Rodent Societies: An Ecological & Evolutionary Perspective

Wauters et al. 1990; Koprowski 1993a, 1993b) and may
actively avoid pursuing males, to mate in some locations
near the ground (Koprowski 1993a). Most ground squirrels
(Sciuridae: Marmotini) mate belowground in burrows, pos-
sibly to reduce exposure to predators and /or reduce the like-
lihood of interruption of copulations by other males (Davis
1982a). Levels of overt aggression at the time of copula-
tion appear reduced in most ground squirrels (e.g., Erpino
1968) and at least one species, the thirteen-lined ground
squirrel (Spermophilus tridecemlineatus), exhibits a queu-
ing convention, in which males wait for opportunities to
copulate (Schwagmeyer and Parker 1987). A comparison
of copulatory times in the Sciuridae provides insight into
the potential selective pressures influencing the duration of
copulation.
Several possible explanations that are notmutually ex-
clusive may account for differences in copulatory times
among the Sciuridae:

1. Copulatory times may have increased in ground squir-
   rels to accrue benefits of prolonged copulation, such
   as increased likelihood of insemination and increased
   number of sperm (Schwagmeyer and Foltz 1990 and
   references within).
   2. Copulatory times have decreased in tree squirrels due
   to other constraints, such as predation risk and /or inter-
   ference by other males.
   3. Copulatory times may be the result of within-lineage
   cycling evolution (Reeve and Sherman 2001).

The large aggregations, intense levels of interaction

(2,500 interactions /h, Farentinos 1972; Steele and Kop-

rowski 2001), and frequent vocalizations (Zelley 1971;

Lishak 1982) characteristic of mating bouts of tree squirrels

make mating activities conspicuous. Males and females in

the conspicuous assemblages may be at considerable risk of

predation from raptors, canids, felids, and mustelids that

are known predators of tree squirrels (Gurnell 1987; Steele

and Koprowski 2001). This may be one of the selective

pressures for ground squirrel copulations to occur below-

ground (Davis 1982a). Although tree squirrels nest in cavi-

ties and leaf nests in branches of trees (however, Tamiasci-

urususe burrows in some populations: Shaw 1936; Yahner

1980), copulation rarely occurs in dens. The small size of

these refuges may be too confining, or risk of interruption

through the nest entrance may be too high, to permit copu-

lation in most arboreal nests. Occupants of nests struggle to

repel individuals from entering when not copulating (Steele

Alternative Reproductive Tactics and Strategies of Tree Squirrels 93

Table 7.1 Duration and location of copulations in the squirrels

Location Sample

Species of mating size Setting Duration Source

Arboreal squirrels

Sciurus aberti T 35 W 72.2 sec Farentinos 1972

S. carolinensis T 65 W 21.9 3.0 sec Koprowski, unpublished manuscript

S. griseus T2W30 sec Ingles 1948; Cross 1969

S. niger T 20 W 18.5 2.6 sec Koprowski 1993b

S. vulgaris T5Z15 sec Raspopov and Isakov 1980

Tamiasciurus douglasii T 10 W 1 to 25 min Koford 1982

G 2 W 4 and 8 min Smith 1968

Semi-arboreal and ground-

dwelling squirrels

Tamias sibricus G L 3 to 135 sec Blake and Gillet 1988

G W 5 to 120 sec Blake and Gillet 1988

T. striatus G 9 W 88 sec Yahner 1978a

G 3 W 1.5 to 3.0 min Wishner 1982

Cynomys gunnisoni G4W1 min Fitizgerald and Lechleitner 1974

Marmota flaviventris G W 1 to 23 min Armitage 1962

M. monax G 3 L 3 to 8 min Hoyt and Hoyt 1950

M. olympus G W 0.5 to 8 min Barash 1989

Spermophilus beldingi G W 10 to 12 min Sherman and Morton 1979, 1984

S. elegans G 1 L 2.5 min Denniston 1957

S. erythrogenys G 1 W 1 to 2 min Ognev 1940

S. richardsonii G 4 W 3 to 4 min Davis 1982a

S. tridecemlineatus G 51 W 4.7 (1–17) min Schwagmeyer and Brown 1983

NOTES: G ground; T tree; L laboratory; W wild; Z zoo.