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Institute of Arctic Biology and Department of Biology and Wildlife, University of Alaska Fairbanks, Fairbanks, Alaska 99775
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 · h1. At Ta 0-20°C, Tb
increased 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.
hibernation; metabolism; arctic ground squirrel; metabolic fuel
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