In October, 1945, when this study was begun, the prairie vole was
bringing forth young. In the winter of 1945-'46 at Lawrence, Kansas,
there was a cessation of reproduction. The reproductive activity was
measured in terms of the fecundity of the subadults and the adults of
both sexes. Figure 2 suggests that the decline was most marked in
December and January; no gravid females were collected in these two
months, although two females trapped in the first week of December were
lactating. In October, November, and December, 85 per cent of the
breeding females were adults. In October, 85 per cent of the adult
females were fecund, and in November, this figure was 80 per cent.
Reproduction at this season, in the females, it appears, was largely a
function of the adults. The proportion of adults to the rest of the
population was calculated for each month; and the monthly changes in
relative numbers of adults is shown in figure 4. In November, December,
and January there was a scarcity of adult voles in the population. The
autumnal decline in reproduction occurred simultaneously with the
disappearance of these adults, and is thought to have been largely a
result of it.
Reproductive activity began in February; and in this month one-third of
the females contained embryos, and 90 per cent of the males were fecund.
Reproduction reached its height in March when fecundity for the females
and males was 77 per cent and 100 per cent respectively. In April both
sexes showed signs of being less productive, and still later in the
spring the percentage of fecundity remained at slightly over 65 for both
sexes, this figure being higher for the males than for the females for
any one month. From January to February there was a 30 per cent increase
in the percentage of adults in the population; and for this period,
there was a 33 per cent increase in the fecundity of both males and
females. In February, 80 per cent of the fecund females were adults. The
breeding in the late winter, as in the fall, is thought to depend upon
the percentage of adults in the population. Hamilton (1937b) noted a
similar correlation between winter breeding and dominance of adults in
_Microtus pennsylvanicus_ in New York. Fisher (1945) found that the
prairie vole continued to breed throughout the winter of 1943-'44 in
Missouri; in such a case, one would expect to find a large proportion of
adults in the population.
[Illustration: FIGURE 4. Seasonal Changes in the Numbers of Adults in
Relation to the Total Population of Prairie Voles.]
Throughout the winter of 1945-'46, at Lawrence, the majority of males
were fecund; but fecundity in the females was much less, and in January,
no females showed signs of reproductive activity. From this it appears
that the females, not the males, limit the breeding season of this
species.