Chytridiomycosis infection and heat compromise sperm quality in a threatened frog.

Environmental change is reshaping wildlife reproduction through increasing temperatures and often leads to the spread of emerging infectious diseases, yet the physiological consequences of managing these stressors remain poorly understood. Amphibians are particularly vulnerable owing to their ectothermy and high susceptibility to chytridiomycosis, caused by Batrachochytrium dendrobatidis (Bd). Here, we examined how Bd infection and thermal treatment interact to influence sperm quality and reproductive investment in male green and golden bell frogs (Ranoidea aurea), a species that has suffered severe population declines. Moderate Bd infection was associated with elevated sperm concentration relative to uninfected and heavily infected males, consistent with increased short-term reproductive investment under elevated mortality risk. However, severe infection led to pronounced reductions in sperm concentration and motility. Thermal treatment successfully eliminated Bd infection but imposed reproductive costs: sperm concentration declined following treatment and remained significantly reduced 6 months later, despite partial recovery of sperm motility and membrane integrity. Our findings reveal that disease and thermal stress jointly shape amphibian reproductive outcomes through context-dependent trade-offs between immune defence and gamete production. Although mild infection may trigger short-lived increases in reproductive output, both severe infection and pathogen clearance via thermal exposure impose lasting constraints on fertility. These results highlight an underappreciated cost of disease mitigation and suggest that increasing thermal extremes associated with climate change may further limit amphibian reproductive resilience, with important implications for conservation management and population persistence.