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

A general tenet in stress physiology is that the hypothalamic-pituitary-adrenal (HPA) axis predominantly produces one glucocorticoid (GC) in response to stressors. However, most vertebrates produce both cortisol and corticosterone, and these steroids show variation across species in absolute levels, relative proportions, and stress responsivity and regulate much more than just stress physiology. Therefore, focusing on a single GC within a species may not capture the whole story. In the present study, we set out to validate non-invasive waterborne hormone measurements in our focal species, the dyeing poison frog Dendrobates tinctorius. In pursuing this goal, we uncovered unexpected patterns of GC abundance within and across species of poison frogs. D. tinctorius had higher amounts of corticosterone than cortisol in both plasma and waterborne samples, and corticosterone was responsive to adrenocorticotropic hormone (ACTH) as canonically assumed. However, corticosterone and cortisol levels were surprisingly similar in D. tinctorius, and cortisol was more abundant than corticosterone in water samples from four additional poison frog species. These results challenge the broadly accepted assumption that corticosterone is always more abundant in amphibians and add to the growing literature highlighting the importance of measuring both GCs to understand (stress) physiology.