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

Abstract

Summary1. 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. 2. 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. 3. 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. 4. 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.

Equipped for Life in the Boreal Forest: The Role of the Stress Axis in Mammals

“…In addition, the primary GC plasma carrier protein, corticosteroid-binding globulin (CBG), plays a major role in modulating the impact of the stress response. CBG binds GCs in blood, as only free GCs appear to enter cells and have biological effects; lower CBG concentrations mean more GCs are free (Breuner et al, 2013). CBG levels can vary during reproduction, are reduced under chronic stress, and vary among species for reasons that are unclear (e.g., from our Yukon work see Boonstra et al, 1998;Boonstra and McColl, 2000;Delehanty and Boonstra, 2011; note that numerous other studies in both the laboratory and field have made similar findings [reviewed in Desantis et al, 2013]).…”

Experimental relationships between levels of corticosterone in plasma and feathers in a free-living bird

“…Additionally, increases in levels of CORT binding globulin (CBG), alone or in concert with increased clearance of CORT from the general circulation, could have reduced levels of free CORT by day 11. How CBGs respond to stressors (or exogenous CORT) is context and species specific (Breuner et al, 2013), and CORT pellets like the ones we used can increase CBG capacity (Müller et al, 2009a). Therefore, it is possible that such an increase in CBG capacity decreased the free CORT available for deposition into feathers.…”

Variation in adrenal and thyroid hormones with life-history stage in juvenile northern elephant seals (Mirounga angustirostris)

“…Catecholamines, such as epinephrine and norepinephrine, are also released as part of the fight-or-flight response to stress. Together, the suite of hormones activated by this response influence many processes in the body including heart rate, metabolic rate, energy allocation, growth, reproduction, immune response, memory, and behavior, allowing an animal to react appropriately to the stress, which ultimately promotes survival (Breuner et al, 2013;Brilion et al, 1995;Connell and Davies, 2005;Khani and Tayek, 2001;Maule et al, 1987;Romero and Butler, 2007;Tilbrook et al, 2000).…”