Meeting Abstract
Cities are rapidly expanding, and global warming is intensified in urban environments due to the urban heat island effect. Thus, urban animals may be particularly susceptible to climate change given their small thermal safety margins. We used two experiments to test three hypotheses related to understanding determinants of critical thermal maxima (Tmax) in urban ants—specifically, that (1) body size, (2) preferred micro-environments, and (3) hydration status influence Tmax. First, we manipulated water access (water provided ad lib. or restricted for 8 h) and determined Tmax (knockdown temperature; range: 26-61°C) for 11 species common to cities in California’s Central Valley that exhibit nearly 300-fold variation in body mass. Larger species had higher Tmax after controlling for phylogeny, and water limitation in some, but not all, species reduced Tmax. However, Tmax was not influenced by the estimated active temperature (Tactive) of ants or ant water content. Second, we used another water limitation treatment level (water restriction for 32 h) and more estimates of Tactive in two focal, similar-sized species with different hydric and thermal sensitivities. Specifically, they varied greatly in Tactive and Tmax (Prenolepis imparis: 22°C and 40°C; Formica moki: 30°C and 51°C; respectively) and their Tmax responded differently to water limitation (8 h water limitation reduced Tmax in P. imparis, but not F. moki). Water limitation for 8 and 32 h similarly reduced Tmax in P. imparis, but only 32 h of limitation reduced Tmax in F. moki. Neither Tactive nor body size affected Tmax in either species. In sum, we found varied support for our hypotheses: inter- (but not intra-) specific variation in body size influenced Tmax, Tactive did not influence Tmax, and the sensitivity of Tmax to water availability varied across species. Our results highlight the importance of water as a valuable resource for animals in the Anthropocene.
