PERSISTENCE OF CIRCADIAN RHYTHMICITY IN HIBERNATING GROUND-SQUIRRELS

The body temperatures (T-b) of golden-mantled ground squirrels maintained under constant dim light (<20 lx red light) at an ambient temperature of 10 degrees C were monitored via telemetry throughout the hibernation season. During euthermia, when T-b ranged from 34 to 39 degrees C, these animals exhibited robust circadian T-b rhythms. During bouts of hibernation, when T-b dropped close to ambient (11-13 degrees C), circadian T-b rhythms persisted, although the amplitudes of the rhythms were considerably dampened compared with euthermia. The periods of the intrabout T-b rhythms were within the ranges observed during euthermia and were stable within an individual bout but varied between hibernation bouts. Arousals from hibernation occurred at a fixed phase angle of the T-b cycle. Once the period of an intrabout T-b rhythm was determined, it was possible to predict the timing of arousal from the hibernation bout to within 1 h of any 24-h period. This study confirms previous speculation about the persistence of circadian rhythms in golden-mantled ground squirrels during deep hibernation and demonstrates that the circadian system is involved in the timing of periodic arousals from hibernation.