Skin Deep: Cortisol Dominance in Juvenile Lesser Sirens (Siren intermedia) Revealed by Dermal and Water-Borne Glucocorticoid Sampling

In vertebrates, corticosterone and cortisol are glucocorticoid (GC) steroid hormones central to the vertebrate stress response, but their relative contribution depends on context and life history. We used water-borne hormone sampling in lesser sirens, Siren intermedia, to examine whether corticosterone or cortisol predominates in stress responses and evaluated whether dermal secretions reliably track acute corticosterone changes. First, we measured corticosterone via dermal swabbing over a 2 h period following a hand-restraint stressor. Dermal corticosterone did not increase relative to pre-stressor baseline levels during the 2 h sampling period, suggesting that dermal measurements may not reflect acute circulating GC changes. We then measured cortisol and corticosterone across the GC profile (baseline, stressed, and recovery) using water-borne sampling techniques in juveniles. Cortisol release rates were significantly higher than corticosterone and were elevated during the recovery phase relative to baseline (p = 0.08), whereas stressed samples did not differ from baseline. Additionally, cortisol release rates were positively associated with temperature, while no effect of temperature was detected on corticosterone. Although cortisol and corticosterone were strongly correlated, cortisol responded to environmental temperatures and was repeatable, whereas corticosterone was not. Finally, we validated the use of water-borne sampling techniques to measure cortisol and corticosterone in S. intermedia. Cortisol was higher in juvenile sirens, highlighting that each GC may play functionally distinct roles across contexts. Together, these findings indicate that cortisol is the more environmentally responsive GC in juvenile S. intermedia and that water-borne sampling provides a more reliable method for assessing acute stress physiology in this species.