On the correlated evolution of ecological lifestyle and thermal tolerance

The breadth of thermal tolerance delineates the upper (critical thermal maximum/upper critical temperature (CT_max/T_uc)) and lower (critical thermal minimum/lower critical temperature (CT_min/T_lc)) temperatures relevant to survival and/or persistence of organisms, and it is a correlate of extinction risk under climate change. Theory suggests that tolerance breadth evolves with the range of environmental temperatures. For instance, a narrow tolerance breadth is classically observed in tropical versus temperate species, and tropical ectotherms may feature increased extinction risk under climate change owing to the proximity of CT_max and mean environmental temperatures. Here, we underscore that an organism's lifestyle influences the extent of thermal fluctuation in its environment. We predict that subterranean species feature a narrower thermal tolerance breadth than surface-dwelling species, as the former evolve under dampened thermal variance. Using thermal limits data, we test this hypothesis in reptiles, mammals and arthropods. Subterranean species (n = 5-37 per taxon) featured reduced tolerance breadths compared with surface-dwelling species, and the difference was significant in reptiles and mammals; additionally, subterranean arthropods featured a significantly lower CT_max and higher CT_min than surface species. Thus, classical theory on thermal tolerance extends beyond patterns of geolocation to species lifestyle, where evolution under dampened thermal variance can reduce thermal tolerance breadth and influence other thermal traits.