males sooner after receiving an ejaculation with a subordi-
nate male (about 55% of females resumed mating in 10
minutes) as compared to females that received an ejacula-
tion from a dominant male (only22% resumed mating in
less than 10 minutes; McClintock, Anisko, and Adler 1982).
Therefore, sperm transport was likely to be decreased when
females received an ejaculate from a subordinate male.
Failure to ovulate
Most female rodents, such as Norway rats and house mice,
ovulate spontaneously (i.e., on a regular cycle), while in oth-
ers, such as voles, ovulation is induced (triggered) by mat-
ing (Milligan 1982). Even in rodents thought to be sponta-
neous ovulators, copulation can cause ovulation to occur
earlier than it would have otherwise or increase ovulation
rates (Zarrow and Clark 1968; Eberhard 1996). If females
respond differentially to different males — for example, not
all copulations result in ovulation or not all ova are ovu-
lated after each copulation — then selection would act on
male traits that increase the probability of ovulation. In-
creasing the likelihood of ovulation, the number of mature
eggs ovulated, or the rapidity of ovulation would increase
male reproductive success. Males may also benefit if ovula-
tion coincides with the release of peak numbers of their own
sperm. If this occurs, the likelihood of previous or subse-
quent males fertilizing most of the eggs is decreased. Al-
though female reduction in ovulation could result in a de-
crease in the number of offspring produced, females may
benefit by producing fewer offspring of higher quality.
The importance of copulatory stimuli in inducing ovu-
lation has been examined in a few studies. A larger num-
ber of preejaculatory thrusts and intromissions resulted in
a greater percentage of females ovulating in prairie voles
(Gray et al. 1974). Similarly, Norway rats were more likely
to ovulate when coital stimulation was increased, and an
increased amount of stimulation resulted in a greater per-
centage of eggs ovulated (Zarrow and Clark 1968).
Failure to prepare uterus for implantation
In some species of rodents, follicles enter a luteal phase
and produce progesterone necessary for endometrial pro-
liferation only in response to the stimulus of copulation
(table 4.3). The initiation of the luteal cycle, which results
in the changes in uterine cells, is necessary for successful im-
plantation to occur. Variation in the number and pacing of
intromissions affects the likelihood that this change in uter-
ine cells will occur. This has been demonstrated in experi-
ments in which the amount of copulatory stimulation re-
ceived by a female is manipulated. Eighty-six percent of
female Norway rats receiving 10-16 intromissions had their
luteal cycle triggered, as compared to 67% of females re-
ceiving 6-9 intromissions (Chester and Zucker 1970). The
critical stimulus can be preejaculatory intromissions (Ad-
ler 1969; Huck and Lisk 1985) or the male ejaculatory re-
sponse, which includes increased thrusting, grabbing the fe-
male with all four legs, shuddering and swelling of the penis
(Land and McGill 1967; McGill et al. 1968). The most di-
rect evidence that this mechanism can be a form of sexual
selection comes from studies of McClintock, Anisko, and
Adler (1982), in which dominant male Norway rats per-
formed 6.1 intromissions as compared to 4.4 performed by
subordinant males. Thus MMM may be a mechanism by
which females help to prepare the uterus for implantation
of blastocysts.Removal of copulatory plugs
In a number of rodents, males deposit substances in the
genital opening of females after mating; these are referred
to as copulatory plugs (Dewsbury and Baumgardner 1981;
Baumgardner et al. 1982). Two proposed functions of these
plugs are to prevent leakage of sperm and reduce MMM.
Female rodents may actively control the length of time that
copulatory plugs are present in their genital tract. For ex-
ample, about one-half to two-thirds of female tree squir-
rels have been observed to remove plugs deposited by some
males but not others (Koprowski 1992). Females either re-
moved plugs immediately after mating or left them in place
for several hours. Thus females may discriminate among
males by selective removal of mating plugs.Raising the Young: Conflict and CooperationFemale rodents continue to invest in offspring after birth.
Females provide nutrition, warmth, and protection prior to
weaning. All three of these forms of investment are critical
in altricial species, and the latter is the most important in
precocial species. These forms of investment can be en-
hanced by maintenance of a territory by the mother, for-
mation of female kin clusters, or formation of cooperative
breeding groups (either extended families or groups with
multiple breeding females). Investment in offspring may
continue even after weaning in some species by bequeathal
of the natal territory to offspring or by allowing offspring to
remain philopatric, that is, at the natal nest.Female territoriality
Two hypotheses have been proposed for the function of ter-
ritoriality in female rodents; both would affect reproductive52 Chapter Four