Persistence of circadian rhythmicity in hibernating ground squirrels.

The body temperatures (Tb) of golden-mantled ground squirrels maintained under constant dim light (< 20 1x red light) at an ambient temperature of 10 degrees C were monitored via telemetry throughout the hibernation season. During euthermia, when Tb ranged from 34 to 39 degrees C, these animals exhibited robust circadian Tb rhythms. During bouts of hibernation, when Tb rhythms persisted, although the amplitudes of the rhythms were considerably dampened compared with euthermia. The periods of the intrabout Tb 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 Tb cycle. Once the period of an intrabout Tb 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.