The 82nd Annual Meeting of the American Association of Physical Anthropologists (2013)

Revisiting the social brain hypothesis: Incorporating within-species group size variation into a comparative analysis


1Department of Anthropology, Univeristy of Michigan, 2Hominid Paleobiology Doctoral Program, The George Washington University

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The social brain hypothesis proposes that the complex social environments of primates selected for increased brain size over time (Dunbar, 1992, Journal of Human Evolution, 20: 469-493). While managing multiple social relationships may require an increase in neocortex size, past comparative studies have neglected to account for the effect of within-species variation in group size on brain size. Data from studies of multiple populations of the same primate species provide an opportunity to investigate whether within-species variation in group size influences the relationship between group size and brain size. In this paper, we implement a re-sampling procedure with phylogenetic generalized least-squares (PGLS) regression to capture within- and between-species variation. Specifically, we tested whether species averages in group size obscured the relationship between group size and brain size. We compiled data on group size from the literature for multiple populations of 26 monkey and ape species from Dunbar’s (1992) sample. We re-sampled group sizes from each species and then conducted 10,000 PGLS regressions to examine the relationship between group size and neocortex ratio. A majority of the regressions was significant, suggesting that group size is correlated with neocortex ratio after controlling for the effect of within-species variation. This result is consistent with the social brain hypothesis. Our procedure accounts for within-species variation and should be used in future phylogenetic comparative studies.

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