ing ground squirrel sociality, in an attempt to: (1) summa-
rize the ecological factors underlying the appearance and
maintenance of sociality, (2) clarify the role that kinship
plays in defining spatial and behavioral relationships among
members of ground squirrel societies, and (3) identify pre-
viously underemphasized correlates of increasing sociality.
These interrelationships are summarized in figure 29.4. Fi-
nally, we identify fruitful avenues for future research.
The Comparative Sociality of Ground Squirrels
David Barash (1973, 1974a) was the first to recognize the
full utility of the comparative method in examining the eco-
logical basis of sociality in the genus Marmota. Barash con-
trasted his own detailed observations of the biology and be-
havior of Olympic marmots (M. olympus) with those made
by other investigators on yellow-bellied marmots (M. flavi-
ventris) and woodchucks (M. monax), and related those
observations to environmental differences experienced by
those North American marmots in nature. Specifically, Ba-
rash (1973) reported a decreasing incidence of aggression,
increasing incidences of “greeting” and play fighting, and
the relaxation of territoriality with increasing social inte-
gration as woodchucks, yellow-bellied and Olympic mar-
mots are considered in series. Advancing sociality was noted
to be coincident with increasing elevation, which Barash
in turn related to the progressively diminishing length of
the growing season, and hence foraging time available for
young of the year of these relatively large-bodied rodents to
amass sufficient fat reserves to survive hibernation (see Ar-
mitage, chap. 30 this volume).
Further, Barash noted that whereas woodchucks would
typically disperse from the natal area during the summer
in which they were born, yellow-bellied and Olympic mar-
mots would typically delay dispersal for 1 or 2 years respec-
tively. From the individual’s perspective, delays in dispersal
are also accompanied by delayed reproductive maturity.
Thus dispersal of young from the natal area was increas-
ingly delayed with increased elevation, and related inversely
to the length of the growing season. Taken together, Barash
(1973, 1974a) concluded that the reduction in the length of
the growing season imposed by increasing elevation selects
for reduced aggression and increasing social cohesion, in-
cluding delayed dispersal as a means of ensuring the sur-
vival of young.
Within the prairie dogs (Cynomys), Hoogland (1981)
contrasted the ecology and behavior of the more highly so-
cial black-tailed prairie dog (C. ludovicianus) with that of
the less social white-tailed prairie dog (C. leucurus). As was
the case for marmots, delays in both dispersal and repro-
duction were evident for the more socially complex spe-
cies, though these differences were not correlated with dif-
ferences in elevation or length of growing season. Rather,
Hoogland (1981) marshaled data suggesting that colonial-
ity in prairie dogs was attributable to antipredator bene-
fits of group living (see fig. 29.1 and Bertram 1978; Alcock
2005 for reviews), and that interspecific differences could
be ascribed to habitat differences promoting different anti-
predator strategies. White-tailed prairie dog habitats of-
fered significantly more protective cover than black-tailed
habitats. Thus white-tailed prairie dogs were interpreted
by Hoogland (1981) to have a greater reliance on avoiding
predation through the use of cover, thereby limiting the size
and density of groups to the number of individuals that
could effectively exploit the cover available in a given area.
By contrast, for black-tailed prairie dogs, without extensive
cover, selection has favored a dependence on the rapid de-
tection of predators (Hoogland 1981), which is enhanced
via increases in the size and density of a social group (Hoog-
land 1979b; Kildaw 1995; Hoogland, chap. 37 this volume).
In a more taxonomically comprehensive effort, Armitage
(1981) examined correlations between an ordinal “sociality
index” and the life-history traits of twenty-four popula-
tions of eighteen different ground-dwelling squirrel species.
While admitting that his sociality index served only as a
first approximation, Armitage categorized the species he in-
cluded in his analysis as Level: 1 solitary, 2 individual
females forming an aggregation or “colony” in suitable
habitat, 3 the expression of male territoriality within a
colony of individual females, 4 the aggregation of female
colony members into a male-controlled harem, and 5 ex-
istence in multiharem colonies. He then reduced the overall
data set to four principal component axes reflecting body
size, seasonality, reproductive effort, and age at reproduc-
tive maturity, and examined correlations between those
axes and his sociality index. Sociality was not correlated346 Chapter Twenty-Nine
Figure 29.1 An alarm-calling Richardson’s ground squirrel (Spermophilus
richardsonii) warns conspecifics of the presence of a predatory threat. Photo
by J. Hare.