Bringing the Lab to the Field: Exploring Water‐Borne Corticosterone as a Physiological Indicator in Captive and Wild Common Frog Larvae (Rana temporaria)

Assessing physiological condition in wild populations is important for understanding how environmental variation shapes organismal performance. Water-borne corticosterone (WB-CORT) sampling shows promise for studying amphibian stress physiology, but its ecological relevance and limitations require evaluation, particularly when methods developed under laboratory conditions are applied to wild individuals. Here, we evaluated WB-CORT sampling in common frog (Rana temporaria) larvae by comparing field-collected and laboratory-reared individuals. We examined whether origin influenced baseline physiological traits by modeling ontogenetic changes in WB-CORT and body mass, an integrative measure of growth. Across ontogeny, laboratory-reared larvae generally showed lower WB-CORT release and higher body mass than field-collected larvae, with distinct developmental trajectories in both traits. Next, we evaluated the sensitivity of WB-CORT and body mass to acute (48 h) nitrate exposure, a widespread pollutant in amphibian breeding ponds. Nitrate treatment did not affect post-exposure WB-CORT release in either origin group. Field-collected larvae lost body mass across all treatments, including controls, whereas laboratory-reared larvae showed nitrate-specific mass loss only at 100 mg/L. Finally, we assessed whether total WB-CORT release reflected internal corticosterone by examining its relationship with tissue CORT concentrations. WB-CORT and tissue CORT were positively related in both groups, supporting WB-CORT as a minimally disruptive physiological measure under the conditions tested. Together, these findings show that WB-CORT captures biologically meaningful variation in internal corticosterone under the conditions tested, but did not detect a sustained response to the acute nitrate challenge. WB-CORT should therefore be viewed as a promising, context-dependent physiological indicator rather than a stand-alone stress biomarker.