Temperature-independence of circannual variations in circadian rhythms of golden-mantled ground squirrels
In golden-mantled ground squirrels, phase angles of entrainment of circadian locomotor activity to a fixed light-dark cycle differ markedly between subjective summer and winter. A change in ambient temperature affects entrainment 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 photocycle at 21 °C. Animals were treated with testosterone implants that eliminated hibernation and prevented the marked winter decline in T(b); these squirrels manifested circannual changes in circadian entrainment indistinguishable from those of untreated animals. Both groups exhibited pronounced changes in phase angle and alpha of circadian wheel-running and T(b) rhythms. Seasonal variation in T(b) is not necessary for circannual changes in circadian organization of golden-mantled ground squirrels.
Journal of Biological Rhythms
Freeman, D., & Zucker, I. (2000). Temperature-independence of circannual variations in circadian rhythms of golden-mantled ground squirrels. Journal of Biological Rhythms, 15 (4), 336-343. https://doi.org/10.1177/074873000129001341