There is sexual activity in every month of the year, but its
incidence varies greatly from one season to another. As has been
indicated by various authors, male voles reach sexual maturity later
than females. It seems that ordinarily the availability of sexually
active males is not a limiting factor, however. While males that are
still well below average adult size produce mature spermatozoa, and
are probably capable of breeding (Jameson, 1947: 145), certain
large old males may sire a disproportionately large percentage of the
litters produced. Observations on males in confinement indicated
that sexual activity tended to be directly proportional to the size
of the testes. Occasional individuals, having much enlarged scrotal
testes were more readily stimulated to sexual activity and more
aggressive toward females than were those in which the testes were
of more nearly typical size or abdominal or were smaller than
normal. The combination of factors controlling size of testes is not
well understood, but males having unusually large testes were caught
most often when food supply was optimum, for instance after a period
of heavy precipitation when an abundant supply of new grass provided
succulent and nutritious food.
In confinement sexual activity was largely inhibited and attempts to
establish a laboratory colony met with failure. Sexual activity was
observed mainly in recently captured males, and their interest was
aroused chiefly by females that had given birth to litters within a
few hours previously. Oestrus is known to follow closely after
parturition. Females found in live-traps with newborn young often
were brought to the laboratory for observation. An apparent instance
of hostility between rival males competing for an oestrus female was
observed on September 2, 1950. The female was found in a trap with
four newborn young, and since the young had not yet attached to her
teats, she was temporarily returned to the trap after recording, to
prevent desertion of the litter. Returning twenty minutes later I
found another adult vole at this trap. It would suddenly emerge from
dense grass nearby, and would move over the trap or around it, with
jerky, halting movements, then would dart back under cover. The
female emerged from the nest box into the trap runway, and sniffed
at the other, and both pressed against the intervening wire barrier.
There was gnawing on the wire by one or both. A third adult vole
appeared. As it moved toward the trap, all three suddenly took alarm
and darted back under cover, the female hiding in the trap nest box.
In a few seconds they again appeared. The two outsiders, presumably
both males, were not individually recognizable, but several times
one was seen to dart at the other, chasing it away momentarily. They
were seldom both in sight at once.
Males confined with post-partum females usually evinced sexual
interest, following them about persistently and nuzzling their
genitalia. The females, however, were often unreceptive perhaps
because they were disturbed by strange surroundings and by the
presence of their litters, so that they usually attempted to escape,
or to rebuff the male's attention. At first the female might flee,
squeaking in protest at the male's pursuit. If he still continued to
follow, she would turn on him, rearing back in the characteristic
threatening pose, and would lunge at him, striking him sharply or
driving him back. After such rebuff, males were usually intimidated
or discouraged so that they temporarily or permanently abandoned
their advances, and small males were more easily rebuffed than were
larger individuals. On several occasions large males having enlarged
testes were not readily rebuffed by females but continued to follow
them. When the female turned upon him, such a male might lunge
against her, throwing her off balance, and causing her to attempt to
escape, and then continuing the pursuit until it ended in copulation
or in more severe fighting. Although not accepted sexually, a
rebuffed male might be readily accepted as a nest-mate, huddling
along with the female and perhaps other individuals of both sexes.
In huddling voles, the most frequently observed type of social
behavior was grooming; one individual would slide its chin or muzzle
through the other's fur with a stroking movement consisting of a
series of rapid forward jerks and the stroking movements might
continue for periods of minutes. The recipient of the grooming
usually made no evident response indicative of either pleasure or
displeasure. Often it seemed to be sleeping while the grooming was
performed. Individuals of both sexes performed this grooming and the
recipient might be of either the same sex or the opposite sex. This
grooming may have some significance as a search for ectoparasites
such as fleas, or mites that often infest the voles. However, after
prolonged grooming by a companion, a vole's fur was of mussed and
disarranged appearance. Although the grooming that occurs between
voles that are resting in nests seems to have no direct significance
as sexual behavior, somewhat similar actions constitute part of the
mating pattern. A sexually aroused male overtaking a receptive
female, slides his chin forward along her back with jerky, stroking
movements. In some observed instances this behavior continued
intermittently for several minutes before actual copulation. In some
other instances it was almost lacking.