Hibernation in the extreme: Burrow and body temperatures, metabolism, and limits to torpor bout length in Arctic ground squirrels

Arctic ground squirrels overwinter in hibernacula with average temperatures of -8 degreesC and minimum temperatures to -26 degreesC, conditions that a re extremely cold compared to those reported for other hibernators. During torpor, metabolic rate of arctic ground squirrels rises proportionally with decreases in ambient temperature (Ta) below 0 degreesC while core body tem perature (Tb) remains constant. At Ta > 0 degreesC, metabolic rate remains minimal and relatively constant while Tb increases to >12 degreesC, implyin g temperature-independent inhibition of metabolism, Maximum torpor bout len gth is greatest at Ta near 0 degreesC and shortens as Ta is either decrease d or increased, giving support to both metabolic rate- and body temperature -linked hypotheses of what constrains torpor duration in mammals. Brain tem perature, if it increases as Ta decreases below 0 degreesC, may ultimately influence torpor duration in arctic ground squirrels.