Hibernation and circadian rhythms of body temperature in free-living arctic ground squirrels

In mammals, the circadian master clock generates daily rhythms of body temperature (T_b) that act to entrain rhythms in peripheral circadian oscillators. The persistence and function of circadian rhythms during mammalian hibernation is contentious, and the factors that contribute to the reestablishment of rhythms after hibernation are unclear. We collected regular measures of core T_b (every 34 min) and ambient light conditions (every 30 s) before, during, and following hibernation in free-living male arctic ground squirrels. Free-running circadian T_b rhythms at euthermic levels of Tb persisted for up to 10 d in constant darkness after animals became sequestered in their hibernacula in fall. During steady state torpor, T_b was constant and arrhythmic for up to 13 d (within the 0.19°C resolution of loggers). In spring, males ended heterothermy but remained in their burrows at euthermic levels of T_b for 22-26 d; patterns of T_b were arrhythmic for the first 10 d of euthermia. One of four squirrels exhibited a significant free-running T_b rhythm (τ =22.1 h) before emergence; this squirrel had been briefly exposed to low-amplitude light before emergence. In all animals, diurnal T_b rhythms were immediately reestablished coincident with emergence to the surface and the resumption of surface activity. Our results support the hypothesis that clock function is inhibited during hibernation and reactivated by exposure to light, although resumption of extended surface activity does not appear to be necessary to reinitiate T_b cycles.