Temperature-independence of circannual variations in circadian rhythms of golden-mantled ground squirrels

In golden-mantled ground squirrels, phase angles of entrainment of circadia n locomotor activity to a fixed light-dark cycle differ markedly between su bjective summer and winter, A change in ambient temperature affects entrain ment only during subjective winter when it also produces pronounced effects on body temperature (T-b). It was previously proposed that variations in T -b are causally related to the circannual rhythm in circadian entrainment. To test this hypothesis, wheel-running activity and T-b were monitored for 12 to 14 months in castrated male ground squirrels housed in a 14:10 LD pho tocycle at 21 degrees C. Animals were treated with testosterone implants th at eliminated hibernation and prevented the marked winter decline in T-b; t hese squirrels manifested circannual changes in circadian entrainment indis tinguishable from those of untreated animals. Both groups exhibited pronoun ced changes in phase angle and alpha of circadian wheel-running and T-b rhy thms. Seasonal variation in T-b is not necessary for circannual changes in circadian organization of golden-mantled ground squirrels.