In mammals, the circadian master clock generates daily rhythms of body temperature (Tb) 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 Tb (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 Tb 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, Tb 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 Tb for 22-26 d; patterns of Tb were arrhythmic for the first 10 d of euthermia. One of four squirrels exhibited a significant free-running Tb rhythm (τ =22.1 h) before emergence; this squirrel had been briefly exposed to low-amplitude light before emergence. In all animals, diurnal Tb 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 Tb cycles.
ASJC Scopus subject areas
- Animal Science and Zoology