13.7 m height for sequestered copulations inS. niger,55.4% Female in estrus
of the 11.2 m mean height for sequestered copulations in S.
carolinensis;Koprowski 1993a, 1993b). In addition, prob-
ability of being attacked by males while copulating de-
creased from 0.375 to 0.245 following a breakaway by
females in eastern gray squirrels but not fox squirrels (Kop-
rowski 1993a, 1993b). Females appear to actively seek lo-
cations that minimize risk of injury due to aggressive inter-
actions by other males and falls from precarious mating
sites (Koprowski 1998).
Copulatory plug removal
Immediately following copulation, females groom their gen-
italia (Koprowski 1993a, 1993b) and frequently remove
copulatory plugs that protrude from beyond the vulva
(72.6% of 22 copulations in S. niger,50.0% of 26 copula-
tions inS. carolinensis,Koprowski 1992). Copulatory plugs
are consumed by female eastern gray squirrels (61.5% of
13 plugs) and fox squirrels (75.0% of 16 plugs) or are dis-
carded (Koprowski 1992).
Multi-male mating
Females of all species of tree squirrels for which reproduc-
tive behavior has been studied are known to copulate on
multiple occasions during a mating chase, with the vast ma-
jority of copulations occuring with different males (2 to
8 males: Farentinos 1972; Koford 1982; Tamura et al. 1988;
Wauters et al. 1990; Arbetan 1993; Koprowski 1993a,
1993b; Koprowski 1998). Multiple paternity of litters has
been confirmed in red squirrels and eastern gray squirrels
(Arbetan 1993; David-Gray et al. 1998, 1999).
Solicitation of copulations
Female choice is a component of tree squirrel mating tac-
tics. Female Abert’s squirrels appear to actively avoid dom-
inant males that compete for access to females, and instead
may solicit subordinate males (Farentinos 1980). Evasive
behavior by females through breakaways, and thus avoid-
ance of dominant males, suggests that females (Koprowski
1993a, 1993b, 1993c) may be choosing to mate with males
other than those most dominant, in addition to preferring
sites with a decreased risk of injury (Steele and Koprow-
ski 2001).
Each female tactic appears to be to the detriment of in-
dividual males participating in the mating bout; however,
it likely provides advantage to estrous females (fig. 7.1). Se-
lection of safe locations for copulations decreases risk of
injury to a female that is just starting her reproductive in-
vestment (Koprowski 1993a, 1993b, 1998). Removal of
copulatory plugs and multi-male mating reduces the repro-
ductive success of individual males. Estrous females may
benefit from multiple mating in several ways, including as-
surance of fertilization, increased uncertainty of paternity,
increased nutrition from consumption of copulatory plugs,
and increased genetic variation in offspring (Koprowski
1992, 1998). At least 9 different hypotheses were proposed
to explain multi-male mating (Wolff and Macdonald 2004).
If assurance of fertilization or increased nutrition were the
cause of multiple mating by females, one might expect fe-
males to mate with the same male frequently during mating
bouts, for these benefits could be accrued simply by increas-
ing number and duration of copulations. Because female
tree squirrels rarely mate repeatedly with a male and multi-
male mating is the rule, hypotheses of increased genetic
variation in offspring and /or masking of paternity to de-
ter infanticide cannot be refuted. Infanticide is reported in
tree squirrels but the frequency of this behavior is not well
known. Among some mammals, multi-male mating ap-
pears to be a tactic to reduce certainty of paternity and de-
ter infanticide (Wolff and Macdonald 2004). Solicitation of
copulations clearly is to the disadvantage of actively pur-
suing males and favors lower-ranking males. This suggests
mate choice or manipulation of activities to a scramble88 Chapter Seven
Figure 7.1 Tactics used by female tree squirrels during mating bouts, with
negative fitness consequences shaded.