Most males lose significant weight during the breeding sea-
son, wounding is common, and mortality related to wound-
ing and weight loss is known to occur (Steele and Koprow-
ski 2001).
Many tactics used by males and females are to the detri-
ment of individuals of the opposite sex, a classic conflict be-
tween the sexes (Koprowski 1998). A common element of
mating chases where male and female strategies are coinci-
dent is in the minimization of the time required for copu-
lation. Evidence suggests that the location and duration of
copulation expose the copulating pair to increased risk of
injury or predation (Koprowski 1992, 1993a, 1993b). Ad-
vantage is likely gained by both sexes if copulatory times
are decreased without a concomitant decrease in effective-
ness of copulations.
Why Do Tree Squirrels Copulate So Quickly?
To compile data on copulatory duration in tree and ground
squirrels, I used observations of uninterrupted intromissions
that include pelvic thrusting, the common behavior pat-
tern during mating in squirrels. Squirrels do not appear to
lock (classes 9 to 12 of Dewsbury 1972) during copulation
(Denniston 1957; Schwagmeyer and Parker 1987; Koprow-
ski 1993a, 1993b), and most form copulatory plugs (re-
viewed by Koprowski 1992). If the observer’s descriptions
were of sufficient detail, I used time from initial pelvic thrust
to cessation and dismount to delineate duration of a single
intromission. Where possible, I report duration of intromis-
sions in natural settings, but I was limited occasionally to
observations of confined pairs (table 7.1). Although many
reproductive parameters may differ drastically between cap-
tive and wild populations, the few comparative data avail-
able for squirrels suggest that copulatory time is conserva-
tive (Davis 1982a; Blake and Gillet 1988; Schwagmeyer
and Foltz 1990). Copulation durations of tree squirrels are
nearly always shorter than those of ground squirrels, and
are generally 1 min (table 7.1).
To examine plasticity of copulatory time in tree squirrels,
I also used data for fox squirrels and eastern gray squirrels
collected on populations in Lawrence, Kansas, and Salem,
Oregon, using the methods of Koprowski (1993a, 1993b).
I recorded the duration of copulations and measures of
safety (height aboveground) and competition (number of
males in the mating bout, competitors present or absent)
through each mating bout. If plasticity exists in copulatory
duration of fox squirrels and eastern gray squirrels, then
copulatory times would be longer when copulations were
in safe locations (near to ground) and where mate competi-
tion was relaxed (low density of competitors, copulations in
seclusion).
Duration of copulation did not vary with levels of com-
petition. Copulatory duration is not correlated (r0.02,
df57, P0.50) with the number of S. carolinensismales
in the mating bout. Copulations last equally long when the
female avoids pursuing males and copulates in seclusion
with a male that has relocated her and when copulation oc-
curs with several males nearby (S. niger: 18.7 1.1 sec and
18.4 1.0 sec, t0.26, df 14, P0.79; S. carolinen-
sis: 22.1 0.9 sec and 21.8 0.9 sec, t0.40, df 37,
P0.69, respectively). Duration of a copulation also was
not correlated with height aboveground (S. niger: r0.05,
df 18, P0.50; S. carolinensis: r0.16, df 52,
0.25). At least in these two species, little plasticity exists
in copulatory duration.
Among the squirrels (Mammalia: Sciuridae), some spe-
cies have short copulatory times (30 sec, Koprowski
1993a, 1993b) while others copulate for much longer peri-
ods (1 to 33.5 min, Schwagmeyer and Foltz 1990). Kop-
rowski (1993a) suggested that the short copulatory times of
eastern gray squirrels may be advantageous and related to
intense overt aggression and vulnerable location of copu-
lations. Females of several arboreal or semi-arboreal squir-
rels appear to prefer to mate in seclusion (Yahner 1978a;92 Chapter Seven
Figure 7.4 Tactics used by male tree squirrels during mating bouts with
changes in competition highlighted. Gray shading indicates scramble competi-
tion, boxes indicate interference competition, and black shading indicates sperm
competition.