Standard Operative Temperatures and Thermal Energetics of the Antelope Ground Squirrel Ammospermophilus leucurus

Abstract

We measured standard operative temperatures (Tes) for the diurnal ground squirrel Ammospermophilus leucurus in its Southern California desert habitat by using pelt-covered metal models of the animals. Values of Tes were used to predict duration of periods of activity, microhabitat use, and energy costs of thermoregulation at various times of the year. For A. leucurus, Tes (a direct measure of heat flow) specifies the thermal environment more realistically than any of the standard micrometeorological indices. In the hot season, Tes in unshaded areas can be 30 C above the upper critical temperature (Tuc) of A. leucurus and 20 C above air temperature for much of the day. Under such conditions maximum tolerable periods of surface activity for A. leucurus are 7-9 min. Even in the shade, Tes often exceeds Tuc, and the only available thermal refugia on hot days are in underground burrows. These findings strongly support the interpretation that the thermoregulatory strategy of A. leucurus involves cycles of transient hyperthermia followed by periods of passive heat dissipation underground. Rates of heat gain from the environment in summer are three to four times the rate of metabolic heat production, even for active individuals. In the cool season Tes in open areas is usually within the thermal neutral zone, and the thermoregulatory responses of A. leucurus follow the standard mammalian pattern.