Integrating phylogeography and physiology reveals divergence of thermal traits between central and peripheral lineages of tropical rainforest lizards

Tropical ectotherms are regarded as being especially threatened by global warming, but the extent to which populations vary in key thermal physiological traits is little known. In general, central and peripheral populations are most likely to differ where divergent selection pressures are un-opposed by gene flow. This leads to the prediction that persistent and long-isolated lineages in peripheral regions, as revealed by phylogeography, may differ physiologically from larger centrally located lineages. We test this prediction through comparative assays of critical thermal limits (minimum and maximum critical thermal limits, CT_min, CT_max) and optimal performance parameters (B80 and T_opt) across central and peripheral lineages of three species of ground-dwelling skinks endemic to the rainforests of northeast Australia. Peripheral lineages show significantly increased optimal performance temperatures (T_opt) relative to central populations as well as elevated CT_min, with the latter trait also inversely related to elevation. CT_max did not vary between central and peripheral lineages, but was higher in a forest edge species than in the forest interior species. The results suggest that long-isolated populations in peripheral rainforests harbour genotypes that confer resilience to future warming, emphasizing the need to protect these as well as larger central habitats.