Does heat tolerance actually predict animals’ geographic thermal limits?

The “climate extremes hypothesis” is a major assumption of geographic studies of thermal tolerance and climatic vulnerability. However, this assumption remains vastly untested for the warm edges of animals’ geographic ranges, and multiple factors may contribute to uncoupling heat tolerance and geographic limits. Herein, we compiled and analyzed multiple types of heat tolerance indexes and of maximum temperatures for each species’ known distribution (hereafter, Tmax). The dataset includes over 1000 entries of heat tolerance and Tmax data distributed across marine fish, terrestrial arthropods, amphibians, non-avian reptiles, birds, and mammals. With it, we first tested if heat tolerance constrains the Tmax of sites where species could be observed. Secondly, we tested if the strength of such restrictions depends on how high Tmax is relative to heat tolerance. Thirdly, we correlated the different estimates of Tmax among them and across species. Restrictions are strong for amphibians, arthropods, and birds; and inconsistent among reptiles and mammals. For marine fish, they describe a non-linear relationship that contrasted with terrestrial groups. Traditional heat tolerance measures in thermal vulnerability studies, like panting temperatures and the upper set point of preferred temperatures, do not predict Tmax or are inversely correlated to it, respectively. Heat tolerance restricts the geographic warm edges more strongly for species that reach sites with higher Tmax for their heat tolerance. These results underline the importance of reliable species’ heat tolerance indexes to identify their thermal vulnerability at their range’s warm edges. Besides, the tight correlations of Tmax estimates across on-land microhabitats support a view of multiple types of thermal challenges simultaneously shaping ranges’ warm edges for on-land species. Differently, the heterogeneous correlation of Tmax estimates in the Ocean supports the view that fish thermoregulation is more limited at coastal zones. Our results undermine arguments proposing that species of any realm (terrestrial or marine) are at systematically higher thermal risk and that heat tolerance is insensitive to geographic thermal gradients. We propose new hypotheses to understand thermal restrictions on animal distribution.