Basking behavior predicts the evolution of heat tolerance in Australian rainforest lizards.

There is pressing urgency to understand how tropical ectotherms can behaviorally and physiologically respond to climate warming. We examine how basking behavior and thermal environment interact to influence evolutionary variation in thermal physiology of multiple species of lygosomine rainforest skinks from the Wet Tropics of northeastern Queensland, Australia (AWT). These tropical lizards are behaviorally specialized to exploit canopy or sun, and are distributed across marked thermal clines in the AWT. Using phylogenetic analyses, we demonstrate that physiological parameters are either associated with changes in local thermal habitat or to basking behavior, but not both. Cold tolerance, the optimal sprint speed, and performance breadth are primarily influenced by local thermal environment. Specifically, montane lizards are more cool tolerant, have broader performance breadths, and higher optimum sprinting temperatures than their lowland counterparts. Heat tolerance, in contrast, is strongly affected by basking behavior: there are two evolutionary optima, with basking species having considerably higher heat tolerance than shade skinks, with no effect of elevation. These distinct responses among traits indicate the multiple selective pressures and constraints that shape the evolution of thermal performance. We discuss how behavior and physiology interact to shape organisms' vulnerability and potential resilience to climate change.