Validating an Immunoassay to Measure Fecal Glucocorticoid Metabolites in Yellow-Bellied Marmots

The yellow-bellied marmot ( Marmota flaviventer ) study at the Rocky Mountain Biological Laboratory near Crested Butte, Colorado, USA is the world’s second longest study of free-living mammals. Quantifying physiological stress is essential for understanding their health, reproductive success, and survival in a variable environment. Historically, we used a validated radioimmunoassay (RIA) to measure fecal glucocorticoid metabolites (FGMs). Given the costs and risks of working with radioisotopes, we have shifted to a more sustainable method. Here we evaluate the suitability of two competitive enzyme-linked immunosorbent assays (ELISA) from Cayman Chemical Company (CCC) and Arbor Assays (AA) to measure corticosterone levels in FGMs. The findings revealed that the AA ELISA, unlike the CCC ELISA, consistently matched the RIA in terms of accuracy across high and low corticosterone concentrations, demonstrated superior assay parameters, showed the highest correlations with RIA results and effectively captured the annual variations in FGM concentrations, indicative of its reliability for use in longitudinal studies. We further analytically validated the usage of the AA ELISA for FGMs, confirming its efficacy without matrix effects, thus establishing its suitability for ongoing and future studies of FGMs in marmots. The transition to the AA ELISA from the RIA ensures continued data integrity while enhancing safety and environmental sustainability. ### Competing Interest Statement The authors have declared no competing interest.