Evaluating stress in natural populations of vertebrates: total CORT is not good enough

Summary Our goal in this review is to discuss how measures beyond simple quantification of total glucocorticoid levels are needed in comparative studies of stress. We need to measure corticosteroid binding globulin – CBG – and further downstream performance metrics to properly evaluate the significance and impact of stress in wild populations. We briefly cover the current literature, discuss methods that may enable detection of chronic stress and point to directions for future research to continue to clarify this field. CBG may regulate access of hormones to tissues, but disagreement remains as to the functional importance of total vs. free vs. bound hormone in the plasma. Here we focus on recent studies providing solid evidence supporting the free hormone hypothesis. These studies unequivocally indicate that the biologically active fraction consists only of the portion that is free (not total levels in the blood). We also present the ‘reservoir hormone hypothesis’, indicating the relevance of CBG ‐bound hormone in the plasma. The vast majority of physiological studies on stress in natural populations only measure blood corticosterone or its faecal metabolites. However, downstream metrics (e.g. immune function, oxidative stress and body mass changes) can assess the physiological impact of the changes in corticosterone and CBG and ultimately of the adaptiveness of these changes. Here we will discuss some of the most promising factors that can be measured downstream of the plasma and the rationale for their measurement, how to do it, and an introduction to the evidence. Although we draw on biomedical findings for some of this insight, we recognized that findings from a few mammalian laboratory species (primarily rats and mice) may not apply to the vast diversity of vertebrate species from fish to birds. In large measure we are still in the natural history phase of the accumulation of fundamental knowledge in terms of trying to extract general principles in the stress biology of natural populations. Our review outlines what is needed to lay the foundation for these principles.