Entrainment of Circadian Activity Rhythms in Squirrels

Entrainment behavior of circadian activity rhythms in squirrels apparently results from the interaction of an endogenous rhythm with environmental parameters. Red squirrels were placed in LD cycles which were out of phase with normal activity time. The time course of entrainment was a gradual, orderly process which often culminated in decreasing overshooting and undershooting. Animals placed in LD conditions which were insufficient to cause entrainment showed the predicted oscillatory free runs. The same analysis technique was used to examine entrainment data from other studies. Responsiveness varied, presumably depending on individual differences and the type and amplitude of the synchronizer. Generally, the stable entrainment-phase angle for diurnal animals was about 180 degrees from that for nocturnal animals. To examine the complex relation between period and phase during the entrainment process, 21 light-pulse experiments were conducted. It was found that, at least with red squirrels, phase and period shifting occur together and presumably act synergistically.