competition where different male traits may be favored
than in a contest competition.
Group nesting occurs in many species (Koprowski 1996,
1998) but does not appear unequivocally related to mating
strategies (Koprowski 1998). Mixed-sex nesting groups,
however, are most common during the winter breeding sea-
son in eastern gray squirrels (Koprowski 1996), fox squir-
rels (Adams 1976; Koprowski 1996), and Abert’s squirrels
(Edelman 2004). No studies have examined the relation-
ship between frequency of nest sharing and mate choice or
copulatory success; thus, the adaptive significance of mixed
sex nesting groups is not yet known.
Male tactics
The spatial and temporal dispersion of estrous females pro-
vides challenges to males. The overall and instantaneous
(i.e., immediate: Dearborn et al. 2001) operational sex
ratios are heavily male biased, suggesting that intrasexual
competition among males is high (Steele and Koprowski
2001). Indices of sexual selection reflect this intensity; vari-
ation among males in reproductive success is considerable,
with a strong positive skew (Koprowski 1993a, 1993b).
Distribution of copulations in tree squirrels demonstrates
a strong positive skew, with a majority of copulations gar-
nered by one or two males (Farentinos 1972; Tamura et al.
1988; Wauters et al. 1990; Koprowski 1993a, 1993b). The
causes of this inequitable distribution of copulatory success
in males are the multiple types of competition experienced
by males due to female spatiotemporal dispersion and tac-
tics during mating chases. Success in scramble, interference,
and sperm competition likely is important in determining
the reproductive success of male tree squirrels.
Scramble competition
An intense scramble competition occurs as males attempt
to locate females at two different spatiotemporal scales. Re-
ceptive females must be located in the greater landscape
during the breeding season if males are to increase mating
opportunities. Males are attracted to estrous females from
distances of nearly 1 km, and monitor females beginning at
least 5 days before estrus (Thompson 1977; Steele and Kop-
rowski 2001). Large home ranges of males relative to fe-
males (by a factor of 1.2 to 2.0: Koprowski 1998), despite
a minimal or lack of sexual dimorphism (5%: Nixon
et al. 1991; Boutin and Larsen 1993), can also be inter-
preted as a tactic of males to track female distribution and
receptivity (Koprowski 1998). Males increase home ranges
in response to decreases in female density (Kenward 1985).
Home ranges of males expand during the breeding sea-
son and account for the significantly larger home ranges of
adult males than those of females (reviewed in Koprowski
1998). Male eastern gray squirrels visited 5.8 1.2 SD fe-
males during the 3 h immediately after emergence from
their nests (n14: Steele and Koprowski 2001). Olfactory
cues are of great import to males in the location of an es-
trous female (Thompson 1977; Gurnell 1987; Koprowski
1991; Steele and Koprowski 2001). Males seem to have par-
ticularly well-developed olfactory skills, and regularly visit
scent mark sites (Koprowski 1993d) and odoriferous fruit-
ing trees (L. N. Brown 1986) relative to females. Success in
this geographic scale scramble competition increases poten-
tial opportunities to mate by increasing the number of mat-
ing chases in which a male is involved.
Other behaviors are also suggested to be reproductive
tactics of males and serve to influence availability of recep-
tive females. Tree squirrels are known to cannibalize conspe-
cifics, including female (Thompson 1976) and male (Holm
1976) eastern gray squirrels, male fox squirrels (Packard
1956), and male bush squirrels (Paraxerus cepapi: de Vil-
liers 1986). Only de Villiers (1986) and Holm (1976) report
the actual killing of young squirrels or suggest infanticide.
The rarity of observation of infanticide in the often well-
concealed arboreal nests of tree squirrels makes further in-
ference difficult; however, the ability of tree squirrels to
enter estrus again in most years (Heaney 1984; Steele and
Koprowski 2001) promotes speculation. Infanticide is sug-
gested to be a male reproductive tactic, but difficulty in doc-
umenting this phenomenon obfuscates conclusions (Viljoen
1977, de Villiers 1986, but see Weissenbacher 1987). The
role of infanticide as a reproductive tactic is discussed fur-
ther in Ebensperger and Blumstein, chap. 23, and Hoog-
land, chap. 37, this volume).
Additionally, on the day of estrus, males must locate the
female as she changes location among males within her
home range. This scramble competition is important in de-
termining immediate access to estrous females. Males that
locate females most quickly have a high likelihood of cop-
ulating (Koprowski 1993a, 1993b, 1993d). The frequent
evasive behavior of females during the mating chase pro-
vides a fitness payoff to males not equipped to succeed in
interference competition, for 69.4% and 54.2% of copu-
lations in eastern gray squirrels and fox squirrels, respec-
tively, occur following the breakaway of a female (Koprow-
ski 1993a, 1993b).Interference competition
Interference competition is also important to obtain access
to a female in estrus. The number of male participants in
mating chases ranges between 2 and 34 individuals chas-
ing a single female (Goodrum 1961; Farentinos 1972; Wau-
ters et al. 1990; Arbetan 1993; Koprowski 1993a, 1993b).Alternative Reproductive Tactics and Strategies of Tree Squirrels 89