Disentangling interactions between mercury, immunity, and infection in a Neotropical bat community

O_LIContaminants such as mercury are pervasive and can have immunosuppressive effects on wildlife. Impaired immunity could be important for forecasting pathogen spillover risks, as many land-use changes that generate mercury contamination can also bring wildlife reservoir hosts into close contact with humans and domestic animals. However, the interactions between contaminants, immunity, and infection are difficult to study in natural systems, and empirical tests of possible directional relationships remain rare. C_LIO_LIWe capitalized on extreme mercury variation in a diverse Neotropical bat community in Belize to test association between contaminants, immunity, and infection. By comparing a previous dataset of bats sampled in 2014 with new data from 2017, we first confirmed bat species with more aquatic prey in their diet had higher fur mercury. We then tested covariation between mercury and cellular immunity and determined if such relationships mediated associations between mercury and common bacterial pathogens (hemoplasmas and Bartonella). Lastly, as bat ecology can dictate exposure to mercury and pathogens, we also assessed species-specific patterns in these mercury-infection relationships. C_LIO_LIAcross the bat community, individuals with higher mercury had fewer neutrophils but not lymphocytes, suggesting stronger associations with innate immunity. However, the odds of infection for both pathogens were generally lowest in bats with high mercury, and relationships between mercury and immunity did not mediate infection patterns. When considering species-specific relationships between mercury and infection, mercury was associated with especially low odds for Pteronotus mesoamericanus and Dermanura phaeotis for hemoplasmas. Yet for Bartonella, mercury was associated with particularly low odds in the genus Pteronotus but especially high odds within the Stenodermatinae. C_LIO_LISynthesis and application: Lower infection risk in bats with high mercury despite weaker innate defense suggests contaminant-driven loss of pathogen habitat (i.e., anemia) or vector mortality as possible causal mechanisms. Strong positive associations between mercury and Bartonella in the Stenodermatinae, however, alongside negative associations between mercury and neutrophils in this clade, suggest these bats could be especially vulnerable to immunosuppression. Greater attention to these potential causal pathways could help disentangle the complex relationships between environmental contaminants, immunity, and infection in natural systems and forecast disease risks. C_LI