Torpor Bout Duration during the Hibernation Season of Two Sciurid Rodents: Interrelations with Temperature and Metabolism

Torpor bouts of mammalian hibernators are generally shorter at the beginning and end and are consistently longer during the main part of the hibernation season. Because it is not known why the duration of torpor bouts changes at the beginning and end of the hibernation season, we studied thisphenomenon in two sciurid rodents: Spermophilus saturatus (200-300g) and Eutamias amoenus (45-60g). We examined the seasonal change in torpor bout duration during hibernation at a constant air temperature (Ta) of20 C in relation to (1) the minimum body temperature (minimum Tb) to which the animals could be experimentally cooled before they maintained a constant Tb or began to arouse, (2) Ta at the time of minimum Tb (minimum Ta), and (3) oxygen consumption (Vo2) of torpid individuals atTa 2" C. Average duration of torpor bouts during the main part of the hibernation season was about 11 d in S. saturatus and 8-9 d in E. amoenus; in response to experimental cooling, minimum Tb of both species declined as low as-0. 2 C. In early and late hibernation, when torpor bouts were short, minimum Tb, minimum T,, and Vo, during torpor were higher than in the main part of the hibernation season. Regression analyses suggest that minimum Tb and minimum Ta are more strongly correlated with torpor bout duration than Vo2 in both species.