“Fitness benefits and emergent division of labor at the onset of group-living”

The initial fitness benefits of group-living are considered the greatest hurdle to the evolution of sociality(1), and theory predicts that they need to arise at very small group sizes(2). Such benefits are thought to emerge partly from scaling effects that increase efficiency as group size increases(3–5). 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 function(1). Theory suggests that DOL can emerge even in relatively small, simple groups(9,10). However, empirical data on the effects of group size on DOL and fitness remain equivocal(6). 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 colony(11)— 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.