Why not both? A case study measuring cortisol and corticosterone in poison frogs

Abstract

AbstractA general tenet in stress physiology is that the hypothalamic-pituitary-adrenal (HPA) axis predominantly produces one glucocorticoid (GC) in response to stressors. However, the two main glucocorticoids produced by vertebrates – cortisol and corticosterone – show variation across species in absolute levels, relative proportions, and stress responsivity. Therefore, focusing on a single GC may not tell the whole story. In this study, we measured both cortisol and corticosterone in water samples collected from five species of Dendrobatid poison frogs. Understanding the full picture of the physiological stress response in poison frogs is of special interest due to their susceptibility to environmental stressors and conservation status. Based on pioneering studies of steroidogenesis in a select few species of amphibians, corticosterone is thought to be dominant and more abundant in all amphibians. Counter to this idea, we found water-borne cortisol was more abundant than corticosterone in three of the five species.Dendrobates tinctoriusandPhyllobates terribilisreleased equal amounts of cortisol and corticosterone. When we stimulated the HPA axis inD. tinctorius, we found water-borne corticosterone was responsive to adrenocorticotropic hormone (ACTH), while cortisol did not change. Comparing plasma and water samples, we found water-borne cortisol was positively correlated with cortisol in plasma and the relationship between plasma and water-borne corticosterone was positive but not statistically significant. These results challenge the broadly accepted assumption that corticosterone is ‘dominant’ in amphibians and add to the growing literature highlighting the importance of measuring both GCs to understand (stress) physiology.