PERSISTENCE OF CIRCADIAN RHYTHMICITY IN HIBERNATING GROUND-SQUIRRELS

The body temperatures (T-b) of golden-mantled ground squirrels maintai ned under constant dim light (<20 lx red light) at an ambient temperat ure of 10 degrees C were monitored via telemetry throughout the hibern ation 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), c ircadian T-b rhythms persisted, although the amplitudes of the rhythms were considerably dampened compared with euthermia. The periods of th e intrabout T-b rhythms were within the ranges observed during eutherm ia and were stable within an individual bout but varied between hibern ation 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 dete rmined, it was possible to predict the timing of arousal from the hibe rnation bout to within 1 h of any 24-h period. This study confirms pre vious speculation about the persistence of circadian rhythms in golden -mantled ground squirrels during deep hibernation and demonstrates tha t the circadian system is involved in the timing of periodic arousals from hibernation.