and Koprowski 2001) suggesting that attack rates on copu-
lating pairs might be high within nests. Predation risk is sat-
isfactory to account for most copulations to occur in trees
out of reach of many terrestrial predators. Thirteen-lined
ground squirrels and Belding’s ground squirrels (Spermo-
philus beldingi) are the only ground squirrels that mate pri-
marily aboveground in open grasslands (Sherman and Mor-
ton 1979; Schwagmeyer and Foltz 1990), yet copulations
are prolonged.
Another influence on copulatory time likely is the level
of male-male competition and vulnerability of a copulating
pair to attack by males. Competition for females is greatest
in members of Sciurusand Tamiascharacterized by domi-
nance polygyny or promiscuity, in which numerous males
accumulate around an estrous female; copulatory times are
shortest in these species. Tamiasciurusare intermediate,
with highly variable copulatory times — perhaps because
competition is limited by spatial attachment of territorial
males, which decreases competition (Koford 1982). Pro-
longed intromission characterizes ground-dwelling squir-
rels with lower levels of competition around estrous fe-
males and spatial attachment of males to females (Marmota
olympus, M. flaviventris), low adult densities and scramble
competition (M. monax, S. tridecemlineatus), and even
queuing conventions (S. tridecemlineatus). The short and
less variable copulatory times of tree squirrels relative to
ground squirrels support trends in mating systems, vulner-
ability, and intermale competition.
Intermale competition is great in tree squirrels, yet time
to copulate is remarkably short compared to many mam-
mals. All tree squirrels described use a nonlocking pattern
of copulation. The locked pattern of copulation, in which
the mechanical connection between penis and vagina holds
the copulating pair together (Dewsbury 1972), likely is
adaptive in species where sites for copulation are safe from
predators or conspecific competitors (Dewsbury 1975);
however, a copulating pair locked together may be at con-
siderable risk if predator pressure and /or conspecific com-
petition are intense (Langtimm and Dewsbury 1991). Non-
locking during copulation may allow increased vigilance
for predators or conspecifics and permit resumption of the
copulatory sequence after disruption (Langtimm and Dews-
bury 1991).
If relative safety of copulatory sites from disruption in-
fluences reproductive success of males and females, then
duration of copulation may also be under selective pressure.
Risk of disturbance may be minimized by loss of a locked
copulatory pattern (Dewsbury 1975) but may decrease fur-
ther by minimizing time necessary for successful copula-
tion. Conversely, fitness-related benefits of prolonged copu-
lations to male rodents include increased paternity (Lanier
et al. 1979; Dewsbury and Hartung 1980; Dewsbury and
Baumgardner 1981; Oglesby et al. 1981; Schwagmeyer and
Foltz 1990). Reasons for increased reproductive success
with prolonged copulations may be due to increased num-
bers of sperm, increased time guarding the female from
competitors, and decreased receptivity of the female to sub-
sequent males (Dewsbury 1984).
Among the ground squirrels, S. beldingiare character-
ized by a dominance-lek based mating system, where levels
of competition in proximity of the female are the greatest
reported among ground squirrels. Nearly 50% of copula-
tion attempts are interrupted by other males, but copulatory
duration is prolonged (10 to 12 min) when uninterrupted
(Sherman and Morton 1979, 1984). Evidence suggests that
male ground squirrels can fertilize females during copula-
tions considerably short of maximum duration (Schwag-
meyer and Foltz 1990). The great variance in ground squir-
rel copulatory times suggests the plasticity of this behavior.
Males may attempt to maximize duration of copulation to
accrue fitness benefits through increased paternity (Schwag-
meyer and Foltz 1990) as a trade-off to possibility of attack.
The extremely localized and high levels of overt aggression
in tree squirrels such as Sciurusare not conducive to pro-
viding the option of prolonged copulations, as in ground
squirrels.
The spatial attachment of Tamiasciurusmales to terri-
tories precludes the higher levels of overt competition char-
acteristic of Sciurusspecies (Koford 1982), and a queuing
convention may also exist (Arbetan 1993). This reduced
level of competition may result in the prolonged and highly
variable copulations in this genus.
Tree squirrels usually copulate high in the canopy; cop-
ulating pairs are attacked in nearly 40% of copulation at-
tempts, and are frequently thrown to the ground (Koprow-
ski 1993a, 1993b). The intensity and frequency of attack,
as well as the vulnerable location of copulations, may have
led to selection for and /or maintenance of short copula-
tions, forgoing potential benefits of prolonged copulation.
Low variance and range in copulation times of Sciurusspe-
cies point to some form of stabilizing selection. Chipmunks
(Tamias) have a similar mating system to tree squirrels,
but intermediate copulatory times, potentially due to the
chipmunks’ habit of mating on the ground. The great
intraspecific variability in copulatory times of the ground
squirrels suggests that strong stabilizing selection has not
occurred.
These results suggest that the constraints of predation
risk, male-male competition, and the female’s risk of injury
may present trade-offs to the benefits of prolonged copula-
tion. Analyses of ground squirrels under varying levels of
risk and competition likely will provide insight into the94 Chapter Seven