294 Otospermophilus beecheyinatural history: The California ground squirrel is her-
bivorous; its diet consists of a broad range of plant mate-
rial—more than 20 species of grasses, legumes, and forbs—
but it frequently includes seeds and nuts (e.g., the acorns of
Quercus) in the diet when available. Both the overall energy
content and the squirrel’s assimilation effi ciency are higher
for seeds than for foliage, but the reduced availability and
lower water content of seeds probably explains the high
percentage of foliage in this species’ diet. Other items con-
sumed occasionally include mosses, lichens, and arthro-
pods. O. beecheyi exhibits sexual dimorphism; males are
slightly but signifi cantly larger than females. Variation in
body mass in California ground squirrels shows a Berg-
mann’s cline, with the smaller animals found in the south-
ern part of the range, in more xeric environments. Such
variation corresponds primarily with precipitation and
probably results from the seasonality of resources, caused
by diff erences in rainfall. Reproduction occurs soon after
emergence from the burrow, the timing of which varies
with latitude. Both males and females mate several times
within a breeding season. The young are able to reproduce
in their fi rst year. Gestation is estimated to be 30 days, and
litter sizes range from 5 to 11, depending on elevation and
latitude. Infanticide is common in O. beecheyi and, where
systematically studied, appears to be performed almost ex-
clusively by neighboring mothers, who frequently cannibal-
ize the victims.
This species is abundant throughout its range; popula-
tion densities are reported to vary between 8 and 92 animals/
ha. Home ranges overlap considerably; the average size var-
ies from 270 to 375 m^2 for males, and 616 to 902 m^2 for fe-
males. Females typically maintain larger home ranges than
males, and home ranges do not change appreciably in size
during and after the breeding season. Numerous carnivo-
rous mammals, raptors, and snakes prey on O. beecheyi. The
young of California ground squirrels are particularly sus-
ceptible to predation by the northern Pacifi c rattlesnake
(Crotalus oreganos oreganos). O. beecheyi has shared a long evo-
lutionary history with this snake and, in response, has ap-
parently evolved a suite of behavioral and physiological ad-
aptations for thwarting successful predation, including the
ability to detoxify the venom with specifi c blood proteins,
the creation of distraction displays by tail fl icking, and the
ability to assess predation risks by the sounds of the rattle.
Squirrels will induce the snake to rattle by kicking sand,
so that they can assess specifi c risk characteristics such as
body temperature, the probability of a strike, and the body
size of the snake, all of which are correlated with the rate at
which the rattle is shaken. Moreover, a California ground
squirrel fl ags its tail in the presence of both rattlesnakes and
gopher snakes (Pituophis melanoleucus), but it increases the
blood fl ow (and thus heat) to the tail only in the presence of
a rattlesnake, which hunts mammalian prey with infrared
detectors. Such a reaction by the squirrel decreases the prob-
ability of a successful attack by rattlesnakes. Diff erences in
alarm calls between adults and juveniles, as well as varia-
tions in the responses of adults to the two call types, are
linked to diff erences in the level of predation risk between
juveniles and adults. The California ground squirrel is an
important reservoir host of Crypotosporidium parvum and the
human bacterial pathogen Bartonella washoensis. In Califor-
nia this species is considered one of the most signifi cant ver-
tebrate pests for agriculture; although it is also thought to be
a competitor with grazing cattle, detailed studies on its diet
and energy consumption suggest otherwise. While non lethal
methods of control (habitat modifi cation and transloca-
tions) have been attempted, large-scale attempts at control-
ling crop damage will most likely require an integrated
method of pest management.general references: Anthony 1928; Atwill et al. 2004;
Biardi, Chien, et al. 2006; Biardi, Coss, et al. 2006; Blois et al.
2008; Boellstorff and Owings 1995; Coss and Biardi 1997;
Hanson and Coss 2001a, 2001b; Helgen et al. 2009; Kosoy et
al. 2003; A. W. Linzey, Timm, et al. 2008a; R. E. Marsh 1994;
Rabin et al. 2006; Schitoskey and Woodmansee 1978; Swais-
good, Owings, et al. 1999; Swaisgood, Rowe, et al. 1999; Van
Vuren et al. 1997.Otospermophilus beecheyi. Photo courtesy A. Coke Smith,
http://www.cokesmithphototravel.com.