bioRxiv2019
Glucocorticoid hormones (GCs) are central mediators of metabolism and the response to challenges. Because circulating levels of GCs increase in response to challenges, within-population variation in GCs could reflect individual variation in condition or experience. At the same time, individual variation in the degree to which GCs increase in response to challenges (which is relatively consistent within individuals over time) could have causal effects on stress coping capacity, and the ability to survive and reproduce. Although a number of studies in vertebrates have tested whether within-population variation in GCs predicts components of fitness, it is not clear whether there are consistent patterns across taxa. Here we present the first phylogenetic meta-analysis testing whether within-population variation in GCs is associated with components of fitness across vertebrates. At the same time, we introduce and test predictions about an overlooked but potentially important mediator of GC-fitness relationships: life history context. We suggest that strong context-dependence in the fitness benefit of maintaining elevated GCs could obscure consistent patterns between GCs and fitness across taxa. Meta-analyses revealed that across vertebrates, baseline and stress-induced GCs were consistently negatively correlated with reproductive success. This relationship did not differ depending on life history context. In contrast, the relationships between GCs and survival were highly context dependent, differing according to life history strategy. Both baseline and stress-induced GCs were more strongly negatively associated with survival in longer-lived populations and species. Stress-induced GCs were also more negatively associated with survival in organisms that engage in relatively more valuable reproductive attempts. Fecal GCs did not predict survival or reproductive success. We also used a meta-analytic approach to test whether experimental increases in GCs had consistent causal effects on fitness. Experimental increases in GCs reduced both survival and reproductive success, although the latter relationship was not significant when accounting for phylogeny. Overall, these results support the prediction that GC-fitness relationships can be strongly context dependent, and suggest that incorporating life history may be particularly important for understanding GC-survival relationships. Future studies that explore the role of other aspects of context (e.g., the nature and frequency of stressors, environmental variation) within and across species could provide important insights how and when variation in GC regulation predicts fitness.