Optimal body temperatures with shuttling: desert antelope ground squirrels

Abstract

A variety of animals periodically shuttle between two sites, such as one containing food and another containing oxygen, water, or conditions for heat exchange. The amount of a non-food resource obtained, together with its rate of use, influences travelling and foraging time. Three criteria are evaluated to examine how control of body temperature may be optimal with shuttling: (1) maximizing proportion of time foraging, (2) maximizing rate of net energy gain for a shuttle cycle and (3) maximizing time travelling and foraging relative to time spent at a heat-exchange site. Detailed information on heat exchange by desert antelope ground squirrels, Ammospermophilus leucurus, cooling in their burrows and heating while travelling and foraging on the hot desert surface is used to calculate body temperatures that maximize each criterion. Results also are discussed for analogous models for intermittent bird incubation and ectotherm basking. The third criterion is rejected based on published body temperature patterns. The other criteria are plausible but assume temperature control in anticipation of environmental conditions. Body temperatures that maximize the first criterion maximize rates of net energy gain when the rate of gross energy gain is constant. The optimal body temperatures of ground squirrels exiting a burrow should decrease with: (1) increasing travel time, (2) higher heating or cooling rates, (3) decreased body temperature upon return to a burrow and (4) higher rates of gross energy gain.