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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.
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