Group size matters: ant cleanliness is next to crowdedness
David James Phair, David James Phair , Cang Hui , Theresa Clair Wossler
Department of Botany and Zoology, Stellenbosch University, Stellenbosch, South Africa; Department of Botany and Zoology, Stellenbosch University, Stellenbosch, South Africa ; Department of Mathematical Sciences, Stellenbosch University, Stellenbosch, South Africa ; Department of Botany and Zoology, Stellenbosch University, Stellenbosch, South Africa
Social insects are numerically and ecologically dominant within ecosystems. Despite social living facilitating their success, eusociality potentially increases pathogen and parasite susceptibility through the high density and genetic relatedness of nestmates. The scarcity of large scale epidemics in these social insects, particularly ants, has been attributed to “social immunity”. Individual level behaviours, such as allogrooming, offer colony-level benefits by preventing or limiting pathogen or parasite spread. This study investigated the rate and frequency of hygienic behaviours between increased densities of nestmates within South African ant species. South African ant species were exposed to spores of a generalist entomopathogenic fungus Metarhizium brunneum. An infected individual was introduced to groups of nestmates increasing in local density from one to 25. We assessed the frequency and duration of behaviours, namely allogrooming and self-grooming involving the focal ant. Focal ants spent much time inactive but the duration and frequency of self- and allogrooming increased with increasing local density implying that exposed ants are more regularly groomed in larger groups. Self-grooming was more common than allogrooming and increased with local density which suggests that exposed individuals may detect their own exposure and act altruistically to reduce their spore load and potential transmission to nestmates. As colony density increases contact between nestmates becomes more frequent increasing the potential of secondary transmission events and ants thus become more reliant on prophylactic measures, such as grooming, to prevent disease epidemics.