Meta-analytical evidence for the endocrine-disrupting properties of mercury exposure in wild birds

Mercury (Hg) contamination represents a major threat to biodiversity. To date, despite endocrine disruption being accepted as one of the main toxicological effects of Hg exposure, the evidence for such physiological impacts within an ecological context remains unclear. We carried out a systematic review and meta-analysis to evaluate the endocrine-disrupting properties of Hg on wild birds, using hormone concentrations as a proxy for endocrine status. Our results suggest an overall non-significant negative association between Hg exposure and hormone levels. We did not find any evidence of direct cytotoxicity on certain endocrine glands and/or of disruption of molecular pathways associated with hormone chemical structure. However, we identified hormone-specific effects of Hg, including a significant negative association with thyroxine and a significant positive association with triiodothyronine. We investigated whether toxicity was influenced by biological and methodological factors, but we found no differences in the effects of Hg i) between adults and chicks, ii) between males and females, iii) amongst breeding stages, iv) between correlational and experimental studies, or v) depending on the matrices used to quantify Hg and/or hormones. Our results support the potential of Hg as an endocrine disruptor under naturally occurring levels of exposure. Further research should focus on assessing multiple hormones simultaneously in both sexes, during different reproductive stages while using the same biological matrix for quantification of Hg and hormones. Although measuring circulating hormones in wildlife might be a relevant biomarker to identify sublethal effects of Hg, further efforts are needed to understand the consequences and mechanisms of hormonal disruption.