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

Abstract

Arctic ground squirrels ( Spermophilus parryii) overwinter in hibernaculum conditions that are substantially below freezing. During torpor, captive arctic ground squirrels displayed ambient temperature (T a)-dependent patterns of core body temperature (T b), metabolic rate (TMR), and metabolic fuel use, as determined by respiratory quotient (RQ). At T a 0 to −16°C, T b remained relatively constant, and TMR rose proportionally with the expanding 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°C, T b increased with T a; however, TMR did not change significantly from T b 0 to 12°C, indicating temperature-independent inhibition of metabolic rate. The overall change in TMR from T b 4 to 20° 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 steady-state torpor at T a 4 and 8°C, RQ averaged 0.70 ± 0.013, indicating exclusive lipid catabolism. At T a −16 and 20°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°C, this relationship supports the hypothesis that availability of nonlipid metabolic fuels limits torpor duration in hibernating mammals; for T a values >0°C, hypotheses linked to body temperature are supported. Because anterior body temperatures differ from core, overall, the duration torpor can be extended in hibernating mammals may be dependent on brain temperature.