Thermal Environments and Prey Availability Impose Seasonal Foraging Constraints on Gila monsters

Gienger, C.M.

Department of Biology and Center of Excellence for Field Biology

Austin Peay State University

Clarksville, TN

Tracy C., Richard

Department of Biology and Program in Ecology

Evolution, and Conservation Biology

University of Nevada – Reno

Reno, NV

Gila monsters are a unique lizard species with many aspects of their ecology differing from what would be expected based upon their body size, habitat, or evolutionary history. Gila monsters are reclusive, but unlike other secretive lizards, which tend to be sedentary and nocturnal, Gila monsters are primarily diurnal in their activity, and they search over large areas for food and mates. They are a desert lizard, but they appear to have physiologies that do not tolerate high body temperatures. High above-ground environmental temperatures limit opportunities for Gila monster activity, yet these lizards are most active during the hottest seasons of the year. We propose that the success of this species is related to the activity patterns of individuals, rather than by the ability of these lizards to thermoregulate precisely by behavioral or physiological means. To forage during late summer, Gila monsters must tolerate body temperatures well outside of their preferred body-temperature range. In fact, Gila monsters seem nearly to abandon thermoregulation late in the activity season, yet they avoid lethal temperatures by temporally shifting activity and becoming nocturnal. Prey availability poses an additional constraint late in the activity season. Gila monster above-ground activity nearly ceases by early fall when their prey become unavailable, even though moderate above-ground temperatures would allow these lizards to be active for 22 hours per day. Thus in general, Gila monsters are not precise thermoregulators, but adopt activity patterns designed (a) to avoid extreme body temperatures and (b) to exploit seasonal availability of food.