Reptiles rely on thermal heat exchange to achieve body temperatures (Tbody) conducive to maintaining homeostasis. Diurnal changes in the thermal environment are therefore liable to influence allostatic mediation of survival processes (e.g., immunity) during environmental challenges or stressors. However, the extent to which Tbody prompts individual variation in physiology remains largely unexplored in reptiles. Our study tested how circulating energy‐mobilizing hormone, energy metabolites, and immunity can vary across basal and stress‐induced allostatic states for plateau side‐blotched lizards (Uta stansburiana uniformis) residing in a heterogeneous thermal environment. We collected baseline and acute stress blood samples from male lizards to compare changes in plasma corticosterone (CORT), glucose, and bacterial killing ability (BKA) in relation to each other and Tbody. We hypothesized each physiological parameter differs between allostatic states, whereby stress‐induced activity increases from baseline. At basal and stress‐induced states, we also hypothesized circulating CORT, glucose, and BKA directly correspond with each other and Tbody. We found both CORT and BKA increased while glucose instead decreased from acute stress. At basal and stress‐induced allostatic states, we found CORT to be directly related to Tbody while BKA was inversely related to CORT. We also found BKA and glucose were directly related at baseline, but inversely related following acute stress. Overall, these results demonstrate allostatic outcomes from acute stress in a free‐living reptile and the role of temperature in mediating energetic state and immunity. Future research on reptilian allostasis should consider multiple environmental conditions and their implications for physiological performance and survival.

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