Repeatability of glucocorticoid hormones in vertebrates: a meta-analysis

Abstract

We often expect that investigations of the patterns, causes, and consequences of among-individual variation in a trait of interest will reveal how selective pressures or ecological conditions influence that trait. However, many endocrine traits, such as concentrations of glucocorticoid (GC) hormones, exhibit adaptive plasticity and, therefore, do not necessarily respond to these pressures as predicted by among-individual phenotypic correlations. To improve our interpretations of among-individual variation in GC concentrations, we need more information about the repeatability of these traits within individuals. Many studies have already estimated the repeatability of baseline, stress-induced, and integrated GC measures, which provides an opportunity to use meta-analytic techniques to investigate (1) whether GC titers are generally repeatable across taxa, and (2) which biological or methodological factors may impact these estimates. From an intensive search of the literature, we collected 91 GC repeatability estimates from 47 studies. Overall, we found evidence that GC levels are repeatable, with mean repeatability estimates across studies ranging from 0.230 for baseline levels to 0.386 for stress-induced levels. We also noted several factors that predicted the magnitude of these estimates, including taxon, sampling season, and lab technique. Amphibians had significantly higher repeatability in baseline and stress-induced GCs than birds, mammals, reptiles, or bony fish. The repeatability of stress-induced GCs was higher when measured within, rather than across, life history stages. Finally, estimates of repeatability in stress-induced and integrated GC measures tended to be lower when GC concentrations were quantified using commercial kit assays rather than in-house assays. The extent to which among-individual variation in GCs may explain variation in organismal performance or fitness (and thereby inform our understanding of the ecological and evolutionary processes driving that variation) depends on whether measures of GC titers accurately reflect how individuals differ overall. Our findings suggest that while GC titers can reflect some degree of consistent differences among individuals, they frequently may not. We discuss how our findings contribute to interpretations of variation in GCs, and suggest routes for the design and analysis of future research.

Glucocorticoid levels are linked to lifetime reproductive success and survival of adult barn owls

“…Further, stress-induced but not baseline levels were positively correlated with female and male barn owl survival. The lack of viability selection on baseline corticosterone levels may be due to reduced statistical power or to the high sensitivity of baseline corticosterone levels to several environmental factors or life history stages (Angelier et al, 2009;Cockrem et al, 2009;Hennessy et al, 2015;Rensel & Schoech, 2011;Schoenemann & Bonier, 2018). Overall, our results suggest that corticosterone levels in the barn owl are under positive directional selection (if at all).…”

Differences in adrenocortical responses between urban and rural burrowing owls: poorly-known underlying mechanisms and their implications for conservation

“…However, our results do not allow us to know whether these differences in the physiological responses of urban and rural individuals are plastic responses toward differences in the degree of human perturbation or result from a non-random distribution of individuals showing fixed, unchanging, physiological profiles. Although some studies on this topic have been conducted ( Schoenemann and Bonier, 2018 ; Taff et al , 2018 ), there is no agreement regarding how much of the stress-induced response of individuals remains constant throughout their lives and how much corresponds to individual responsiveness to environmental variation. This information is paramount to understanding the mechanisms underlying the differences observed between urban and rural individuals, as natural selection and matching habitat choice rely on the assumption that the traits of interest remain constant through an individual’s lifespan and are heritable, while habituation (phenotypic plasticity) assumes that organisms are able to respond to variation in the environment by modifying their phenotype (although there are studies showing that plasticity per se can also be heritable and genetically fixed; Hallsson and Björklund, 2012 ; Scheiner et al , 2012 ; Gomez-Mestre and Jovani, 2013 ).…”

The effects of body region, season and external arsenic application on hair cortisol concentration

“…The ICC of 0.19 also indicates that there is little tendency for measurements within the same marmot to be similar, meaning that there is low repeatability of GC measurements in this population. In a recent meta-analysis, Schoenemann and Bonier (2018) found that the mean repeatability of GC measurements in other vertebrates (when measured using substrates such as hair and feathers) was 0.32, with a 95% confidence interval of 0.24–0.41. Thus, our results suggest below average repeatability.…”