Mercury contamination impairs the hypothalamic-pituitary-adrenal axis of a neotropical caiman

The capacity to regulate corticosterone release following a stressful event is an essential component of the physiological flexibility required to maintain homeostasis and ensure adaptive responses to environmental change. We show that wild smooth-fronted caimans with elevated blood mercury (Hg) concentrations exhibited reduced peak corticosterone levels during standardized stress tests, indicating a dampened maximal stress response. Hg blood concentrations were negatively correlated with corticosterone responsiveness, suggesting impairment of hypothalamic-pituitary-adrenal axis function. These results demonstrate that environmental Hg contamination is associated with reduced stress responsiveness and physiological flexibility, which likely limits the ability of Hg-contaminated individuals to cope with novel environmental stressors. We found that a methylmercury-sensitive cysteine in the glucocorticoid receptor is highly conserved across vertebrates, suggesting a shared molecular mechanism for mercury effects on glucocorticoid signaling. Given the widespread Hg pollution in tropical ecosystems, these findings emphasize the role of Hg as an endocrine-disrupting chemical in crocodylians.