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In the Nature Ecology and Evolution study, the researchers, who also included Scott Williams, an assistant professor of anthropology at NYU, examined more than 140 primate species—or more than three times as many as previous studies—and incorporated more recent evolutionary trees, or phylogenies. They took into account food consumption across the studied species—folivores (leaves), frugivores (fruit), frugivores/folivores, and omnivores (addition of animal protein)—as well as several measures of sociality, such as group size, social system, and mating system.

Their results showed that brain size is predicted by diet rather than by the various measures of sociality—after controlling for body size and phylogeny. Notably, frugivores and frugivore/folivores exhibit significantly larger brains than folivores and, to a lesser extent, omnivores show significantly larger brains than folivores.

“Fruit is patchier in space and time in the environment, and the consumption of it often involves extraction from difficult-to-reach-places or protective skins,” observes DeCasien. “Together, these factors may lead to the need for relatively greater cognitive complexity and flexibility in frugivorous species.”