Macrostress: do large‐scale ecological patterns exist in the glucocorticoid stress response of vertebrates?

Summary Disturbances, or stressors, are pervasive in nature and profoundly influence fitness of individuals and, ultimately, species extinction risk. Glucocorticoid (GC) hormones affect multiple physiological and behavioural traits regulating the responses of vertebrates to stressors. Current understanding of what influencesGCstress response variation among species is poor. However, identifying environmental causes of species‐level variation in theGCstress response is important to understanding macroevolutionary patterns in this key physiological system and potentially offers a metric for evaluating species risk to global change. We tested whetherGCstress response of 22 reptile and 66 bird species exposed to a uniform capture stress protocol could be predicted from aGCstressor space model. This model predicts that species‐level variation inGCs results from multivariate selection from environmental gradients and species‐specific metabolic sensitivities. Thus, these multivariate selection interactions delimit the cumulative selection intensity causingGCstress response variation among vertebrates. Using phylogenetic generalized least square regression analysis, we ranked models incorporating putative variables pertaining to the corticosterone stress response (CSR) for reptiles and birds separately. The top‐ranked models explained between 14% and 33% of species variation in theCSR. Models indicated that combinations of variables including increasing body mass, net primary productivity and latitude influenced GC stress responsiveness in reptiles and birds. We conclude thatGCstress response variation among species represents, in part, evolution to species‐specific traits and large‐scale environmental variation. Further improvement to predicting species variation inGCstress response could be achieved by expanding our stressor space model to integrate microevolutionary and ecological processes. There is a pressing need to identify physiological traits that predict species responses to environmental change. Conceptually, the use of plasmaGClevels as a species‐specific marker for environmental tolerance could be plausible. However, we identify three major knowledge gaps and ensuing research areas to aid understanding of the utility of variation inGCstress response as a metric for inferring how species might respond to global change.