1. The ecology of endotherms is driven by their great energetic need for thermoregulation, which renders mammals and birds particularly vulnerable to environmental temperature and resource fluctuations. Important outstanding questions are whether populations are specialized to their particular climate, and to what extent gene × environment interactions determine thermal responses.
2. Here, we show that phylogenetic relatedness and climate interact to determine metabolic capacities, body temperature and morphology in a wild rat (Rattus fuscipes).
3. Mitochondrial metabolic capacities are greater in warm climate populations, indicating that these responses are not the result of cold adaptation. However, glycolytic capacities, fur thickness and capacity for nonshivering thermogenesis are greater in cool climate populations. In populations from cooler climates, body temperatures are lower, but more variable. Together, these changes lead to substantial energy savings in cool climate populations, although all traits are constrained by phylogenetic relatedness.
4. We demonstrate for the first time that gene × environment interactions determine thermal responses in wild mammal populations, and we suggest that physiological variability among populations may render the species more resilient to climate change because it increases whole-species performance breadth. Climate envelope modelling is therefore insufficient to predict the future impact of climate change.