ulation of proteins in semen (Mossman et al. 1955; Mann
and Lutwak-Mann 1981; Koprowski 1992). Voss (1979)
suggested five potential functions for copulatory plugs: stor-
age of sperm, prevention of sperm leakage, induction of
pseudopregnancy, facilitation of sperm transport across
the cervix, and prevention of subsequent intromissions, or
“chastity enforcement.” Copulatory plugs of tree squirrels
consist of an opaque, white, rubbery acellular core; a few
spermatozoa and a thin layer of epithelial cells are often
found on the external surfaces after removal from the fe-
male (Koprowski 1992). Available evidence suggests that
the chastity-enforcement hypothesis is the most likely expla-
nation for rodent copulatory plugs (Voss 1979; Koprow-
ski 1992). Additionally, males guard females for 80 min
following ejaculation in several species of tree squirrels, in-
cluding fox squirrels (McCloskey and Shaw 1977; Koprow-
ski 1993a), eastern gray squirrels (1993b), Abert’s squirrels
(Farentinos 1972), Eurasian red squirrels (Ognev 1940;
Raspopov and Isakov 1980; Wauters et al. 1990), and red
squirrels (Arbetan 1993). Unfortunately, data of sufficient
detail are not available to examine the relationship between
duration of guarding with measures of fitness. These tac-
tics function to provide time that may convey advantage
in sperm competition. The aggregation of the sperm of fox
squirrels and flying squirrels into rouleaux (Martan et al.
1970; Martan and Hruban 1970) may also be important
in sperm competition. Unfortunately, there is a dearth of
knowledge about sperm competition in the tree squirrels,
and order of mating effects are unknown for tree squir-
rels. The vigorous guarding of females after mating (Faren-
tinos 1972; McCloskey and Shaw 1977; Thompson 1977;
Tamura et al. 1988; Wauters et al. 1990; Koprowski 1993a,
1993b) and deposition of copulatory plugs (Koprowski
1992) suggest a last male advantage for most tree squirrels
(Koprowski 1992), as reported for Idaho ground squirrels
(Spermophilus brunneus), in which males tenaciously guard
females after copulation (Sherman 1989).
The reproductive success of males is essentially the
summation of success in these three types of competition
(fig. 7.4). Older dominant, active pursuit males accrue
more copulations per capita than subordinates, suggesting
the relative advantage of this tactic. Young, subordinate an-
imals that adopt the satellite tactic are less successful over
the course of a breeding season, but are more successful
than predicted by their low rank. The costs of extensive
searching for dispersed females, monitoring of female re-
productive condition, location of estrous females, intensive
intermale physical contests and avoidance of these contests,
and postcopulatory guarding, production of proteinaceous
copulatory plugs, and sperm competition appear extensive.Alternative Reproductive Tactics and Strategies of Tree Squirrels 91Figure 7.3 Alternative reproductive tactics of male fox squirrels (Sciurus niger). The estrous female is located inside a cavity at the base of the
tree to the left. Active pursuit males are found at the base of the tree. Satellite males are foraging in the grass to the right.