Animals adapted to extreme environmental conditions often show rhythmic activity with stable periodicity. We studied the daily activity pattern of European ground squirrels (Spermophilus citellus) to determine effects of environmental factors on this pattern. Activity was evaluated by counting the number of squirrels visible above ground in 3 manipulated groups. To test the hypothesis that midday air temperature contributes to the bimodal activity pattern, we manipulated ambient temperature perception by altering the coloration of their head fur. Significantly fewer dark-headed squirrels were visible on the surface in midday on sunny days than were light-headed animals. Because ambient temperature perception is strongly affected by the darkness of the top of the head, we concluded that extremely high midday ambient temperatures play a central role in contributing to the observed midday rest phase in ground squirrels.