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Stepwise evolution of stable sociality in primates


Although much attention has been focused on explaining and describing the diversity of social grouping patterns among primates1,2,3, less effort has been devoted to understanding the evolutionary history of social living4. This is partly because social behaviours do not fossilize, making it difficult to infer changes over evolutionary time. However, primate social behaviour shows strong evidence for phylogenetic inertia, permitting the use of Bayesian comparative methods to infer changes in social behaviour through time, thereby allowing us to evaluate alternative models of social evolution. Here we present a model of primate social evolution, whereby sociality progresses from solitary foraging individuals directly to large multi-male/multi-female aggregations (approximately 52 million years (Myr) ago), with pair-living (approximately 16 Myr ago) or single-male harem systems (approximately 16 Myr ago) derivative from this second stage. This model fits the data significantly better than the two widely accepted alternatives (an unstructured model implied by the socioecological hypothesis or a model that allows linear stepwise changes in social complexity through time). We also find strong support for the co-evolution of social living with a change from nocturnal to diurnal activity patterns, but not with sex-biased dispersal. This supports suggestions that social living may arise because of increased predation risk associated with diurnal activity. Sociality based on loose aggregation is followed by a second shift to stable or bonded groups. This structuring facilitates the evolution of cooperative behaviours5 and may provide the scaffold for other distinctive anthropoid traits including coalition formation, cooperative resource defence and large brains.

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Figure 1: Primate phylogeny showing ancestral state reconstructions for sociality under the reversible-jump Markov chain Monte Carlo-derived model of evolution.
Figure 2: Alternative evolutionary models of primate social evolution.
Figure 3: Estimated transition rates for co-evolution of social living.
Figure 4: Alternative evolutionary models for the evolution of stable grouping patterns.


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S.S. is supported by a Royal Society Dorothy Hodgkin Fellowship. We thank R.I.M. Dunbar for comments on the manuscript.

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S.S. designed the study, compiled the data and executed analyses. C.O. executed analyses. Q.A. was involved in study design and advised on statistical analyses. All authors contributed to the manuscript.

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Correspondence to Susanne Shultz.

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The authors declare no competing financial interests.

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Shultz, S., Opie, C. & Atkinson, Q. Stepwise evolution of stable sociality in primates. Nature 479, 219–222 (2011).

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