Abstract
Compared to most other mammals and birds, anthropoid primates have unusually complex societies characterised by bonded social groups. Among primates, this effect is encapsulated in the social brain hypothesis: the robust correlation between various indices of social complexity (social group size, grooming clique size, tactical behaviour, coalition formation) and brain size. Hitherto, this has always been interpreted as a simple, unitary relationship. Using data for five different indices of brain volume from four independent brain databases, we show that the distribution of group size plotted against brain size is best described as a set of four distinct, very narrowly defined grades which are unrelated to phylogeny. The allocation of genera to these grades is highly consistent across the different data sets and brain indices. We show that these grades correspond to the progressive evolution of bonded social groups. In addition, we show, for those species that live in multilevel social systems, that the typical sizes of the different grouping levels in each case coincide with different grades. This suggests that the grades correspond to demographic attractors that are especially stable. Using five different cognitive indices, we show that the grades correlate with increasing social cognitive skills, suggesting that the cognitive demands of managing group cohesion increase progressively across grades. We argue that the grades themselves represent glass ceilings on animals' capacity to maintain social and spatial coherence during foraging and that, in order to evolve more highly bonded groups, species have to be able to invest in costly forms of cognition.
Highlights
Anthropoid primates differ from other mammals and birds in the extent to which they live in stable social groups (Shultz & Dunbar, 2007, 2010a)
These dyadic relationships are embedded within complex networks of relationships in which third- and fourth-party relationships become important and influence the outcome of dyadic interactions (e.g. Datta, 1983)
We plotted the observed mean size of the different groupings for each taxon of interest against the grade regression lines determined by the analysis in Section II.2a, and asked whether these values fit the grade lines or are randomly where Bk is the group size predicted by the equation for grade k, p(XjBk) is the likelihood that the observed value, X, is the same as the predicted value in any given case with pprior = 0.25 on the default assumption that, as priors, all four options are likely
Summary
Anthropoid primates differ from other mammals and birds in the extent to which they live in stable (bonded) social groups (Shultz & Dunbar, 2007, 2010a). Other species are known to form large, unstable groupings that may consist of more stable sub-groups: examples include Papio papio (Guinea baboon: Dunbar & Nathan, 1972; Patzelt et al, 2014), Mandrillus (drills and mandrills) (Gartlan, 1970; Hoshino et al, 1984; Abernethy, White & Wickings, 2002; Brockmeyer et al, 2015), Nasalis (Yeager, 1992) and Rhinopithecus (Kirkpatrick et al, 1998) This fractal pattern is likely to be cognitively challenging if the higher-level groupings are to have any degree of temporal cohesion and stability (Amici, Aureli & Call, 2008). Second, do the grades correspond to phase shifts in cognitive abilities that reflect increasing neurological processing demands?
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