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 (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 O₂ · 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 Q₁₀ of 2.4, but within this range of T(b), Q₁₀ 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.