advantages for males in committing infanticide have been
discussed extensively for murid rodents (e.g., Labov et al.
1985; Ebensperger 1998c; Wolff and Macdonald 2004).
Females that lose their young during early lactation typi-
cally recycle and mate again sooner than if they had nursed
their young to weaning. Thus infanticide can provide a mat-
ing opportunity for males. In Peromyscusspp., males that
had not mated in a given area (new immigrants) would kill
pups, whereas resident males that had mated did not, sup-
porting the sexual selection hypothesis for male infanticide
(Wolff and Cicirello 1991). Similar observations of males
killing unrelated offspring have been reported for other spe-
cies of rodents and support infanticide as a male reproduc-
tive strategy (e.g., Wolff and Macdonald 2004; Ebensperger
and Blumstein, chap. 23, this volume). A counter-strategy
by females to male infanticide is to mate multiply with sev-
eral males to confuse paternity, which in turn deters males
from killing offspring (Agrell et al. 1998; Wolff and Mac-
donald 2004; Ebensperger 1998c; Solomon and Keane,
chap. 4, this volume).
Another counterstrategy by females against infanticide
is to prevent or terminate pregnancy. Dewsbury (1982c)
points out the difference between pregnancy blockage, in
which implantation is prevented, and the Bruce effect, where
a successful fertilization is terminated by exposure to a new
male. This strategy also benefits males, because females that
terminate a pregnancy will re-ovulate sooner than if they
carried the young to term. The Bruce effect has been dem-
onstrated in the laboratory in many species of Muridae
(e.g., Schwagmeyer 1979; Mallory and Brooks 1980; Huck
1982), in which male pheromones apparently cause abor-
tion or resorption of embryos (Storey 1986b). The Bruce ef-
fect has even been reported in Alpine marmots, in which
pregnancies were terminated with the takeover of the terri-
tory by a new male (Hacklander and Arnold 1999). How-
ever, recent evidence from experimental field studies failed
to support any form of pregnancy disruption in gray-tailed
voles (Microtus canicaudus;de la Maza et al. 1999), and
only marginal or weak evidence exists in prairie voles (Ma-
hady and Wolff 2002). De la Maza et al. (1999) suggested
that multiple mating by females deters infanticide and thus
functionally negates the benefits of the Bruce effect. Both
field studies concluded that the Bruce effect might be an ar-
tifact of laboratory design and not an evolved reproductive
strategy, at least in murid rodents.
Male Mate Choice
A major assumption in current theories of mating strategies
is that males have an unlimited supply of sperm and that
they are selected to maximize its use. However, Dewsbury
(1982b, 1984) points out that there are physiological costs
to ejaculation. In rats, reproductive behavior is reduced for
a few days after satiation of mating, and prolonged copu-
lation depletes sperm from the seminal vesicles, which can
take 3 to 6 days to completely refill (Dewsbury 1984). Sperm
production is limited and males must optimize ejaculate ex-
penditure to achieve maximum reproductive payoff (Stock-
ley 1997). Patterns of mate order may influence which fe-
males are preferred as mates, how males compete for access
to those females, and how copulatory behaviors are used
to achieve fertilization success (Lacey et al. 1997). Mate
choice may be particularly important to males in species
in which there is a first male effect and females are willing
to re-mate. In thirteen-lined ground squirrels, males reject
females that have already mated, and their chances of ac-
tually fertilizing the female are so low they are better off
spending their energy to find new mates (Schwagmeyer and
Parker 1990). Similarly, Arctic ground squirrels were able
to distinguish between mated and unmated partners (Lacey
et al. 1997). Male mate choice should receive greater atten-
tion as a reproductive strategy as more information on mul-
tiple paternity and mate order effects become available.SummaryStrategies that maximize male reproductive success can
vary under different population densities and distributions.
Within the rodents, almost every type of mating system has
been described, ranging from monogamy to promiscuity.
Even within a species, variation in mating systems and mat-
ing strategies occur. Monogamy, although rare, appears to
have evolved when either parental care is critical to offspring
survival or when males are unable to access more than a
single female because of environmental factors (fig. 3.1).
Not only is the distribution of females or resources critical
to a male’s ability to access additional females, but actual
density of both males and females appears to have a large
influence. These variables (density and dispersion in space
and time) also affect whether males gain access to addi-
tional females through overt conflict (territories or domi-
nance hierarchies) or competitive searching (scramble com-
petition). Many of the rodents described as polygynous
routinely have females that multiply mate. Multiple mating
by females can confound the reproductive strategies of
males.
Copulatory success with females will not necessarily re-
flect fertilization success. Studies that examine whether a
species is genetically monogamous or polygynous have shed
light on the importance of sperm competition in male mat-40 Chapter Three