Abstract:
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.
