Glucocorticoids and Fitness
Glucocorticoids influence allocation of resources between reproduction and survival. Current theory posits three hypotheses for GC regulation of allocation strategies. The CORT-tradeoff hypothesis predicts that increasing GCs will suppress reproduction while enhancing survival. The CORT-fitness hypothesis, however, predicts that as GCs increase, all fitness metrics will decline. The CORT-adaptation hypothesis predicts that increased GCs will enhance reproduction. Tests of these hypotheses have produced highly variable results. Data from our lab support the CORT-fitness hypothesis, demonstrating that elevated glucocorticoids in free-living white-crowned sparrows are associated with reduced reproduction and increased survival. However, new data in bluebirds indicates that elevated glucocorticoids increase feather color at a cost to feather structure (enhancing likely reproductive success at a cost to predator escape/migration efficiency). Generally we believe these tests are confounded by variation in resource availability, and so current work in our lab is testing between these three hypotheses within a resource framework.
Mechanisms of Hormone Action
Glucocorticoids are organismal regulators influencing broad physiological systems. However, GC secretion is only one step in the process from perception to functional organismal change. Glucocorticoid physiology can be heavily influenced by plasma binding globulins, and tissue-specific levels of activating/deactivating enzymes and hormone receptors. These ‘downstream’ aspects of stress physiology can drive very different patterns in organismal output, and cannot be predicted based on hormone patterns alone. Incorporating these downstream components into studies of the organismal stress response is an important component to clarify the role of ‘stress physiology’ as a regulator of organismal function. We incorporate measure of binding golubilins into the majority of GC studies in the lab, and focus on tissue levels of receptors across many of our studies.
The developmental environment has strong and pervasive effects on animal phenotype. Exposure to stress during development (in the form of elevated glucocorticoid hormones or nutritional deprivation) is one environmental cue that can have strong formative effects on morphology, physiology, and behavior. Although many of the effects of developmental stress appear negative, there is increasing evidence for an adaptive role of developmental stress in shaping animal phenotype. Several graduate students in the lab have focused on stress during development, and the long term phenotypic changes associated with it.