Urocitellus brunneus 353natural history: Food habits have only been studied in
U. b. brunneus. Its diet, similar to that of U. columbianus, in-
cludes grasses, leaves, fl owers, roots, bulbs, and some in-
sects and fungi. Three types of burrows are produced: for
hibernation, for rearing their young, and shallow auxiliary
burrows. The Idaho ground squirrel exhibits sexual dimor-
phism; body size and cranial dimensions are about 2.4–3.3
percent larger for males than for females. The breeding sea-
son lasts 12–13 days. Detailed studies have been conducted
on the breeding behavior of U. b. brunneus. Males emerge
two weeks prior to females. Within two days of the females’
emergence, mature males court, guard, and copulate with
females and then continue to guard females if the density of
receptive females is low. Larger males can often displace
smaller males, and paternity analyses show that the males
that were the last to guard a female and did so the longest
fathered 66–100 percent of the young. Mating occurs below-
ground, and copulatory plugs are produced after mating.
One litter is produced annually; average litter size at wean-
ing is 5.2 ± 1.4 (n = 59). Littermates emerge between late
March and early April in U. b. endemicus. In contrast, emer-
gence of the young is shifted about fi ve to six weeks later in
U. b. brunneus. Aboveground activity in U. b. brunneus is lim-
ited, lasting from late March to early August, and it is usu-
ally not more than four months for each individual.
U. brunneus appears to be excluded from its preferred
habitat by U. columbianus. One simple alarm call is used
when this species is threatened. Two mammals (long-tailed
weasels [Mustela frenata] and badgers [Taxidea taxus]) and
several raptors (Prairie Falcons [Falco mexicanus], Cooper’s
Hawks [Accipiter cooperi], Goshawks [Accipiter gentilis], Red-
Tailed Hawks [Buteo jamaicensis], and Northern Harriers
[Circus cyaneus]) are reported to prey on Idaho ground squir-
rels. Parasites include numerous ticks, fl eas, mites, and lice,
although U. brunneus—and U. b. brunneus in particular—are
reported to have fewer ectoparasites than other species of
ground squirrels, possibly owing to their fragmented and
isolated populations. Detailed demographic studies of a
population of U. b. brunneus from 1987 to 1989 showed a drop
from 272 individuals to 10. The population’s decline was due
to the loss of older breeding females, which was attributable
to a reduction in the food supply, especially of seeds, which
are important for hibernation. These food shortages were
ultimately caused by habitat alterations, due to fi re suppres-
sion and, in turn, changes in plant-species composition. Ge-
netic studies of 11 populations of U. b. brunneus indicate con-
siderable variation in their genetic structure (due to genetic
drift) and a lack of gene fl ow (due to signifi cant habitat frag-
mentation and isolation). Four neighboring populations
showed no diff erentiation, while seven others were each
distinct. The primary factors contributing to the decline of
this subspecies are habitat loss as a result of fi re suppression,
timbering, the replacement of native grasslands by exotic
invasives, and the conversion of native steppe-brush habitatUrocitellus brunneus endemicus. Photo courtesy Eric Yensen.
Urocitellus brunneus brunneus. Photo courtesy Roblyn Stitt.