Dehydration enhances multiple physiological defense mechanisms in a desert lizard, Heloderma suspectum .

The physiological challenges associated with dehydration can induce an increase in plasma glucocorticoid concentrations, a response thought to provide the mechanism for dehydration suppressing immune function. However, a comprehensive examination of the inter-relationship of dehydration, stress and immune function has not been conducted within a single species. We previously demonstrated that Gila monsters ( Heloderma suspectum ), which inhabit a xeric environment with a predictable seasonal drought, have enhanced measures of innate immunity when dehydrated. These results suggest that, in this species, dehydration may not induce a glucocorticoid response, but, instead, enhances physiological defense mechanisms. To explore this possibility, we examined multiple measures of innate immunity as well as initial and reactive plasma concentrations of glucocorticoids in captive and free-ranging Gila monsters at various hydration states. Our results show that, in this species, dehydration alone does not cause a substantial increase in plasma glucocorticoids, and we provide broader evidence that dehydration enhances defensive mechanisms including stress reactivity and various measures of innate immune function. These findings suggest that physiological responses to dehydration may depend heavily on an organism's ecology. More research on the effects of dehydration on the glucocorticoid response and immunity will help clarify the interactive roles they play in response to hydration challenges and whether adaptations to water-limited environments influence these interactions.