depletion in males and bet hedging against sterility in the
first male), gaining material benefits (i.e., more paternal in-
vestment from males), getting better genes for offspring, in-
creasing the probability of mating with high-quality males,
mate compatibility, increasing the genetic variability of off-
spring, and preventing harassment by males or infanticide
of offspring. We have discussed some of these hypotheses
previously and do not review them here because there are
already very good reviews in the literature (see Schwag-
meyer 1984; Agrell et al. 1998; Wolff and MacDonald 2004
for more details on these hypotheses).
The observations of MMM by females might suggest that
female mating preferences are more complex than has been
assumed by sexual selection theory (Schwagmeyer 1984).
Some of these assumptions of sexual selection theory are
being reexamined with the increasing evidence for benefits
to females from mating with more than one male.
Based on our literature search, the intraspecific fre-
quency of MMM varies from 12% in red squirrels (Sciurus
vulgaris,Wauters et al. 1990) to 79% in meadow voles
(Berteaux et al.1999) and multiple paternity within litters
varies from 16% in Columbian ground squirrels (Spermo-
philus columbianus,Murie 1995) to 90% in the yellow-
toothed cavy (Hohoff et al. 2003). There are also a few data
showing interpopulation variability in multiple paternity
(table 4.2). Although little is known about temporal vari-
ation in multiple paternity within a population, multiple
paternity can vary from none (0%) to a high level (50%)
within the same population in different years (Schwagmeyer
and Brown 1983).
Effects of multi-male mating on female
reproductive success
Even though more social than solitary species are reported
to have MMM (Møller and Birkhead 1989), MMM is re-
ported just as frequently in socially monogamous mammals
as in socially polygynous mammals. As predicted by the
active female hypothesis, females solicit multiple matings
in all species for which data are available (Wolff and Mac-
Donald 2004). In most species, we still do not know with
whom females mate. These data are critical for evaluating
proposed hypotheses.
There is conflicting evidence on the effects of MMM on
the probability of producing a litter. In Gunnison’s prairie
dogs (Cynomys gunnisoni), there is increased probability of
producing a litter following MMM (Hoogland 1998b). An
increased probability of conceiving following MMM may
be due to fertility assurance, genetic incompatibility avoid-
ance, or mate choice for good genes via sperm competition
(Murie 1995; Stockley 2003). On the other hand, a de-
creased probability in conceiving has been detected in two
species following MMM (deermice, Peromyscus manicula-
tus,and Djungarian hamsters, Dewsbury 1982b; Wynne-
Edwards and Lisk 1984). Such a decrease following MMM
may be the result of a strange-male-induced pregnancy block
(Dewsbury 1982b; Wynne-Edwards and Lisk 1984). Fi-
nally, MMM had no effect on conception in four species
(13-lined ground squirrels, Spermophilus tridecemlinea-
tus,Schwagmeyer 1986; Columbian ground squirrels, Mu-
rie 1995; black-tailed prairie dogs, Cynomys ludovicianus,
Hoogland 1995; prairie voles, Wolff and Dunlap 2002).
Although litter size tends not to differ at birth (only 1 of
5 studies showed a significant difference), there was a non-
significant tendency toward increased offspring survivorship
for female Columbian ground squirrels that mated with
more males (Murie 1995). Furthermore, female yellow-
toothed cavies weaned more young if they mated with four
males than with only one male, because more pups survived
between birth and weaning (Keil and Sacher 1998).Postcopulatory Cryptic Female ChoiceMate choice can occur after copulation in many species of
animals. Eberhard (1996) reviewed available data on post-
copulatory (cryptic) female choice. Cryptic female choice is
defined as processes that occur after copulation and influ-
ence paternity in favor of particular males. This type of fe-
male choice is referred to as cryptic because it cannot be ob-
served (Bolhuis and Giraldeau 2005). Cryptic female choice
is considered to be relatively common in some animal taxa
while still remaining relatively unexplored in others. Of the
numerous possible mechanisms by which cryptic female
choice may occur (see the following) some are not likely to
occur in rodents, but we will review evidence for several
that are more likely (table 4.3).Failure to transport sperm to the site of fertilization
In rodents, male genitalia do not reach the site where ova
are fertilized; females actively transport the sperm (Hunter
1975). In mammals, sperm transport is often triggered by
copulation. For example, at least one preejaculatory intro-
mission is needed to stimulate a female Norway rat (Rat-
tus norvegicus) to transport sperm to the oviducts (Adler
1969). Sperm are not transported past the cervix if a female
receives additional intromissions too soon after receiving
the first ejaculation (Adler and Zoloth 1970). It takes a
minimum of 10 minutes before sperm are completely trans-
ported to the site of fertilization (Matthews and Adler
1977). Female Norway rats solicited intromissions fromReproductive Strategies in Female Rodents 49