In female voles that are sexually quiescent, both those that have
not yet attained breeding maturity, and those that have undergone
regression after attainment of sexual maturity, the vaginal orifice
is not evident. The canal is sealed externally by a membranous
layer of epithelium. Presence of a vaginal orifice indicates that
the individual is in some active stage of the breeding cycle. The
appearance of the orifice varies between different females, and it
changes in the same female from day to day or even from hour to
hour. Presumably these changes in the vaginal orifice are cyclical
and are closely correlated with oestrus, but attempts to trace them
were unsuccessful largely because the normal cycle was rapidly
suppressed in captive voles, which soon became sexually quiescent.
Individual voles living under natural conditions were not trapped
with sufficient regularity to permit tracing the details of changes
in their genitalia.
In those females having the vaginal orifice most developed, the
margins are turgid and slightly inflamed. The circular opening gapes
1.0 to 1.5 mm. in diameter when the tail is raised. A female may
remain in this condition for two days or more. Vaginal smears at
this stage often showed nucleated cells characteristic of oestrus.
Subsequently the margins of the orifice become less prominent and
the opening becomes smaller. The dorsal and ventral walls adhere
until an opening is no longer evident unless the adjacent skin is
stretched.
In pregnancy the orifice is occasionally sealed, but usually is
evident. It is, however, less prominent than in oestrus, and does
not gape. The margins are less turgid than in oestrus, and the
opening is in the form of a transverse slit through which the
purplish epithelial lining of the dorsal wall of the vagina can be
seen. After parturition, placentae and bloody discharge often are in
evidence in the vaginal canal. Females that have not given birth to
young recently may also have bloody mucous discharge. Its
significance has not been determined. In females that are undergoing
sexual regression, the margins of the vaginal orifice become
shrunken and pale, and the orifice becomes partly or wholly sealed.
Bodenheimer and Sulman (1946:255) concluded from their study of
_Microtus guentheri_ that in this species, as in "the cat," "the
rabbit," "the ferret," and a few other mammals, ovulation is induced
by copulation, and that there is no regular vaginal cycle. Hoyte
(1955:412) disagreed with these conclusions for other species of
_Microtus_, as he trapped individuals of _M. oeconomus_ that had
recently ovulated without copulation (at least no sperm were found
in the genital tracts). In _M. ochrogaster_ oestrus seems to be
controlled largely by the food supply, at least the incidence of
perforate females was found to fluctuate irregularly tending to
follow the trend of rainfall, and, probably in more direct
correlation, the amount of new grass present (see Table 1, and
Martin, 1956:383-384). It therefore seems unlikely that in this
species ovulation is dependent on copulation.
In females that have not yet produced young the teats are minute and
well concealed in the fur, so that they are difficult to find, but
in lactation they become conspicuous. In early lactation the teats
are typically about 1 mm. in diameter and 2.5 mm. in length. As
lactation progresses, they become thickened to nearly twice the
original diameter. After lactation, as inversion occurs, they shrink
to scabrous low prominences, 2 mm. to 3 mm. in diameter, surrounded
by bare skin. There are three pairs of mammae, one pair pectoral and
the other two abdominal. As mentioned by Jameson (1947:146), the
pectoral mammae show little evidence of use in lactating prairie
voles. Probably they are not used at all except in females with more
than the four young in a litter accommodated by the abdominal
mammae. As in various other rodents, the suckling young may cling to
the female's teats and may be dragged over the ground as she moves
about. When the female forages near the nest, she may drag the young
with her instead of leaving them, but she can detach them instantly
if she so desires. On many occasions females found in live-traps had
young that were several days old clinging to their teats. In some
instances young that had their eyes open may have followed the
female into the trap and attached afterward.