Stress in a conservation context: A discussion of glucocorticoid actions and how levels change with conservation-relevant variables

Over the past few decades, there has been a steep increase in the number of conservation-related field studies that measure glucocorticoid hormones (corticosterone or cortisol) as a marker for stress. Endocrine tools show great potential for informing conservation, however interpretation of results is often complicated. This paper reviews the role of glucocorticoids for non-physiologists, evaluates select applications of glucocorticoid measures to conservation field studies, and proposes a theoretical model to focus future research. Because levels of glucocorticoids typically increase in response to challenge and sometimes predict mortality or decrease reproduction, it is often assumed that high levels of glucocorticoids indicate stress. However, literature review suggests that glucocorticoid measures fail to change consistently in a predictable manner with adverse conditions and do not always show a linear correlation with survival or reproductive success. Inconsistencies relate in part to methodological problems but also have a physiological basis. We propose that relationships between (1) glucocorticoids and fitness and (2) glucocorticoids and disturbance may be more log quadratic rather than linear. We hope these models will be useful in generating predictions for future studies and resolving the inconsistencies that currently complicate interpretation for conservation endocrinologists.