mammal populations.
+Jour. Wildlife Mgmt., 12:153-161.</p>
+
+<p><span class="smcap">Strecker, J. K.</span></p>
+
+<p class="indnt">1929. Notes on the Texas cotton and Atwater wood rats. Jour.
+Mamm., 10:216-220.</p>
+
+<p><span class="smcap">Summerhayes, V. S.</span></p>
+
+<p class="indnt">1941. The effects of voles (<i>Microtus agrestis</i>) on
+vegetation. Jour. Ecol., 29:14-48.</p>
+
+<p><span class="smcap">Svihla, A.</span></p>
+
+<p class="indnt">1929. Life history notes on <i>Sigmodon hispidus hispidus</i>.
+Jour. Mamm., 10:352-353.</p>
+
+<p><span class="smcap">Townsend, M. T.</span></p>
+
+<p class="indnt">1935. Studies on some small mammals of central New York.
+Roosevelt Wildlife Annals, 4:1-120.</p>
+
+<p><span class="smcap">Uhler, F. M.</span>, <span class="smcap">C. Cottam</span>, and <span class="smcap">T. E. Clarke</span>.</p>
+
+<p class="indnt">1939. Food of the snakes of George Washington National Forest,
+Virginia. Trans. 4th N. A. Wildlife Conf., 605-622.</p>
+
+<p><span class="smcap">Wooster, L. D.</span></p>
+
+<p class="indnt">1935. Notes on the effects of drought on animal populations
+in western Kansas. Trans. Kansas Acad. Sci., 38:351-352.</p>
+<p><span class="pagenum"><a name="Page_416" id="Page_416">[416]</a></span></p>
+<p class="indnt">1936. The contents of owl pellets as indicators of habitat
+preferences of small mammals. Trans. Kansas Acad. Sci.,
+39:395-397.</p>
+
+<p class="indnt">1939. An ecological evaluation of predatees on a mixed prairie
+area in western Kansas. Trans. Kansas Acad. Sci., 42:515-517.</p>
+
+
+<p><i>Transmitted May 19, 1955.</i></p>
+
+
+
+<hr class="chap" />
+<h2><a name="UNIVERSITY_OF_KANSAS_PUBLICATIONS_MUSEUM_OF_NATURAL_HISTORY" id="UNIVERSITY_OF_KANSAS_PUBLICATIONS_MUSEUM_OF_NATURAL_HISTORY">UNIVERSITY OF KANSAS PUBLICATIONS, MUSEUM OF NATURAL HISTORY</a></h2>
+
+
+<p>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.</p>
+
+<p>* 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:</p>
+
+<p>Vol. 1.</p>
+
+<p class="indnt">Nos. 1-26 and index. Pp. 1-638, 1946-1950.</p>
+
+<p class="indnt">Index. Pp. 605-638.</p>
+
+<p>*Vol. 2.</p>
+
+<p class="indnt">(Complete) Mammals of Washington. By Walter W. Dalquest. Pp.
+1-444, 140 figures in text. April 9, 1948.</p>
+
+<p>Vol. 3.</p>
+
+<p class="indnt">*1. The avifauna of Micronesia, its origin, evolution, and
+distribution. By Rollin H. Baker. Pp. 1-359, 16 figures in
+text. June 12, 1951.</p>
+
+<p class="indnt">*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.</p>
+
+<p class="indnt">3. Phylogeny of the waxwings and allied birds. By M. Dale
+Arvey. Pp. 473-530, 49 figures in text, 13 tables. October 10,
+1951.</p>
+
+<p class="indnt">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.</p>
+
+<p class="indnt">Index. Pp. 651-681.</p>
+
+<p>*Vol. 4.</p>
+
+<p class="indnt">(Complete) American weasels. By E. Raymond Hall. Pp. 1-466, 41
+plates, 31 figures in text. December 27, 1951.</p>
+
+<p>Vol. 5.</p>
+
+<p class="indnt">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.</p>
+
+<p class="indnt">2. Two new moles (Genus Scalopus) from Mexico and Texas. By
+Rollin H. Baker. Pp. 17-24. February 28, 1951.</p>
+
+<p class="indnt">3. Two new pocket gophers from Wyoming and Colorado. By E.
+Raymond Hall and H. Gordon Montague. Pp. 25-32. February 28,
+1951.</p>
+
+<p class="indnt">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.</p>
+
+<p class="indnt">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.</p>
+
+<p class="indnt">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.</p>
+
+<p class="indnt">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.</p>
+
+<p class="indnt">8. A new pocket gopher (Genus Thomomys) from eastern Colorado.
+By E. Raymond Hall. Pp. 81-85. October 1, 1951.</p>
+
+<p class="indnt">9. Mammals taken along the Alaskan Highway. By Rollin H.
+Baker. Pp. 87-117, 1 figure in text. November 28, 1951.</p>
+
+<p class="indnt">*10. A synopsis of the North American Lagomorpha. By E.
