Brain size and social complexity: a computed tomography study in Hyaenidae

The social brain hypothesis posits that the demands of living in complex social groups require increased neural processing, and that this underlies the expansion of brain areas involved in mediation of complex social behavior. However, much of the support for the social brain hypothesis is derived from comparative studies in primates. If large brains evolved as a result of selection pressures imposed by life within complex societies, as the social brain hypothesis predicts, then gregarious nonprimate species should possess large brains and exhibit comparable expansion of brain areas mediating social behavior. Our purpose here was to test a prediction of the social brain hypothesis-- that increased brain size is related to social complexity --by examining species in the carnivore family Hyaenidae. Hyaenidae contains 4 extant species that span a spectrum of social complexity: the aardwolf (Proteles cristata) is solitary during the nonbreeding season, and forms monogamous pairs during the breeding season; the striped hyena (Hyaena hyaena) lives solitarily or in small groups; the brown hyena (Parahyaena brunnea) lives in groups of up to 14 individuals; and the spotted hyena (Crocuta crocuta) lives in complex hierarchically organized groups containing up to 90 animals. Computed tomography was used to create three-dimensional endocasts based on serial analysis of coronal sections of the adult endocranium. The largest brain volume, relative to body size, is found in the spotted hyena. We found no significant variation in relative brain volume among striped hyenas, brown hyenas, and aardwolves. The spotted hyena also possesses a larger anterior cerebrum volume relative to total brain volume than is found in the other hyena species; this region is composed primarily of frontal cortex. These data are consistent with the idea that expansion of the frontal cortex is driven by the demands of processing cognitive information associated with complex social lives, but other factors may drive the evolution of large brains in hyaenids.