Effects of ambient temperature on metabolic rate, respiratory quotient, and torpor in an arctic hibernator

Arctic ground squirrels (Spermophilus parryii) overwinter in hibernaculum c onditions that are substantially below freezing. During torpor, captive arc tic ground squirrels displayed ambient temperature (T-a)-dependent patterns of core body temperature (T-b), metabolic rate (TMR), and metabolic fuel u se, as determined by respiratory quotient (RQ). At T-a 0 to -16 degrees C. T-b remained relatively constant, and TMR rose proportionally with the expa nding gradient between T-b and T-a, increasing > 15-fold from a minimum of 0.0115 +/- 0.0012 ml O-2 . g(-1) . h(-1). At T-a 0-20 degrees C, T-b increa sed with T-a; however, TMR did not change significantly from T-b 0 to 12 de grees C, indicating temperature-independent inhibition of metabolic rate. T he overall change in TMR from T-b 4 to 20 degrees equates to a Q(10) of 2.4 , but within this range of T-b, Q(10) changed from 1.0 to 14.1. During stea dy-state torpor at T-a 4 and 8 degrees C, RQ averaged 0.70 +/- 0.013, indic ating exclusive lipid catabolism. At T-a -16 and 20 degrees C, RQ increased significantly to >0.85, consistent with recruitment of nonlipid fuels. RQ was negatively correlated with maximum torpor bout length. For T-a values > 0 degrees C, this relationship supports the hypothesis that availability of nonlipid metabolic fuels limits torpor duration in hibernating mammals; fo r T-a values >0 degrees C, hypotheses linked to body temperature are suppor ted. Because anterior body temperatures differ from core, overall, the dura tion torpor can be extended in hibernating mammals may be dependent on brai n temperature.