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

Assessing physiological stress in wild populations is essential for detecting early signs of environmental deterioration and for informing conservation strategies before population declines become evident. Non-invasive tools such as water-borne corticosterone (WB CORT) sampling offer a promising means to monitor stress responses in free-living amphibians. However, a central concern in conservation physiology is whether methods validated in laboratory settings provide ecologically relevant estimates when applied to wild individuals exposed to complex environments. Here, we tested the utility and ecological relevance of WB CORT sampling in larvae of the European common frog ( Rana temporaria) by comparing wild-caught and lab-reared larvae. First, we examined whether environmental origin influenced baseline physiological traits by modeling changes in WB CORT and body mass across ontogeny. Wild larvae exhibited significantly higher baseline WB CORT levels and lower body mass than their lab-reared counterparts, suggesting increased allostatic load under field conditions and divergent growth trajectories. We evaluated the sensitivity of these traits to acute (48 h) exposure to nitrate, a widespread agricultural pollutant commonly found in amphibian breeding ponds. While CORT responses were not significantly altered, nitrate exposure led to a significant reduction in body mass in wild but not lab-reared larvae, indicating origin-specific sensitivity. Finally, we evaluated the validity of WB CORT as a proxy for tissue CORT. WB CORT and tissue CORT levels were positively correlated in both origin groups, with no significant difference in slope or strength of correlation, supporting the reliability of WB CORT as a non-invasive measure of physiological state in both wild and lab-reared larvae. Our findings demonstrate that WB CORT sampling captures biologically meaningful variation in amphibian stress physiology and is a valid tool for monitoring across environmental contexts. Integrating such physiological metrics into field assessments can help identify vulnerable populations and guide proactive amphibian conservation strategies.