Embryonic exposure to mercury contamination is associated with elevated corticosterone and lower body condition in wild Smooth-fronted caiman hatchlings

Mercury (Hg) is a worldwide contaminant that affects all ecosystems, yet most evidence of its biological impacts comes from fish, birds, and mammals, leaving reptiles comparatively understudied. Crocodylians may be particularly vulnerable to Hg due to their long lifespans, high trophic positions, and occurrence in tropical and subtropical regions affected by artisanal and small-scale gold mining. We investigated the effect of Hg contamination during embryonic development in the Smooth-fronted caiman (Paleosuchus trigonatus, n = 15), using claws, scutes and red blood cells (RBC)as a non-invasive proxy for in-ovo exposure. We found a positive relationship between plasma corticosterone (CORT) concentration and claw (R2 = 0.65, p < 0.001), scute (R2 = 0.67, p < 0.001) and RBCs (R2 = 0.39, p = 0.018) Hg concentrations, suggesting that even low embryonic Hg exposures may alter the endocrine status of hatchlings. We further assessed body condition in relation to CORT concentration to explore potential phenotypic effects and found a negative relationship (R2 = 0.48, p = 0.004). Our findings support the use of keratinized tissues as a minimally invasive biomonitoring matrix for assessing in-ovo Hg exposure in crocodylians and also highlight the need to integrate contaminant dynamics into conservation strategies for species inhabiting Hg-impacted ecosystems.