Fitness Benefits and Emergent Division of Labour at the Onset of Group-Living - Yuko Ulrich (University of Lausanne)

Sociality evolved repeatedly, yet the factors promoting group-living remain poorly understood. A proposed advantage of group-living is that it enables division of labour (DOL) among group members, leading to higher efficiency in larger groups. However, empirical data on such effects, and on the origins of the behavioural diversity underlying DOL, are lacking. Here, we use long-term automated behavioural tracking in colonies of the clonal raider ant Ooceraea biroi, combined with mathematical modelling, to show that increases in social group size generate behavioural variation and lasting specialization among genetically identical workers. Thus, the between-individual behavioural diversity underlying DOL can arise from changes in the social environment alone. These behavioural changes translate into large increases in per capita fitness, arguably via increases in colony-level homeostasis. This concomitant emergence of DOL and increased fitness already occurred at group sizes corresponding to the onset of group-living. The ability of DOL to self-organize in small, genetically homogeneous social groups as they grow can thus promote social cohesion at incipient stages of group-living.