The evolutionary endocrinology of circulating glucocorticoids in free-living vertebrates: Recent advances and future directions across scales of study

Abstract

Circulating glucocorticoid hormone concentrations are dynamic, flexible, and promote adaptive responses following perturbations in an animal’s environment. As a result, circulating glucocorticoid levels are thought to shape fitness and have been suggested to be a key trait for predicting how species will cope with novel environmental change. Nevertheless, the factors that shape variation in glucocorticoid-mediated coping mechanisms remain unclear because the evolutionary underpinnings of the function and regulation of these hormones are poorly understood. Here, I summarize recent advances in our understanding of the evolution of circulating glucocorticoids, which have included (i) longitudinal studies exploring microevolutionary processes that shape within- and between-individual variation in glucocorticoids, (ii) interspecific comparative studies highlighting macro-evolutionary patterns of among-species variation in glucocorticoids, and (iii) intraspecific comparative studies which help to disentangle the relative roles of environment, life-history, and behavior in shaping among-population variation in glucocorticoids. Important avenues for future research will include exploring how natural selection may act on different components of the hypothalamus–pituitary–adrenal axis, characterizing patterns of phenotypic plasticity in circulating glucocorticoids across populations and species, as well as exploring how microevolutionary processes differ across taxa or gradients of environmental conditions.