the majority of offspring are sired by the first male to copu-
late), a last male advantage, and no mate order advantage
(Foltz and Schwagmeyer 1989). For example, first male ef-
fects have been found in thirteen-lined ground squirrels
(Foltz and Schwagmeyer 1989), Belding’s ground squirrels
(Hanken and Sherman 1981), and in Arctic ground squir-
rels (Lacey et al. 1997). In thirteen-lined ground squirrels
there is a 75% chance of the first male siring the offspring,
while in Arctic ground squirrels first males sire 90% of the
litter (Foltz and Schwagmeyer 1989; Lacey et al. 1997).
This precedence for mating (first, last) will have large im-
pacts on the mating strategies of males. Mate-order effects
can influence male searching strategies (whether to stay and
copulate with a female or continue searching), the number
and duration of copulations, and if the male should try to
guard the female after copulation. Belding’s ground squir-
rels, which have a first male advantage, resume searching
for females as soon as copulation is completed (Sherman
1989). But if there is an extreme bias toward first males fer-
tilizing the offspring, as in Arctic ground squirrels, subse-
quent males should not waste their time and energy on
these females, with which they are unlikely to sire offspring
(Lacey et al. 1997). If second or third mates are likely to sire
offspring, then males should try to mate with any females
they can. If there is a last male advantage, other mecha-
nisms like mate guarding may be used to reduce sperm com-
petition (Sherman 1989).
Precopulatory strategies
The most obvious precopulatory mechanism by which males
can monopolize females is via overt conflict. This conflict
can be through territoriality or dominance (as discussed
earlier), or males can also attempt to prevent rival access to
mates by mate guarding. Mate guarding is not predicted to
occur if there is a first male advantage. For instance, mate
guarding does not occur in Belding’s ground squirrels, in
which there is a clear first male advantage, but does oc-
cur in the Idaho ground squirrel (Spermophilus brunneus),
which has a last male advantage (Sherman 1989). Mate
guarding can be used by males to deter females from re-
mating or to force females to accept them as a mate (Stock-
ley 1997). This strategy can conflict with female reproduc-
tive strategies if harassment affects foraging efficiency or
prevents females from mating with preferred or alterna-
tive males (Stockley 1997; Wolff and Macdonald 2004).
However, females can occasionally escape guarding males,
as seen in gray squirrels (Koprowski 1993a), undermin-
ing male attempts to monopolize the estrus. Mate guarding
also occurs in California ground squirrels, but females were
still able to successfully re-mate (Boellstorff et al. 1994).
Mate guarding can also have the disadvantage of reducing
male opportunities to resume mate searching.
Another means by which a male can reduce the chances
of re-mating by a female is by depositing a copulatory plug.
This method has the advantage to the male that he can leave
and search for new mates. Copulatory plugs have been de-
scribed in many species of murids (Hartung and Dewsbury
1978) and sciurids (Murie and McLean 1980; Koprowski
1992); however, they are not always effective. In rats (Rat-
tus norvegicus), 69% of copulatory plugs were dislodged
with subsequent intromissions (Dewsbury 1984). In Co-
lumbian ground squirrels, 16 of 34 females had plugs and
7 were very loose (Murie and McLean 1980). Female fox
squirrels and eastern gray squirrels were able to remove
plugs within 30 seconds of the end of copulation, suggest-
ing that plugs can be ineffective at restricting female re-
mating opportunities (Koprowski 1992).
Thus the effectiveness of precopulatory mechanisms de-
pends, in part, on the benefits of multiple mating to females,
which will influence their incentives to seek out other mates
(Wolff and Macdonald 2004; Solomon and Keane, chap. 4,
Koprowski, chap. 7, this volume).Postcopulatory strategies
If females are able to re-mate, males may pursue postcopu-
latory strategies. Males that fail to prevent rivals from ini-
tiating copulation with their mate can prevent fertilization
by disrupting copulations. If a male gray squirrel detects
a copulating pair he will attack, bite, and sometimes knock
them from the tree (Koprowski 1993a). In the Cape ground
squirrel, dominant males usually copulate with the female
underground, where disruptions are rare (Waterman 1998).
Disruptions can also occur within the female’s reproductive
tract, where ejaculate from one male could disrupt the
sperm transport of a rival (Dewsbury et al. 1992). Ejaculate
disruption can also interfere with males’ own sperm trans-
port if they re-copulate too soon, which may give incentive
for males to use mate guarding or copulatory plugs to delay
female re-mating.
Males may compete with rival males by altering sperm
investment. Males may influence who fertilizes the ova
through sperm quality or increased sperm quantity. Sperm
morphology in rodents is extremely variable, in both size
and length (Roldan et al. 1992). Species in which females
mate with multiple males have longer sperm than in non-
multiple mating species, and sperm length is positively
correlated with maximum sperm velocity (Gomendio and
Roldan 1991). Other factors that could influence fertiliza-
tion success include sperm viability, sperm concentration,
or ejaculate volume (Dewsbury 1984). Fertilization success38 Chapter Three