cally live in female groups and associate with one or more
males (Nutt 2003). Unimale groups were the most com-
mon association, and there was no genetic evidence of mul-
tiple paternity, even though some females did appear to
mate outside of their social group. In bushy-tailed wood-
rats, males and females occupy large, overlapping home
ranges during the breeding season, and genetic data suggest
that males gain exclusive access to females during estrus
with no multiple paternity within litters (Topping and Mil-
lar 1999).
One problem with using multiple paternity to document
multiple mating by females is that since copulations do not
always accomplish fertilization, paternity examinations nec-
essarily underestimate the occurrence of multiple mating.
Furthermore, in some species of rodents, multiple-mated
females can have a lower probability of pregnancy (Dews-
bury 1982c; Wolff 1989). In addition, older paternity stud-
ies underestimated the amount of multiple mating by fe-
males because of the low probability of detecting multiple
paternity (Wolff 1989). However, current methods allow
more resolution of genetic relationships with a greater de-
gree of confidence (Travis et al. 1996).
Promiscuity
Promiscuity, in which both sexes mate with multiple part-
ners, is common in the small rodents, as the frequency of
multiple paternity of litters demonstrates (table 3.1). In a
moderately dense population of white-footed mice, up to
30% of litters were sired by multiple males (Wolff 1989) and
multiple paternity has been well-documented in deer mice
(Birdsall and Nash 1973; Merritt and Wu 1975; Ribble and
Millar 1996). Multiple paternity of litters was common in
one population of meadow voles (Microtus pennsylvani-
cus), in which males were nonterritorial with no fixed home
range (Boonstra et al. 1993). Low variability in male com-
petitive ability (cohorts are similar because of their short
life span and seasonality in breeding) and habitat that re-
duced the ability of males to detect rivals made it more
difficult for males to monopolize females (Boonstra et al.
1993).
Most of the diurnal ground squirrels described previ-
ously as polygynous routinely have females that multiply
mate. However, direct observations of their mating behav-
iors historically focused on male overt conflict, ignoring fe-
male behavior. Research over the past two decades has re-
vealed that the majority of sciurid rodents are promiscuous,
with both sexes pursuing strategies that often conflict with
each other. Extra-pair copulations have been documented
in the monogamous Alpine marmot (Marmota marmota;
Goossens et al. 1998). Travis et al. (1996) found that in ter-
ritorial Gunnison’s prairie dogs there was a very high oc-
currence of multiple paternity of litters; 61% of offspring
were sired by extra-territory males. Travis et al. (1996) sug-
gest that the mating system be reclassified from female de-
fense polygyny to overlap promiscuity, as Boellstorff et al.
(1994) suggested for California ground squirrels, where in-
dividual females mated with an average of six males. Simi-
larly, in Columbian ground squirrels, once classified as fe-
male defense polygyny, females mate on average with four
males, and a minimum of 16% of litters are multiply sired
(Murie 1995). In Belding’s ground squirrels, 78% of litters
were multiply sired (Hanken and Sherman 1981), support-
ing the notion that polygyny is not an appropriate descrip-
tion of the mating system. Multiple mating and multiply
sired litters have also been reported in the purportedly mo-
nogamous prairie voles (Solomon et al. 2004).Effects of Sperm Competition on Male StrategiesWhenever females mate with more than one male, intra-
sexual competition can occur in the reproductive tract of
the females. Sperm competition describes competition be-
tween the sperm of more than a single male for fertilization
of ova within the reproductive tract of the female (Parker
1970). This sort of competition between males may be less
obvious to observers, but may have profound implications
for male reproductive tactics.
The degree to which sperm competition can affect the
mating strategies of males depends on the effectiveness of
strategies to prevent fertilization by other males. Parker
(1970) identified two major male strategies when dealing
with sperm competition: first, those that prevent rival males
from copulating with a female (precopulatory) and second,
those that increase a male’s fertilization chances when deal-
ing with a previously mated female (postcopulatory).
Countering sperm competition may reduce the oppor-
tunities to gain access to additional mates, but if the likeli-
hood of finding another mate is low, staying and guarding
a current female has a higher payoff. Another considera-
tion is the cost of the time /energy investment of preventing
subsequent re-mating, and how effective these strategies are
(Parker 1970; Dewsbury 1982b; Sherman 1989; Schwag-
meyer 1990). These considerations will be influenced by the
current mate’s opportunity to re-mate, which is affected by
the time it takes for other males to find the female or the
length of time between copulation and fertilization.
The influence of sperm competition on male mating
strategies is also influenced by sperm precedence or mate
order (nonrandom differential fertilization success; Lewis
and Austad 1994). There are three possible mate order ef-
fects: a first male advantage or bias in fertilization (in whichMale Mating Strategies in Rodents 37