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.