High thermal variation in maximum temperatures invert Brett's heat-invariant rule at fine spatial scales.

Discovering how species' thermal limits evolve and vary spatially is crucial for predicting their vulnerability to ongoing environmental warming. Current evidence indicates that heat tolerance is less spatially variable than cold tolerance among species, presenting a major concern for organismal vulnerability in a rapidly warming world. This asymmetry in thermal limits has been supported by large-scale geographic studies, across latitudinal and elevational gradients (known as Brett's heat-invariant rule). Yet, how critical limits vary across finer spatial scales (e.g., across microenvironments) is less understood. Here, we show that minimum temperatures are more variable than maximum temperatures at large geographic scales (across latitude/elevation) but are less variable at local scales (within sites), in turn guiding spatial asymmetries in thermal tolerances. Using thermal tolerance measurements from amphibians, insects, and reptiles, we confirm the invariance of heat tolerance at large spatial scales and also find more variable heat than cold tolerances at local scales (an inverted Brett's heat-invariant rule at fine spatial scales). Our results suggest that regional- or global-level studies will likely obscure fine-scale structuring in thermal habitats and corresponding patterns of local heat tolerance adaptation. We emphasize that inferences based on broadscale geographic patterns obscure fine-scale variation in thermal physiology. For instance, a genetic basis for fine-scale variation in thermal physiology may reshuffle spatial and phylogenetic patterns of vulnerability.