Primate social group sizes exhibit a regular scaling pattern with natural attractors.

R I M Dunbar,Padraig Mac Carron,Susanne Shultz

doi:10.1098/rsbl.2017.0490

R I M Dunbar, Padraig Mac Carron + Show 1 more

Open Access

https://doi.org/10.1098/rsbl.2017.0490

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Abstract

Primate groups vary considerably in size across species. Nonetheless, the distribution of mean species group size has a regular scaling pattern with preferred sizes approximating 2.5, 5, 15, 30 and 50 individuals (although strepsirrhines lack the latter two), with a scaling ratio of approximately 2.5 similar to that observed in human social networks. These clusters appear to form distinct social grades that are associated with rapid evolutionary change, presumably in response to intense environmental selection pressures. These findings may have wider implications for other highly social mammal taxa.

Highlights

Mammals live in a variety of social systems with group sizes that vary, both within and between species, from one to several hundred individuals [1]
While group size within a species varies as a function of well-known environmental and demographic processes [3,4], there is no general explanation for why group sizes vary so much between species
This raises the question as to whether, at the taxon level, primate group sizes consist of a single distribution or several discrete distributions

Summary

Introduction

Mammals live in a variety of social systems with group sizes that vary, both within and between species, from one to several hundred individuals [1]. While group size within a species varies as a function of well-known environmental and demographic processes [3,4], there is no general explanation for why group sizes vary so much between species ( there is a long held assumption that ecology plays a central role [5,6,7]) This raises the question as to whether, at the taxon level, primate group sizes consist of a single distribution or several discrete distributions (each with its own optimal value, representing some kind of social grade). The former may be favoured where group sizes are flexible and respond facultatively to extrinsic ecological drivers, as predicted by the socio-ecological model [5,6,7]. The latter might be favoured if social evolution has followed a stepwise pattern [8], the fact that primate social evolution is predictable does not necessarily tell us anything about resulting group sizes

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