Rodent Societies: An Ecological & Evolutionary Perspective

However, burrow use and diets place strong energetic
constraints on these animals, and it appears that these
“bottom-up” factors are often limiting (except in cases of
episodic plague and anthropogenic control efforts).
Burrows are essential for both active and torpid ground
squirrels for thermoregulation and predator protection,
and anchor much of the above-ground activity. Burrow sys-
tems are not completely safe, and may be invaded by small-
bodied snakes as well as larger-bodied digging predators
such as badgers (Taxidea taxus). Badgers take ground squir-
rels regardless of their size or torpor status (Michener 2000;
Michener and Iwaniuk 2001). Ground squirrels are thus
spatially restricted, but their largely herbaceous diets (e.g.,
Van Horne et al. 1998) require a high volume of consump-
tion. Ranges in adult male weights are 135 – 625 g for Sper-
mophilusand 600 –750 g for Cynomys. A 400-g Richard-
son’s ground squirrel requires 30.6 g of fresh gypsyflower
(Cynoglossum officinale) leaves per day, or 7.65% of its
body mass, to meet its metabolic requirements (Blintz
1984). At this rate, ground squirrels and the larger-bodied
prairie dogs can consume a large fraction of the preferred
foods near their burrows, which, when risk of predation is
high and food productivity is low, can create a spatially re-
stricted food limitation.
For hibernators, caloric intake must also support a steep
increase in body mass through the active season. Body mass
of Piute ground squirrels (S. mollis,formerly S. townsendii)
generally doubled during their 4 –5-month active season
(Van Horne et al. 1997). The types of foods consumed by

ground squirrels generally cannot be stored for long periods

in moist burrow systems. Indeed, caching behavior has not

been reported; caching is found in ground-dwelling mam-

mals in very dry environments, or in moist environments

where caches can be kept relatively dry and /or seed coats

are very hard.

Small mammals generally use favorable microclimates to

avoid exposure to inclement weather. For ground squirrels,

these favorable microclimates often are available only in the

burrow systems, where food is unavailable. When weather

conditions are such that foraging requires more energy than

can be gained, animals must use stored energy. Under these

conditions, food availability can easily become limiting.

Adaptations to such conditions, particularly in very hot

or very cold climates, involve combinations of maximizing

food storage (mostly as fat) and minimizing metabolic en-

ergy expenditure through the use of seasonal or temporary

heterothermy (Lehmer et al. 2001).

So it seems that energy budgeting could be challenging

for these species. What evidence do we have that energy

availability actually limits populations? Surely the perva-

siveness of heterothermy among these species and popu-

lations indicates the need to conserve energy. Metabolic

adaptations to the problem of obtaining energy in ground

squirrels vary from obligate hibernation to opportunistic

use of torpor bouts. Heterothermy occurs in response to

lack of moisture and /or low temperatures. Facultative tor-

por in prairie dogs is influenced by body condition, as it oc-

curs at warmer temperatures in late winter when animals

468 Chapter Thirty-Nine

Table 39.2 Conservation status of all North American ground squirrel subspecies known to be endangered, imperiled, or vulnerable

Common name Scientific name Taxon conservation status rank

Northern Idaho ground squirrel S. brunneus brunneus Imperiled

Southern Idaho ground squirrel S. brunneus endemicus Imperiled

Wyoming ground squirrel S. elegans nevadensis Apparently secure

Wind river golden-mantled ground squirrel S. lateralis lateralis Apparently secure

Golden-mantled ground squirrel S. lateralis wortmani Apparently secure

Kodiak island arctic ground squirrel S. parryii kodiacensis Vulnerable

St. Lawrence Island ground squirrel S. parryii lyratus Vulnerable

Shumagin Islands Arctic ground squirrel S. parryii nebulicola Vulnerable

Odgood’s arctic ground squirrel S. parryii osgoodi Vulnerable (uncertain)

Spotted ground squirrel S. spilosoma marginatus Unranked

S. spilosoma obsoletus Unranked

Coachella round-tailed ground squirrel S. tereticaudus chlorus Critically imperiled or Imperiled

Allen’s thirteen-lined ground squirrel S. tridecemlineatus alleni Possibly extinct, Questionable taxonomic status

Ground squirrel S. tridecemlineatus arenicola Unranked

Ground squirrel S. tridecemlineatus blanca Vulnerable

White mountains ground squirrel S. tridecemlineatus monticola Vulnerable

Ground squirrel S. tridecemlineatus pallidus Unranked

Ground squirrel S. tridecemlineatus parvus Unranked

Rock squirrel S. variegatus tularosae Critically imperiled, Imperiled, or Vulnerable

SOURCE: NatureServe (2004).

NOTES: S.Spermophilus.For infraspecific taxon rank (NatureServe), see table 41.1.