Ants express risk-adjusted sanitary care
Being cared for when sick is a benefit of sociality that can reduce disease and improve survival of group members. However, individuals providing care risk contracting infectious diseases themselves. If they contract a low pathogen dose, they may develop micro-infections that do not cause disease, but still affect host immunity by either decreasing or increasing the hosts vulnerability to subsequent pathogen infections. Caring for contagious individuals can thus significantly alter the future disease susceptibility of caregivers. Using ants and their fungal pathogens as a model system, we here tested if the altered disease susceptibility of experienced caregivers, in turn, affects their expression of sanitary care behaviour. We found that micro-infections contracted during sanitary care had protective or neutral effects upon secondary exposure to the same (homologous) pathogen, but consistently induced high mortality upon super-infection with a different (heterologous) pathogen. In response to this risk, the ants selectively adjusted the expression of their sanitary care. Specifically, the ants performed less grooming yet more antimicrobial disinfection, when caring for nestmates contaminated with heterologous pathogens as compared to homologous ones. By modulating the components of sanitary care in this way, the ants reduced their probability of contracting super-infections of the harmful heterologous pathogens. The performance of risk-adjusted sanitary care reveals the remarkable capacity of ants to react to changes in their disease susceptibility, according to their own infection history, and to flexibly adjust collective care to individual risk.