Androgenic agricultural pollution suppresses immune function and alters reproductive allocation in eastern mosquitofish.

Anthropogenic pollution poses a serious threat to wildlife. Particularly concerning are endocrine-disrupting chemicals (EDCs) due to their ability to mimic or interfere with hormone signalling. Given the fundamental role of hormones in regulating immunity and reproduction, androgenic EDCs are predicted to impact immunocompetence and associated life-history trade-offs. Yet, despite their potency and environmental ubiquity, the effects of androgenic EDCs on immune function and reproductive allocation remain poorly understood. Here, we tested how exposure to the androgenic agricultural pollutant 17β-trenbolone affects immunity and reproductive investment in eastern mosquitofish (Gambusia holbrooki). Adult mosquitofish were exposed to environmentally relevant concentrations of trenbolone (control: 0 ng/l, low: 11 ± 2.9 ng/l, high: 101 ± 13.6 ng/l) for 45 days. Immune function was assessed using phytohaemagglutinin (PHA)-induced skin swelling, white blood cell (WBC) counts and neutrophil-lymphocyte ratio (NLR), while male reproductive investment was quantified by sperm production. Trenbolone exposure significantly impaired immunity, reducing PHA-induced swelling by ~25%, decreasing total WBC counts and increasing NLRs, a biomarker of inflammation and immune response. In contrast, trenbolone exposure increased sperm number, and we found an overall negative correlation between immune response and sperm production. Our findings highlight how androgenic pollutants can simultaneously compromise immunity and alter reproductive allocation, enforcing evolutionary trade-offs that could negatively affect individual fitness and population resilience in contaminated environments.