The prairie vole is non-territorial and somewhat social. Several or
many individuals of both sexes and various sizes may use the same
system of surface runways and burrows and even the same nest. In
general, members of such a group are mutually tolerant. A strange
vole may provoke some hostility at first, but may soon be accepted
as a member of a new group. Consequently, there are frequent shifts
from one home base to another. Sexual relations are probably more or
less promiscuous, although a male and female may rest and travel
together in a semi-permanent association. In confinement only those
males having markedly enlarged scrotal testes showed interest in
females that were in oestrus. Post-partum females especially were
eagerly pursued by such males. Anoestrus females are imperforate,
and a vaginal orifice is present only during an active oestral cycle
or in pregnancy. The perforate condition therefore, is a crude index
of breeding activity in the population. In adult females the ratio
of those that were perforate usually fluctuated between one-fourth
and three-fourths of the total. Only in severe summer drought did
the numbers decline below 24 per cent. Normally, breeding continues
the year around, but it is temporarily inhibited in unusually cold
weather or drought. The highest incidence of pregnancy normally is
in late spring and early summer. The ratio of juveniles in the
population from month to month and year to year is far more stable
than the actual population density.
Gestation is 21 days or a little less. The mean litter is 3.37
+- .075 young. Three is the most frequent number per litter, with
four, two, and five in that order of frequency. Larger and older
females have more young per litter, on the average. Average size
is greater in those litters having fewer young. At birth, young
are between 40 and 50 mm. in length (typically, 47 mm.), and weigh
2.9 +- .05 grams.
At an age of nine days the young have their eyes open, and they may
be weaned at an age of approximately three weeks. Young suckle
chiefly from the four abdominal teats. The pectoral mammae seem to
be inadequately developed, with the result that in exceptionally
large litters of five, six or seven young, usually no more than four
survive. Until weaning the young spend much of their time attached
to the female's teats. She may even drag them behind as she forages.
Females that have suckling young become much less tolerant of other
voles. Attacks on young, and cannibalism, are common. Adult males,
especially, are liable to eat the newborn young. The acquisition of
cannibalistic habits by individuals, and seasonal lack of adequately
nutritious plant foods may result in the killing off of young in
such numbers that the population level is held down.
In young females sterile oestral cycles often begin at about the
time of weaning. Earliest pregnancies occur when females are
approximately one month old, but most are several weeks older before
they become pregnant. Rate of growth declines steadily from a length
increment of approximately 2 mm. per day in voles less than two
weeks old to an increment of approximately one-fourth mm. per day in
subadults. Growth rate is highly variable among individuals at all
stages, and especially in those that have attained adult size. Even
adults tend to gain in length, slowly, as well as in weight, and the
largest individuals are all many months old.