The social brain hypothesis (SBH) posits that the demands imposed on individuals by living in cohesive social groups exert a selection pressure favouring the evolution of large brains and complex cognitive abilities. Using volumetry and the isotropic fractionator to determine the size of and numbers of neurons in specific brain regions, here we test this hypothesis in African mole-rats (Bathyergidae). These subterranean rodents exhibit a broad spectrum of social complexity, ranging from strictly solitary through to eusocial cooperative breeders, but feature similar ecologies and life history traits. We found no positive association between sociality and neuroanatomical correlates of information-processing capacity. Solitary species are larger, tend to have greater absolute brain size and have more neurons in the forebrain than social species. The neocortex ratio and neuronal counts correlate negatively with social group size. These results are clearly inconsistent with the SBH and show that the challenges coupled with sociality in this group of rodents do not require brain enlargement or fundamental reorganization. These findings suggest that group living or pair bonding per se does not select strongly for brain enlargement unless coupled with Machiavellian interactions affecting individual fitness.

Department of Biological Sciences University of Cape Town 7701 Rondebosch South Africa

Department of General Zoology Faculty for Biology University of Duisburg Essen 45117 Essen Germany

Department of Zoology Faculty of Science Charles University Viničná 7 CZ 128 44 Praha 2 Czech Republic

Department of Zoology Faculty of Science University of South Bohemia Branišovská 1760 CZ 370 05 České Budějovice Czech Republic

Mammal Research Institute Department of Zoology and Entomology University of Pretoria Pretoria 0002 South Africa

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