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

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 influences GC stress response variation among species is poor. However, identifying environmental causes of species-level variation in the GC stress 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 whether GC stress response of 22 reptile and 66 bird species exposed to a uniform capture stress protocol could be predicted from a GC stressor space model. This model predicts that species-level variation in GCs results from multivariate selection from environmental gradients and species-specific metabolic sensitivities. Thus, these multivariate selection interactions delimit the cumulative selection intensity causing GC stress 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 the CSR. 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 that GC stress response variation among species represents, in part, evolution to species-specific traits and large-scale environmental variation. Further improvement to predicting species variation in GC stress 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 plasma GC levels 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 in GC stress response as a metric for inferring how species might respond to global change.