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.