Effects of dehydration on thermoregulatory behavior and thermal tolerance limits of Lithobates catesbeianus (Anura: Ranidae)

Foreseeing the effects of high environmental temperatures and drought on populations requires understanding how these conditions will influence the thermoregulatory behavior and thermal tolerance of organisms. Heath (1970) developed a model of behavioral thermoregulation in which ectotherms show fine-tuned (proportional) thermoregulation responses and all-or-none responses to avoid overheating. While scattered evidence suggests that dehydration alters the performance and thermoregulation of ectotherms, these effects have not been used to update such a model. To do that, we evaluated the effects of hydration level (HL) on the behavioral thermoregulation and physiological thermal limits of the “Bullfrog” (Lithobates catesbeianus), a model organism and important invader species. To examine the effects of dehydration on proportional responses, we compared the Preferred Body Temperatures (PBT) of frogs with free access to water with other frogs having restricted access to water. To observe the effect of dehydration on all-or-none responses, we measured the Voluntary Thermal Maximum (VTMax) at different hydration levels (100%, 90%, 80% of body weight at complete hydration). To understand the effect of dehydration on physiological thermal tolerance, we also measured the CTMax of frogs at the same hydration levels. Our results update Heath´s thermoregulation model showing disproportionally larger reductions on the PBT than on all-or-none responses and on the thermal limits. Besides, severely dehydrated individuals may lose their VTMax. We suggest that the observed dehydration effects should be included in mechanistic models of species distribution in order to improve climatic vulnerability assessments.