Thermoregulatory behaviour explains countergradient variation in the upper thermal limit of a rainforest skink

The vulnerability of a terrestrial ectotherm to high environmental temperatures depends on the animal's thermal physiology and thermoregulatory behaviour. These variables – environment, physiology, and behaviour – interact with each other, complicating assessment of species vulnerability to global warming. We previously uncovered a counterintuitive pattern in rainforest sunskinks Lampropholis coggeri : a negative relationship between their critical thermal maximum (CT max ) and the temperature of their environment. Could this result be explained by a three‐way interaction between environment, physiology, and behaviour? Here we find that sunskink thermal preference is correlated positively with CT max , but, importantly, skinks from hotter environments prefer lower temperatures than conspecifics from cooler environments. In an acclimation experiment, we find that CT max is plastic and shifts in alignment with acclimation temperature. We also found heritable variation in this trait in a common garden study, but this variation was small relative to the plastic shifts observed in CT max . Thus, our previous observation of a negative correlation between field CT max and temperature is explained, at least in part, by the lizard's thermoregulatory behaviour: lizards from hot environments preferentially choose cool microenvironments, and their physiology acclimates to these cooler experienced temperatures. Our results suggest that behavioural adjustments to the environment can produce countergradient variation in physiological traits. More broadly, our work underscores the importance of interactions between environment, behaviour, and physiology in ectotherms. Understanding these interactions will be crucial in assessing vulnerability to climate change.