The initial fitness benefits of group living are considered to be the greatest hurdle to the evolution of sociality1, and evolutionary theory predicts that these benefits need to arise at very small group sizes2. Such benefits are thought to emerge partly from scaling effects that increase efficiency as group size increases3,4,5. In social insects and other taxa, the benefits of group living have been proposed to stem from division of labour5,6,7,8, which is characterized by between-individual variability and within-individual consistency (specialization) in task performance. However, at the onset of sociality groups were probably small and composed of similar individuals with potentially redundant—rather than complementary—function1. Self-organization theory suggests that division of labour can emerge even in relatively small, simple groups9,10. However, empirical data on the effects of group size on division of labour and on fitness remain equivocal6. Here we use long-term automated behavioural tracking in clonal ant colonies, combined with mathematical modelling, to show that increases in the size of social groups can generate division of labour among extremely similar workers, in groups as small as six individuals. These early effects on behaviour 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 a higher division of labour. Our results indicate that division of labour, increased homeostasis and higher fitness can emerge naturally in social groups that are small and homogeneous, and that scaling effects associated with increasing group size can thus promote social cohesion at the incipient stages of group living.
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We thank A. Gal for advice on data analysis, O. Feinerman and M. Liu for contributions to the tracking algorithms, S. Leibler, Z. Frentz, and D. Jordan for helpful discussions. This work was supported by grant 1DP2GM105454-01 from the NIH, a Searle Scholar Award, a Klingenstein-Simons Fellowship Award in the Neurosciences, and a Pew Biomedical Scholar Award to D.J.C.K.; Swiss National Science Foundation Early Postdoc.Mobility (PBEZP3-140156) and Advanced Postdoc.Mobility (P300P3-147900) fellowships, and a Rockefeller University Women & Science fellowship to Y.U.; a Kravis Fellowship to J.S.; the National Science Foundation Graduate Research Fellowship under Grant No. DGE1656466 to C.K.T. This is Clonal Raider Ant Project paper number 8.
Nature thanks J. O’Dwyer and the other anonymous reviewer(s) for their contribution to the peer review of this work.