356 Urocitellus columbianusdoes not. Mean litter sizes vary from 2.1 to 4.2 in the wild
and from 3.0 to 4.6 in captivity; and the range of litter sizes
(estimated from embryos and placental scars) is from 2.7 to
7.0. Litter sizes are negatively correlated with both elevation
and latitude. Gestation is 24 days. In U. columbianus, the
young are born hairless and weigh between 6.8 and 11.4 g.
Full adult weight is not reached until the second year. The
young enter hibernation at a lower mass (only 60% of adult
mass) than similar species, and they show a higher survival
rate during hibernation, even though overwinter survival
appears to increase with an animal’s body mass. Sex ratios
are often skewed toward females. Compared with several
congeners, the overall life expectancy for U. columbianus is
greater.
Population densities from diverse locations range from
24.7/ha to 61.7/ha, and juvenile densities have been reported
to vary from 4.6 to 20.7/ha. Home ranges are estimated to
average 4200 m^2 for males but only 1000 m^2 for females. Male
home ranges overlap considerably, but males are distinctly
territorial around the core areas in which they defend
breeding females. This male aggression, however, declines
after the breeding season. Adult females are also territorial,
which may help protect juveniles until emergence. Females
establish territories close to their natal burrows and show
a considerable degree of site fi delity for several years. It is
postulated that this may increase patterns of familiarity
and social interactions, but it also heightens inbreeding,
especially between daughters and fathers. U. columbianus is
highly social and appears to develop social bonds earlier
than other species. Both empirical and theoretical evidence
suggest that the Columbian ground squirrels’ social system
functions as a social network (society) that is infl uenced by
some individuals more than others. Cross-fostering experi-
ments indicate that social interactions (recognition of kin
and agonistic behavior) are based on experience in the nest
and therefore are not directly determined by the degree of
relatedness, as was previously proposed. Infanticide is re-
ported; in one study it was as high as 7.6 percent of all ju-
veniles (and 12.5% of all litters) over a two-year period.
Infanticide in Columbian ground squirrels is generally not
between close relatives. Dispersal between colonies is much
higher in males than in females. This appears to be due to a
higher level of adult aggression toward young males and a
greater acceptance of daughters by adult females. There is
little evidence that patterns of dispersal are specifi cally
driven by the risk of depression from inbreeding. Dispersal
distances in one study were usually less than 4 km, with a
maximum of 8.5 km. Hence it is likely that migration pat-
terns, rather than dispersal per se, accounted for the low
genetic diff erentiation observed between populations
across 183 km in this same study. Studies of patch coloniza-
tion, however, indicate that patch occupancy is related to
the distance between patches, not to the area of the patches,
and that Columbian ground squirrels do not regularly colo-
nize new habitats. Details on locomotion, self-grooming,
allogrooming (which occurs but is rare), agonistic interac-
tions, greeting behavior, basking behavior, courtship be-
havior, and scent-marking are all described in the literature.
The Columbian ground squirrel uses a broader range of vo-
calizations than is typically observed in other species (e.g.,
U. armatus, U. elegans, U. richardsonii). Its vocalizations fol-
lowing copulation are nearly identical in structure to alarm
calls but appear to function in mate guarding. These mating
calls do not seem to provoke the same vigilance response
as alarm calls, yet the animals typically react to playbacks
of mating calls outside the breeding season as they would to
alarm calls. Alarm calls in response to avian predators are
more complex and involve a more rapid succession of calls
than those used for ground predators.
Mortality in the young can be particularly high if heavy
snows occur during the mating season. A nine-year study,
however, demonstrated that the number of young survivingUrocitellus columbianus. Photo courtesy Phil Myers, Animal
Diversity Web, animaldiversity.org.