species (Steele and Koprowski 2001) relative to sociality of
the ground-dwelling squirrels (Armitage 1981). A contin-
uum of space-use patterns exists in the tree squirrels (Kop-
rowski 1998). Many species of tree squirrels in the genera
Sciurus, Glaucomys,and Callosciurusdo not exhibit terri-
torial defense by either sex and thus have overlapping home
ranges (Gurnell 1987). Females in some species defend small
exclusive core areas around nests (Havera and Nixon 1978;
Tamura et al. 1989; Koprowski 1998) and Tamiasciurus
are predominantly territorial (Smith 1968; Steele and Kop-
rowski 2001). As a result, females are often dispersed in
space. Adults of nearly all tree squirrels are known to nest
in single-sex and mixed-sex groups, particularly but not ex-
clusively in winter (Layne and Raymond 1994; Koprowski
1998; Layne 1998). In eastern gray squirrels (Sciurus caro-
linensis), nesting groups often consist of two adult females
and their offspring (Koprowski 1996; Steele and Koprowski
2001). Visual cues, vocalizations, and scent marks appear
important components of communication in social systems
of tree squirrels (reviewed by Gurnell 1987; Steele and Kop-
rowski 2001). In eastern gray squirrels, the most social spe-
cies of tree squirrel known, natal philopatry of daughters
results in formation of kin clusters of related females (Kop-
rowski 1996). Philopatry of both males and females is un-
common in red squirrels (Tamiasciurus hudsonicus: Larsen
and Boutin 1998); indeed, natal dispersal of offspring is
typically the rule for the few species of tree squirrels stud-
ied to date (Koprowski 1998). Retention of kin in natal ar-
eas leads to formation of the fundamental social unit in
ground-dwelling squirrel societies, the female-female dyad,
and appears related to body size energetics (Armitage 1981,
1999a). I suggest elsewhere that the evolution of sociality in
tree squirrels may involve a similar relationship, with body
size energetics favoring the overwinter retention of females
in their natal area (Koprowski 1998).
A General Review of Tree Squirrel
Reproductive Biology: Setting the Stage
Tree squirrels typically do not breed until 1 year of age
(Gurnell 1987; Koprowski 1994a, 1994b); however, in ex-
tremely good years of food availability, precocious breeding
at ages as young as 5.5 months has been reported (Smith and
Barkalow 1967; Koprowski 1994a, 1994b). Reproductive
life is at least 8.0 years and possibly 12.0 years (Barkalow
and Soots 1975; Koprowski et al. 1988). Most tree squir-
rels are spontaneous ovulators (Layne 1954; Millar 1970;
Gurnell 1987). Females are typically in estrus for 1 day
and likely 8 h (Smith 1968; Dolan and Carter 1977; Ko-
ford 1982; Wells-Gosling and Heaney 1984; Koprowski
1993a, 1993b) during breeding seasons that extend over a
4 to 12 week period (Gurnell 1987; Steele and Koprowski
2001). As a result, estrous females are dispersed in both
time and space. Females may attract males for several days
prior to estrus (Thompson 1977). On the morning of a fe-
male’s day of estrus, males congregate outside of the nest.
Aggregations of males pursue the female throughout her
day of estrus, usually with intense intrasexual and intersex-
ual aggression (Farentinos 1972; Gurnell 1987; Koprowski
1993a, 1993b, 1998; Steele and Koprowski 2001). Most
species of Sciurus, Tamiasciurus,and Glaucomysonly pro-
duce a single litter each year after a 30 – 45 day gestation;
however, in many species a second litter can be produced
(Heaney 1984; Gurnell 1987). Some Sciurusfrom the neo-
tropics, such as S. granatensis,can produce three litters
per year (Heaney 1984). Modal litter size is 2 to 4 young
(Heaney 1984; Gurnell 1987). No paternal care is known
for any tree squirrels.Female tactics
Tactics of females during a single day of estrus have received
less attention than those of males; however, a number of be-
haviors executed by females are suggested to be reproduc-
tive tactics. A detailed analysis of female behavior during
mating bouts suggests that females use several tactics to in-
fluence the type and outcome of competition among males
(Koprowski 1998).Evasive behavior
During mating chases, males aggregate around the female
early on the morning of her day of estrus (Bakken 1959;
Koprowski 1993a, 1993b). Aggression amongst males is
intense and often exceeds 2,500 interactions /hour (Faren-
tinos 1972; Koprowski 1993a). Females avoid this intense
aggression and break away from males distracted by inter-
male interactions (Koprowski 1993a, 1993b). In eastern
gray squirrels and fox squirrels (Koprowski 1993a, 1993b),
the majority of copulations follow a breakaway (69.4%
and 54.2% of all copulations in these species, respectively).
The remainder of copulations occurs while the female is se-
questered by a dominant male. Thus a majority of copula-
tions follow evasive tactics, and suggest that factors other
than male dominance are necessary for male success.Selection of locations for mating
The location of matings does not appear to be random in
eastern gray squirrels (Koprowski 1993a) or fox squirrels
(Koprowski 1993b). Copulations following evasion of males
occur in sites closer to the ground than those when males
sequester females at the end of branches (30.7% of theAlternative Reproductive Tactics and Strategies of Tree Squirrels 87