Photoperiod and Entrainment of Circadian Activity in the Antelope Ground Squirrel

The antelope ground squirrel (Ammosper mophilus leucurus) is highly diurnal, both in natural habitats and in a variety of laboratory light-dark schedules. The pattern results from the interaction of an endogenous rhythm and environmental parameters. Experimental manipulation of light-dark ratios showed that entrained phase angle is sensitive to changes in sunrise and sunset, and probably depends, at least in part, on the integral action of light.