ABSTRACT
The acute glucocorticoid response is a key mediator of the coordinated vertebrate response to unpredictable challenges. Rapid increases in glucocorticoids initiate a series of changes that can allow animals to effectively cope with or avoid stressors. It has become clear that the scope of the GC response—defined here as the absolute increase in GCs—is often associated with among-individual differences in performance and fitness and varies across species based on environment and life history. In addition to varying in scope, GC responses can differ enormously in speed; however, relatively little is known about whether speed and scope covary or how selection shapes variation in speed. We used a database of corticosterone samples collected at 5 time points from 1,750 individuals of 58 species of birds to ask i) how the speed and scope of the GC response covary among individuals and species and ii) whether variation among species in the speed of the response is predicted by environmental context or key life history traits. As predicted by a recent optimality model, faster absolute GC responses were strongly associated with a larger scope both among-individuals and among-species. Despite this covariation, the relative speed of the GC response (as a percentage of scope) varied independently of scope, suggesting that selection could operate on both features of the response independently. Species with faster relative GC responses lived in locations with more intra-season variation in temperature and had shorter lifespans. Our results suggest that rapid changes associated with the speed of the GC response, such as those occurring through non-genomic receptors, might be an important determinant of coping ability and we emphasize the need for studies explicitly designed to measure speed independently of scope.
Competing Interest Statement
The authors have declared no competing interest.
