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

Arctic ground squirrels ( Spermophilus parryii) overwinter in hibernaculum conditions that are substantially below freezing. During torpor, captive arctic ground squirrels displayed ambient temperature (Ta)-dependent patterns of core body temperature (Tb), metabolic rate (TMR), and metabolic fuel use, as determined by respiratory quotient (RQ). At Ta 0 to −16°C, Tb remained relatively constant, and TMR rose proportionally with the expanding gradient between Tb and Ta, increasing >15-fold from a minimum of 0.0115 ± 0.0012 ml O2 · g−1 · h−1. At Ta 0–20°C, Tbincreased with Ta; however, TMR did not change significantly from Tb 0 to 12°C, indicating temperature-independent inhibition of metabolic rate. The overall change in TMR from Tb 4 to 20° equates to a Q10 of 2.4, but within this range of Tb, Q10 changed from 1.0 to 14.1. During steady-state torpor at Ta 4 and 8°C, RQ averaged 0.70 ± 0.013, indicating exclusive lipid catabolism. At Ta −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 Ta values <0°C, this relationship supports the hypothesis that availability of nonlipid metabolic fuels limits torpor duration in hibernating mammals; for Ta 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.