O’Connor, M.P.*; Zimmerman, L.C.; Spotila, J.R.: Effects of behavior, physiology, and habitat on thermoregulation in desert tortoises.

Desert tortoises are herbivorous ectothermic denizens of the Mojave and Sonoran deserts whose thermal ecology constrains activity and resource acquisition on both seasonal and daily time scales. We used behavioral observations of individual tortoises with temperature sensitive telemeters, operative temperature models, and recently developed analytic techniques to estimate both operative and available body temperatures to study the thermal ecology of free-ranging tortoises near Las Vegas, NV. We particularly addressed the roles of habitat and microhabitat choices, thermal inertia (due to relatively large body size), and the physiological control of heating and cooling in controlling body temperatures. Analyses of available operative temperatures suggested that thermal constraints on activity varied both seasonally and day-to-day. They also reveal a role for burrow use as a thermal refugium and a possible role for selection among surface microhabitats in controlling body temperatures. Microsites under shrubs warmed significantly slower than other microhabitats. Contrary to a priori predictions, sites near, but not under, shrubs were often among the hottest available sites. Much of the microsite-based diversity in operative temperature occurred during the hottest parts of the day when tortoises could not be active, but some diversity was available at times when tortoises were active. Both observations of telemetered tortoises and body temperatures predicted from operative temperatures using a model of heating and cooling in tortoises support an important role for the tortoise’s thermal inertia in buffering body temperatures from rapidly rising and falling operative temperatures. The role and importance of physiological control of warming and cooling is much less clear.