Bringing the Lab to the Field: Validating Water-Borne Corticosterone as a Conservation Tool in Captive and Wild Amphibian Larvae

Assessing physiological conditions in wild populations is critical for detecting early environmental deterioration and informing conservation before population declines occur. Non-invasive tools such as water-borne corticosterone (WB-CORT) sampling show promise for monitoring amphibian stress physiology, but their ecological relevance and limitations must be carefully evaluated, particularly when laboratory-developed methods are applied to wild individuals in complex environments. Here, we tested the utility of WB-CORT sampling in larvae of the European common frog (Rana temporaria) by comparing wild-caught and laboratory-reared individuals. First, we examined whether environmental origin influenced baseline physiological traits by modelling ontogenetic changes in WB-CORT and body mass, an integrative measure of growth. Wild larvae exhibited higher baseline WB-CORT levels and lower body mass than their laboratory-reared counterparts, suggesting divergent corticosterone profiles and growth trajectories between both environments. Next, we evaluated the sensitivity of WB-CORT and body mass to acute (48h) nitrate exposure, a widespread agricultural pollutant commonly found in amphibian breeding ponds. Nitrate exposure did not alter WB-CORT levels in either origin-group. Body mass declined in wild larvae across all treatments, including controls, whereas laboratory-reared larvae maintained stable mass except at the highest nitrate concentration, indicating origin-dependent responses not mirrored by corticosterone release. Finally, we explored WB-CORT as a proxy for internal corticosterone by examining its relationship with tissue CORT concentrations. WB-CORT and tissue levels were positively related in both wild and laboratory-reared larvae. This association supports the use of WB-CORT as a non-invasive measure of corticosterone under the specific conditions tested. Together, our findings show that WB-CORT can reflect variation in corticosterone levels between laboratory-reared and field-collected larvae, but that its responsiveness to environmental challenges is context dependent, highlighting both the potential and limitations of WB-CORT. We conclude by emphasizing the need for further evaluation across species, life stages, and stressors before broader application in amphibian conservation. Lay summaryWe tested water-borne corticosterone (WB-CORT) as a non-invasive indicator of corticosterone levels in wild and laboratory-reared tadpoles exposed to the pollutant nitrate. Wild tadpoles showed declining body mass during the experiment regardless of pollutant exposure, with no corresponding change in corticosterone. WB-CORT reflected internal hormone levels but requires broader validation.