Prophylaxis in ant societies: a social response to pathogen vulnerability?

Social insects rely on multiple defence mechanisms to limit pathogen transmission within colonies. Prophylactic sanitary behaviours are performed in the absence of immediate pathogenic threats and contribute to routine disease prevention. Among these behaviours, grooming and necrophoresis help manage potentially harmful biological material, but how their expression relates to variation in pathogen susceptibility among colonies remains poorly understood. We investigated the relationship between susceptibility to the entomopathogenic fungus Beauveria bassiana and the expression of prophylactic grooming and necrophoresis in the ant Lasius flavus. We found strong and consistent intercolonial differences in pathogen susceptibility, as well as marked variation in the expression of prophylactic sanitary behaviours. None of the grooming behaviours examined were associated with pathogen susceptibility. However, more pathogen-susceptible colonies tended to remove corpses more rapidly during the early phase of necrophoresis. This pattern is consistent with a possible compensatory relationship among defence components, but may also arise from demographic and organizational differences among colonies, as worker age structure and task allocation can influence both susceptibility to infection and corpse removal behaviour. Overall, our results reveal substantial intercolonial variation in multiple components of disease defence but provide little support for a compensatory relationship between pathogen susceptibility and prophylactic sanitary behaviour.