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.
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