Influence of Thermal Conditions on the Surface Activity of Thirteen‐Lined Ground Squirrels

We examined the relation between surface activity and the surface microclimate in thirteen—lined ground squirrels. We statistically isolated the effects of thermal conditions on squirrel activity and tested the suitability of two optimization models for predicting thermoregulatory behavior. Metabolic and microclimatic data were used to represent thermal conditions as standard operative temperature. The proportion of time spent on the surface was highest at standard operative temperatures between 30° and 40°C. This range approximately matched the squirrel's physiological thermal neutral zone. This pattern was not qualitatively altered by using a multiple—regression model to remove the effects of potential confounding variables (e.g., time of day or time of year). At low temperature, the observed reduction in surface activity resulted in lost foraging benefits °6 times as great as the energy savings in thermoregulatory and activity costs. Therefore, we believe that these squirrels cannot be considered daily energy maximizers. Rather, optimization must be considered in the context of longer time periods. At high temperatures, the observed reduction in surface activity was described by a constrained thermal model of shuttling thermoregulation, which assumed squirrels maximized surface feeding time. The model also explained a systematic decline in body temperature when exiting and entering the burrow as surface operative temperatures increased.