+Raymond Hall. Pp. 119-202, 68 figures in text. December 15,
+1951.</p>
+
+<p class="indnt">11. A new pocket mouse (Genus Perognathus) from Kansas. By E.
+Lendell Cockrum. Pp. 203-206. December 15, 1951.</p>
+
+<p class="indnt">12. Mammals from Tamaulipas, Mexico. By Rollin H. Baker. Pp.
+207-218. December 15, 1951.</p>
+
+<p class="indnt">13. A new pocket gopher (Genus Thomomys) from Wyoming and
+Colorado. By E. Raymond Hall. Pp. 219-222. December 15, 1951.</p>
+
+<p class="indnt">14. A new name for the Mexican red bat. By E. Raymond Hall.
+Pp. 223-226. December 15, 1951.</p>
+
+<p class="indnt">15. Taxonomic notes on Mexican bats of the Genus Rhogeëssa. By
+E. Raymond Hall. Pp. 227-232. April 10, 1952.</p>
+
+<p class="indnt">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.</p>
+
+<p class="indnt">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.</p>
+
+<p class="indnt">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.</p>
+
+<p class="indnt">19. A new chipmunk (Genus Eutamias) from the Black Hills. By
+John A. White. Pp. 259-262. April 10, 1952.</p>
+
+<p class="indnt">20. A new piñon mouse (Peromyscus truei) from Durango, Mexico.
+By Robert B. Finley, Jr. Pp. 263-267. May 23, 1952.</p>
+
+<p class="indnt">21. An annotated checklist of Nebraskan bats. By Olin L. Webb
+and J. Knox Jones, Jr. Pp. 269-279. May 31, 1952.</p>
+
+<p class="indnt">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.</p>
+
+<p class="indnt">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.</p>
+
+<p class="indnt">24. The subspecific status of two Central American sloths. By
+E. Raymond Hall and Keith R. Kelson. Pp. 313-317. November 21,
+1952.</p>
+
+<p class="indnt">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.</p>
+
+<p class="indnt">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.</p>
+
+<p class="indnt">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.</p>
+
+<p class="indnt">28. The pocket gophers (Genus Thomomys) of Coahuila, Mexico.
+By Rollin H. Baker. Pp. 499-514, 1 figure in text. June 1,
+1953.</p>
+
+<p class="indnt">29. Geographic distribution of the pocket mouse, Perognathus
+fasciatus. By J. Knox Jones, Jr. Pp. 515-526, 7 figures in
+text. August 1, 1953.</p>
+
+<p class="indnt">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.</p>
+
+<p class="indnt">31. Four new pocket gophers of the genus Cratogeomys from
+Jalisco, Mexico. By Robert J. Russell. Pp. 535-542. October
+15, 1953.</p>
+
+<p class="indnt">32. Genera and subgenera of chipmunks. By John A. White. Pp.
+543-561, 12 figures in text. December 1, 1953.</p>
+
+<p class="indnt">33. Taxonomy of the chipmunks, Eutamias quadrivittatus and
+Eutamias umbrinus. By John A. White. Pp. 563-582, 6 figures in
+text. December 1, 1953.</p>
+
+<p class="indnt">34. Geographic distribution and taxonomy of the chipmunks of
+Wyoming. By John A. White. Pp. 584-610, 3 figures in text.
+December 1, 1953.</p>
+
+<p class="indnt">35. The baculum of the chipmunks of western North America. By
+John A. White. Pp. 611-631, 19 figures in text. December 1,
+1953.</p>
+
+<p class="indnt">36. Pleistocene Soricidae from San Josecito Cave, Nuevo Leon,
+Mexico. By James S. Findley. Pp. 633-639. December 1, 1953.</p>
+
+<p class="indnt">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.</p>
+
+<p class="indnt">Index. Pp. 647-676.</p>
+
+<p>*Vol. 6.</p>
+
+<p class="indnt">(Complete) Mammals of Utah, <i>taxonomy and distribution</i>. By
+Stephen D. Durrant. Pp. 1-549, 91 figures in text, 30 tables.
+August 10, 1952.</p>
+
+<p>Vol. 7.</p>
+
+<p class="indnt">*1. Mammals of Kansas. By E. Lendell Cockrum. Pp. 1-303, 73
+figures in text, 37 tables. August 25, 1952.</p>
+
+<p class="indnt">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.</p>
+
+<p class="indnt">3. The silky pocket mice (Perognathus flavus) of Mexico. By
+Rollin H. Baker. Pp. 339-347, 1 figure in text. February 15,
+1954.</p>
+
+<p class="indnt">4. North American jumping mice (Genus Zapus). By Philip H.
