City limits: Heat tolerance is influenced by body size and hydration state in an urban ant community

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 (CTmax) in urban ants—specifically, that (a) body size, (b) hydration status, and (c) chosen microenvironments influence CTmax. We further tested a fourth hypothesis that native species are particularly physiologically vulnerable in urban environments. We manipulated water access and determined CTmax 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 CTmax, and inter (but not intra) specific variation in body size influenced CTmax where larger species had higher CTmax. The sensitivity of ants’ CTmax to water availability exhibited species‐specific thresholds where short‐term water limitation (8 hr) reduced CTmax and body water content in some species while longer‐term water limitation (32 hr) was required to reduce these traits in other species. However, CTmax was not related to the temperatures chosen by ants during activity. Further, we found support for our fourth hypothesis because CTmax 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.

Does social thermal regulation constrain individual thermal tolerance in an ant species?

“…It is not known whether these size differences are genetically determined, or whether they result from phenotypic plasticity. In contrast, studies in other ant species did not find a significant relationship between thermal tolerance and body size (Foucaud et al, 2013;Johnson & Stahlschmidt, 2020). When Modlmeier, Pamminger, Foitzik, and Scharf (2012) studied plasticity in cold tolerance in the ant Temnothorax nylanderi, they found no correlation between worker size and the ability to recover from a chill-induced coma; they did, however, highlight the importance of acclimation and suggested that cold tolerance was determined more by differences in physiology than by differences in behaviour or worker size.…”

Life-history traits predict responses of wild bees to climate variation

“…Based on our findings, increasing summer temperatures in montane systems will likely be associated with a decrease in the relative abundance of larger bodied bees. Body size of insects can have profound effects on their physiologies and thermal heat tolerances [56,57]. Bees have been shown to have species-specific differences in thermal heat tolerances [58–60], and some of this variation can be linked to species traits, such as body size.…”

Critical thermal limits in ants and their implications under climate change

“…Analyses conducted at much smaller geographical scales have found little or no phylogenetic signal. For urban ants in California only a moderate phylogenetic signal was detected in CT max for 11 species (Johnson & Stahlschmidt, 2020). Bujan et al (2020b) studied an ant community in the USA across different seasons and found no phylogenetic signal in CT max for six genera active in the winter, 17 genera active in the spring, and 13 genera active in the summer.…”

“…Analyses conducted at much smaller geographical scales have found little or no phylogenetic signal. For urban ants in California only a moderate phylogenetic signal was detected in CT max for 11 species (Johnson & Stahlschmidt, 2020). Bujan et al .…”