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).