Males access the female using intense aggression directed
at other males and occasionally the female (Bakken 1959;
Thompson 1977; Tamura et al. 1989; Wauters et al. 1990;
Koprowski 1993a, 1993b); interaction rates are often exces-
sive (Farentinos 1972; Wauters et al. 1990; Steele and Kop-
rowski 2001). Dominance in tree squirrels is determined
by an interaction between age and body size, with older,
larger males dominating young, small males (Farentinos
1972; Pack et al. 1967; Benson 1980; Allen and Aspey 1986;
Wauters and Dhondt 1989; Wauters et al. 1990). Older or
larger males typically remain in proximity to the estrous fe-
male during a mating bout (fig. 7.2: Wauters et al. 1990;
Koprowski 1993a, 1993b).
The result of shifts that occur between interference and
scramble competition in each mating chase produce two
tactics: active pursuit and satellite (fig. 7.3), which ap-
pear to be adopted by male eastern gray squirrels (Bakken
1959; Koprowski 1993a), fox squirrels (Koprowski 1993b),
Abert’s squirrels (Farentinos 1972, 1980; Koprowski, pers.
observ.), Eurasian red squirrels (Wauters et al. 1990), Mexi-
can fox squirrels (S. nayaritensis), and red squirrels (Kop-
rowski, pers. observ.). Males that adopt the active pursuit
tactic attempt to sequester the female and copulate while
repelling other males. This tactic is adopted by 30.3% of
eastern gray squirrel males and 45% of fox squirrel males
(Koprowski 1993a, 1993b). Males that adopt the active
pursuit tactic are the most dominant and tend to be the
oldest members participating in mating chases (Koprowski
1993a, 1993b). In mating chases with few participants, all
males may use this tactic; it is the only one reported for
some low-density populations (Wauters et al. 1990; Kop-
rowski 1993a, 1993b, pers. obs.). The satellite tactic is
adopted by the remaining majority of males that tend to be
young and hold low dominance rank (Koprowski 1993a,
1993b). Satellite males remain in the vicinity of the estrous
female but avoid active pursuit males, often foraging, feed-
ing, or resting, and thus appearing to not be involved in the
mating chase (Koprowski 1993a, 1993b; Steele and Kop-
rowski 2001).
Why do low-ranking males remain in the vicinity of es-
trous females when they cannot compete with dominant
males that sequester the female? The entire fitness payoff
for satellite males results from the evasive behavior of fe-
males (Farentinos 1980; Koprowski 1993a, 1993b). Satel-
lite males are unable to compete successfully in interfer-
ence competition due to their low rank, and no copulations
have been observed by satellites under these conditions
(Koprowski 1993a, 1993b). When the female avoids dom-
inant males in active pursuit, however, satellite males are
able to copulate. As a group, satellites accrue 73.5% of the
copulations in eastern gray squirrels (Koprowski 1993a)
and 69.2% of the copulations in fox squirrels (Koprowski
1993b) following a breakaway, which is similar to the fre-
quency of males that adopt the satellite tactic in the popu-
lation. The tactics are equally likely to copulate first and
last in a mating chase (Koprowski 1993a). Success of satel-
lites (0.13 to 0.25 copulations /male /chase) is significantly
less than that of active pursuit males (0.32 to 0.83 copula-
tions /male /chase; Koprowski 1993a, 1993b). Measures of
variation also suggest that copulatory success is most vari-
able among satellite males (Koprowski 1993a, 1993b). As
a result of this differential reproductive success per male,
the benefits of actively competing for access to the estrous
female are clear.
The alternative reproductive tactics of males are not the
result of genetic polymorphism in the population. Males
switch tactics dependent on characteristics of attending
males in a mating chase (Koprowski 1993a, 1998), and in
chases with few males only the active pursuit tactic may be
used (Wauters et al. 1990; Koprowski, pers. observ.). Ad-
ditionally, tactics do not appear to be a mixed evolution-
ary stable strategy for the fitness payoffs are not equivalent.
Higher payoffs for active pursuit make this tactic profitable
to adopt when capable of success in interference com-
petition, while lower-ranking males “make the best of a
bad job” by adopting a satellite tactic and scrounging cop-
ulations based upon female behavior (Koprowski 1993a,
1993b; Wolff, in press).Sperm competition
Sperm competition also occurs; however, virtually nothing
is known about competition at this level for tree squirrels.
Copulatory plugs are common in rodents, including tree
squirrels (Hartung and Dewsbury 1978; Voss 1979; Baum-
gardner et al. 1982; Koprowski 1992). Plugs form by coag-90 Chapter Seven
Figure 7.2 Female fox squirrel (Sciurus niger) in estrus sequestered at the end
of a branch by the dominant male using the active pursuit tactic.