Thirst Responders: Effects of Hydration State on Body Temperature and Immune Responses to an Antigen Challenge

Physiological performance is highly temperature sensitive, and enhanced physiological activity (e.g., digestion, reproductive, immune activation) often leads to a thermophilic response that elevates body temperature. Previous studies have shown that the hydration state of the organism may influence the extent of the thermophily, and other studies have shown that hydration state can alter physiological performance. However, it is rare for studies to comprehensively examine the interaction among hydration state, physiological performance, and the thermophilic response. We simultaneously examined the effects of hydration state and an immune challenge on thermophily and plasma-based innate immune responses in Gila monsters (Heloderma suspectum). Our results show that water deprivation led to progressive dehydration and reduction of body temperature. Upon receiving a lipopolysaccharide injection, dehydrated but not hydrated Gila monsters showed a thermophilic response, and multiple reactive immunity metrics were elevated in dehydrated individuals, particularly at 7 days post injection. Interestingly, this reactive immune response and the thermophily were not temporally correlated. Also, the reactive immune response of Gila monsters appears to be more responsive to dehydration than is the constitutive immune response, as enhancement of the reactive response was seen at moderated dehydration whereas previous work has demonstrated that enhancement of constitutive immune function does not occur until more severe dehydration. Future research should further investigate the interrelationship of hydration state, body temperature, and both constitutive and reactive immune function, especially in species where dehydration is a mechanism used to tolerate seasonal drought.