Many consider squirrels to be model mammals for the study
of behavior and ecology. Although some may disagree, we
strongly support this assertion, and submit the more than
fi ve decades of ecological and evolutionary studies on squir-
rels as evidence for their importance to such key research
areas as mammalian social behavior, kin selection, foraging
ecology, food hoarding, and seed dispersal.
Their relevance to the study of behavior and ecology is
due in part to the simple fact that squirrels occupy a variety
of niches throughout the temperate, subtropical, and tropi-
cal regions of the world. With the exception of fl ying squir-
rels, most squirrels are diurnal and easily observed; in fact,
they are one of the few mammals that can be readily marked
and directly studied for extended periods of time. In the
case of social ground squirrels and prairie dogs, for exam-
ple, nearly every individual in a colony can be marked, ob-
served, and followed to measure characteristics such as dis-
persal, longevity, relatedness, and lifetime reproductive
success. Such opportunities are clearly lacking in many
other systems. Moreover, squirrels often function as key-
stone herbivores, seed predators, or seed dispersers; as
such, they may provide critical services that are essential to
ecosystem function. Our current state of knowledge is
based upon temperate and holarctic species, and many new
discoveries probably await us in the grasslands and forests
of the tropics. Here, we provide a brief overview of the ecol-
ogy and behavior of squirrels by highlighting some of the
critical areas in which they have been studied.
Population Ecology
The modest body sizes of the Sciuridae—ranging from the
15 g African pygmy squirrel (Myosciurus pumilio) to the 8000 g
Ta rba ga n ma r mot (Marmota sibirica)—in combination with
high levels of aboveground activity and their often diurnal
habits, have made squirrels the focus of many important
studies in population ecology. Most of the Sciuridae mature
in their fi rst year of life; however, some of the larger-bodied
ground squirrels, such as marmots (Marmota) and prairie
dogs (Cynomys), often have delayed maturity that extends
until 2 to 4 years of age, when fi rst reproduction takes place.
Hibernation of up to eight months a year occurs only in the
Marmotini and, importantly, constrains resource use to the
brief active season. High-latitude marmots, prairie dogs,
ground squirrels, and chipmunks cannot complete repro-
duction late in this short season, whereas tree, fl ying, and
tropical ground squirrels do not have such severe temporal
constraints and often rely on cached food items and fat
stores to remain active overwinter, with only occasional
bouts of short-duration torpor.
A few signifi cant patterns emerge from the recent phyloge-
netic assessments by V. Hayssen in characterizing the repro-
ductive performance of the Sciuridae, despite variations
such as litter sizes that range from 1.0 to 9.7 young (mean =
3.8 young) in some antelope ground squirrels (Ammosper-
mophilus). Gestation (pregnancy) extends from 22 to 80 days
(mean = 34.6 days), and females nurse from 21 to 105 days
(mean = 45.0 days). Tree squirrels invest more into reproduc-
tion than do most ground squirrels and often have multiple
modest-sized (two to fi ve young) litters each year, especially
in the tropics, as evidenced from the few species that have
been studied. Flying squirrels have smaller litter sizes (one
to four young) than most tree squirrels and ground squir-
rels. Marmotine ground squirrels typically produce a single
large litter of small young and thus have a modest annual
investment, despite a sizeable energetic investment in this
single large litter, and some large-bodied ground squirrels