Rodent Societies: An Ecological & Evolutionary Perspective

could be influenced by the delay between competing males’
copulations and timing of copulation relative to female
ovulation.
The time interval between different multiple-male mating
partners in thirteen-lined ground squirrels influenced the
second male’s ability to sire offspring (Schwagmeyer and
Foltz 1990). A longer interval between matings by multiple
males gave the first male a reproductive advantage for pa-
ternity. No such relationship with delay and fertilization
was evident in house mice (Dewsbury 1984). In Belding’s
and thirteen-lined ground squirrels, which have first male
advantages for siring offspring, second males may be able
to get some portion of paternity by frequent re-mating with
the female and increasing the duration of copulations (Sher-
man 1989; Schwagmeyer and Foltz 1990). These strategies
of secondary males do not seem to work in Arctic ground
squirrels. For a male Arctic ground squirrel the only way
to increase paternity is to increase the number of females
mated with first; number of matings per se does not seem to
increase probability of paternity (Lacey et al. 1997).
Another postcopulatory option is to increase sperm vol-
ume (Schwagmeyer and Foltz 1990). In laboratory rats,
golden hamsters (Mesocricetus auratus), and deer mice, the
probability of fertilization is increased by larger volumes of
ejaculate (Lanier et al. 1979; Dewsbury and Hartung 1980;
Dewsbury 1984). Male meadow voles increase the amount
of sperm in their ejaculates when they mate in the presence
of another male’s odors (delBarco-Trillo and Ferkin 2004).
In rodents in general, there appears to be an increase in re-
matings (number of ejaculates per female) with increasing
potential for sperm competition (inferred by relative testes
size; Stockley and Preston 2004). This increase in the num-
ber of ejaculations is suggested to increase the number of
sperm transferred, but could also be related to the timing
of insemination. If the exact time of ovulation is unknown,
these repeated copulations could increase the likelihood of
their sperm being present and viable at the time of ovula-
tion (Parker 1984b).
Stockley and Preston (2004) used relative testes size as a
measure of the intensity of sperm competition. In species in
which females mate promiscuously, presumably resulting in
sperm competition, males have relatively larger testes size
than in monogamous or strictly polygynous species (Ke-
nagy and Trombulak 1986; table 4.1). For North American
voles, monogamous and polygynous species have small tes-
tes compared to promiscuous species (Heske and Ostfeld
1990). In the promiscuous Cape ground squirrel, with an
average OSR of 10 males to a single female, testes size is
nearly 20% of a male’s head-body length (Waterman 1998;
fig. 3.4). Males in this species masturbate to ejaculation,
which may be related to intense sperm competition (Water-

man unpubl. data). Large testes size is presumably a mech-

anism for producing large volumes of sperm, increasing the

probability of fertilizing eggs compared to males with less

sperm volume; however, there are few data from wild popu-

lations to support this idea.

Another strategy for males to increase their fitness is to

kill the young sired by another male, and thus provide them-

selves with a reproductive opportunity (Hrdy 1979; Eben-

sperger 1998c; Ebensperger and Blumstein, chap. 23, this

volume). Theoretical and empirical evidence of reproductive

Male Mating Strategies in Rodents 39

Figure 3.4 In the promiscuous Cape ground squirrel, the potential for sperm

competition is high and testes size is nearly 20% of a male’s head-body length

(a). Males will often masturbate to ejaculation (a) and (b), which may be related

to intense sperm competition. Photo (a) by A. Rasa and (b) by J. Waterman.

A

B