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

Abstract

We measured standard operative temperatures ($T_{es}$) for the diurnal ground squirrel Ammospermophilus leucurus in its Southern California desert habitat by using pelt-covered metal models of the animals. Values of $T_{es}$ 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, $T_{es}$ (a direct measure of heat flow) specifies the thermal environment more realistically than any of the standard micrometeorological indices. In the hot season, $T_{es}$ in unshaded areas can be 30 C above the upper critical temperature ($T_{uc}$) 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, $T_{es}$ often exceeds $T_{uc}$, 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...