Abstract
Cities are rapidly expanding, and global warming is intensified in urban environments due to the urban heat island effect. Therefore, urban animals may be particularly susceptible to warming associated with ongoing climate change. We used a comparative and manipulative approach to test three related hypotheses about the determinants of heat tolerance or critical thermal maximum (CT (max)) in urban ants-specifically, that (a) body size, (b) hydration status, and (c) chosen microenvironments influence CT (max). We further tested a fourth hypothesis that native species are particularly physiologically vulnerable in urban environments. We manipulated water access and determined CT (max) for 11 species common to cities in California's Central Valley that exhibit nearly 300-fold variation in body size. There was a moderate phylogenetic signal influencing CT (max), and inter (but not intra) specific variation in body size influenced CT (max) where larger species had higher CT (max). The sensitivity of ants' CT (max) to water availability exhibited species-specific thresholds where short-term water limitation (8 hr) reduced CT (max) and body water content in some species while longer-term water limitation (32 hr) was required to reduce these traits in other species. However, CT (max) was not related to the temperatures chosen by ants during activity. Further, we found support for our fourth hypothesis because CT (max) and estimates of thermal safety margin in native species were more sensitive to water availability relative to non-native species. In sum, we provide evidence of links between heat tolerance and water availability, which will become critically important in an increasingly warm, dry, and urbanized world that others have shown may be selecting for smaller (not larger) body size.