Using the OSR as a predictor of mating systems by look-
ing at its effect on only one mating strategy (overt conflict)
may be too simplistic. Other factors may influence male
and female mating strategies. As most species of ground-
dwelling squirrels are promiscuous (table 3.1), increased in-
tensity of sexual selection among males may be apparent in
other more subtle forms of sexual selection besides overt
competition (see the following).
Defense polygyny
In defense polygyny, dominance is usually site-related
(Schwagmeyer 1990), where males win fights in their own
area and lose outside of this area (see also Wolff et al. 1983
for Peromyscus). In yellow-bellied marmots, resident males
have never been observed to lose an encounter with invad-
ing males that attempt to take over their territory (Armitage
1998). Similarly, in white-footed mice and deer mice resi-
dents, resident males win the majority of encounters with
intruding males (Wolff et al. 1983).
Severe wounding during territorial battles has been de-
scribed in a number of species in which territorial males
directly defend resident females by overt conflict. In Cali-
fornia ground squirrels (Spermophilus beecheyi), 93% of
males were wounded during the mating season (Boellstorff
et al. 1994), and severe wounding has also been reported
for Columbian ground squirrels (S. columbianus;Steiner
1970) and Richardson’s ground squirrels (S. richardsonii;
Michener 1983a). However, less costly means of exclud-
ing other males from a territory are also used, including
scent marking, patrolling, and parallel running (Schwag-
meyer 1990; Roberts, chap. 22, and Busher, chap. 24, this
volume).
Non-defense polygyny
In non-defense polygyny there is no site-specific dominance.
Males usually roam over wide areas and access to females
(or at least first access) is determined by who wins encoun-
ters. There are two important forms of pre-copulatory
male-male competition in non-defense polygyny: establish-
ing dominance hierarchies (through overt conflict or dis-
placements) and competitive searching.
With dominance hierarchies, more dominant males
should have greater reproductive success. Dominance is cor-
related with reproductive success in Belding’s ground squir-
rels (Spermophilus beldingi;Sherman 1976 ) and round-
tailed ground squirrels (S. tereticaudus;Dunford 1977b),
where larger males have an advantage. Even in species in
which size does not affect dominance status, more dominant
individuals have greater reproductive success, as in Cape
ground squirrels (Xerus inauris;Waterman 1998; fig. 3.3)
and bushy-tailed woodrats (Neotoma cinerea;Topping and
Millar 1999).
Most tree squirrels, which breed very asynchronously
within a local population, also form dominance hierar-
chies to determine access to receptive females (Koprowski,
chap. 7, this volume). Males in these species gather near
an estrous female and determine dominance relationships
through highly agonistic interactions, but do not maintain
their aggregations beyond the estrous period (e.g., tassle-
eared tree squirrel, Sciurus aberti,Farentinos 1972; Eur-
asian red squirrel, S. vulgaris,Wauters et al. 1990; eastern
gray squirrel, S. carolinensis,Koprowski 1993a). In eastern
gray squirrels the female can be pursued by 4 to 8 males in
a single mating bout, and adult male gray and fox squir-
rels (S. niger) aggressively compete for copulations during
mating bouts (Thompson 1977; Koprowski 1993a,b). How-
ever, there appear to be two alternative reproductive strat-
egies in gray squirrels that are condition dependent (Ko-
prowski 1993a,b). The males participating in the activeMale Mating Strategies in Rodents 35Figure 3.3 Displacement interactions between two male Cape ground squir-
rels involves nose-nose recognition and assessment but no overt conflict (top).
Males use these displacements to establish a linear dominance hierarchy within
their all-male social groups (bottom). Photos by J. Waterman.