Fitness benefits and emergent division of labour at the onset of group living.

Abstract

The initial fitness benefits of group-living are considered the greatest hurdle to the evolution of sociality1, and theory predicts that they need to arise at very small group sizes2. Such benefits are thought to emerge partly from scaling effects that increase efficiency as group size increases3–5. In social insects and other taxa, they have been proposed to stem from division of labor (DOL)5–8, which is characterized by between-individual variability and within-individual consistency (specialization) in task performance. At the onset of sociality, however, groups were likely small and composed of similar individuals with potentially redundant rather than complementary function1. Theory suggests that DOL can emerge even in relatively small, simple groups9,10. However, empirical data on the effects of group size on DOL and fitness remain equivocal6. Here, we use long-term automated behavioral tracking in clonal ant colonies, combined with mathematical modeling, to show that increases in social-group size can generate DOL among extremely similar workers, in groups as small as six individuals. These early effects on behavior were associated with large increases in homeostasis—the maintenance of stable conditions in the colony11— and per capita fitness. Our model suggests that increases in homeostasis are primarily driven by increases in group size itself, and, to a smaller extent, by higher DOL. Overall, our results indicate that DOL, increased homeostasis, and higher fitness can naturally emerge in small, homogeneous social groups, and that scaling effects associated with increasing group size can thus promote social cohesion at incipient stages of group-living.