Exploring the effects of horizontal pathogen transmission on mortality and behaviour in a cooperatively breeding spider

Social interactions are a driving force behind disease outbreaks in animal societies. As groups become more complex and build permanent nests, they run the risk of introducing more routes of potential pathogen exposure. Eusocial societies have well-established ‘superorganism immunity’ that protects against disease. However, fewer studies have investigated social immunity in cooperative breeders, which may provide insight into the evolution of immunity as a form of collective behaviour during major evolutionary transitions in sociality. We exposed groups of social spiders (Stegodyphus dumicola) to a generalist entomopathogenic fungus (Metarhizium robertsii) using three modes of exposure: directly onto a groupmate, mechanically vectored by prey and nest exposure. We compared spider mortality between exposed groups and pathogen-free controls. We measured space use and colony fragmentation to test whether pathogen exposure route and disease severity affected spiders' aggregation and polydomy behaviour. We found that route of exposure greatly affected spider mortality, with direct exposure resulting in the most rapid mortality. Individuals from groups with a directly exposed spider were also more likely to be observed outside their nest compared to all other treatments. Exposure did not result in differences in colony fragmentation. These data demonstrate that the route of pathogen introduction not only affects the severity of disease outbreaks but can also alter behaviours relevant for social immunity. This study enhances our understanding of the transitional steps in social immunity among cooperative breeders that may lead to the evolution of superorganism immunity.