12 Ecology, Behavior, and Conservationdo not reproduce each year. Non-marmotine ground squir-
rels have a reproductive pattern similar to that of tree squir-
rels, although typically with smaller litter sizes and slower
growth.
The ecological longevity of squirrels is typically 5–10
years; however, in captivity (physiological longevity) many
squirrels can survive 10–20 years. Survivorship in the wild
is poor during the fi rst year of life but increases after natal
dispersal and upon reaching adulthood. Studies on natal
dispersal in yellow-bellied marmots (Marmota fl aviventris)
suggest that survival during dispersal may not be as meager
as once hypothesized. Classic studies conducted on several
ground squirrels (Belding’s ground squirrels [Urocitellus
beldingi] and Columbian ground squirrels [U. columbianus])
and yellow-bellied marmots tell us that attaining an appro-
priate body mass is a key proximate cue that triggers natal
dispersal; ultimately, however, inbreeding avoidance and
reproductive opportunity appear to play an important role
in dispersal from the natal area. Sex diff erences in patterns
of space use, reproductive success, and annual mortality
(greater for males than for females) are paralleled by sexual
dimorphism in body size in some of the ground squirrels
(particularly the marmotine ground squirrels, including
Marmota and Cynomys), whereas most tree and fl ying squir-
rels demonstrate little if any dimorphism in body size but
often exhibit these same ecological diff erences between the
sexes. Chipmunks (Tamia s), however, demonstrate an in-
triguing pattern of reversed sexual dimorphism, with the
females being larger than the males, which is rare among
mammals; the cause of this pattern is unclear, but females
accrue advantages in dominance and resource acquisition.Communication
Individuals communicate using several sensory modalities.
The excellent vision of most squirrels allows them to use
visual cues. Posture, foot stamping, and rapid movements of
the tail appear to convey information on dominance status
between individuals, as well as to indicate the level of alarm
among conspecifi cs and heterospecifi cs. Unfortunately, we
know very little about the messages conveyed by such vi-A litter of round-tailed ground squirrels (Xerospermophilus tereticaudus). Photo courtesy Karen Munroe.