+Krutzsch. Pp. 349-472, 47 figures in text, 4 tables. April 21,
+1954.</p>
+
+<p class="indnt">5. Mammals from Southeastern Alaska. By Rollin H. Baker and
+James S. Findley. Pp. 473-477. April 21, 1954.</p>
+
+<p class="indnt">6. Distribution of some Nebraskan Mammals. By J. Knox Jones,
+Jr. Pp. 479-487. April 21, 1954.</p>
+
+<p class="indnt">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.</p>
+
+<p class="indnt">8. A new subspecies of bat (Myotis velifer) from southeastern
+California and Arizona. By Terry A. Vaughn. Pp. 507-512. July
+23, 1954.</p>
+
+<p class="indnt">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.</p>
+
+<p class="indnt">10. A new bat (Genus Pipistrellus) from northeastern Mexico.
+By Rollin H. Baker. Pp. 583-586. November 15, 1954.</p>
+
+<p class="indnt">11. A new subspecies of pocket mouse from Kansas. By E.
+Raymond Hall. Pp. 587-590. November 15, 1954.</p>
+
+<p class="indnt">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.</p>
+
+<p class="indnt">13. A new cottontail (Sylvilagus floridanus) from northeastern
+Mexico. By Rollin H. Baker. Pp. 609-612. April 8, 1955.</p>
+
+<p class="indnt">14. Taxonomy and distribution of some American shrews. By
+James S. Findley. Pp. 613-618. June 10, 1955.</p>
+
+<p class="indnt">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.</p>
+
+<p class="indnt">Index. Pp. 625-651.</p>
+
+<p>Vol. 8.</p>
+
+<p class="indnt">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.</p>
+
+<p class="indnt">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.</p>
+
+<p class="indnt">3. An ecological study of the collared lizard (Crotaphytus
+collaris). By Henry S. Fitch. Pp. 213-274, 10 figures in text.
+February 10, 1956.</p>
+
+<p class="indnt">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.</p>
+
+<p class="indnt">5. Check-list of the birds of Kansas. By Harrison B. Tordoff.
+Pp. 307-359, 1 figure in text. March 10, 1956.</p>
+
+<p class="indnt">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.</p>
+
+<p class="indnt">More numbers will appear in volume 8.</p>
+
+<p>Vol. 9.</p>
+
+<p class="indnt">1. Speciation of the wandering shrew. By James S. Findley. Pp.
+1-68, 18 figures in text. December 10, 1955.</p>
+
+<p class="indnt">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.</p>
+
+<p class="indnt">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.</p>
+
+<p class="indnt">More numbers will appear in volume 9.</p>
+
+
+<hr class="chap" />
+<div class='tnote'>
+<h2><a name="Transcribers_note" id="Transcribers_note">Transcriber's note:</a></h2>
+
+<p>A Table of Contents has been added to this ebook for the reader's
+convenience.</p>
+
+<p>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>
+
+<p>p. 372, in "A more homogeneous vegetation would tend to pass"
+homogenous has been changed to <a href="#homogeneous">homogeneous</a>.</p>
+
+<p>p. 415, "1953. Foods, and dens of the opossum ..." has been changed to
+"1953. <a href="#fooddens">Food and dens</a> of the opossum ..."</p>
+
+<p>In <a href="#img011"><span class="smcap">Fig. 11</span></a> the bottommost
+y-axis label in the scale of gms. is probably an <a href="#weighterror">error</a>: 45 should be 35.
+</p>
+
+<p>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.  Missing page numbers correspond to
+moved full-page illustrations.
+ 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.</p>
+
+<p>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.</p>
+
+<p>Because the cover of the original document contained text exactly
+duplicated on the title page, this cover information has been omitted.</p>
+</div>
+
+
+
+
+
+
+
+<pre>
+
+
+
+
+
+End of the Project Gutenberg EBook of A Population Study of the Prairie Vole
+(Microtus ochrogaster) in Northeastern Kansas, by Edwin P. Martin
+
+*** END OF THIS PROJECT GUTENBERG EBOOK POPULATION STUDY OF PRAIRIE VOLE ***
+
+***** This file should be named 39396-h.htm or 39396-h.zip *****
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@@ -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: ASCII
+
+*** 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 deg.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 x 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 x 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
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+
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+
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+
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+
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+
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+
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+
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+
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+
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+
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+
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+
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+
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+
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+
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+
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+
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+
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+
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+
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+
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+
+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
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+
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+     in western Kansas. Trans. Kansas Acad. Sci., 42:515-517.
+
+
+     _Transmitted May 19, 1955._
+
+
+
+
+UNIVERSITY OF KANSAS PUBLICATIONS, MUSEUM OF NATURAL HISTORY
+
+
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+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 Rhogeessa. 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 pinon 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. Martin
+
+*** END OF THIS PROJECT GUTENBERG EBOOK POPULATION STUDY OF PRAIRIE VOLE ***
+
+***** This file should be named 39396.txt or 39396.zip *****
+This and all associated files of various formats will be found in:
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