Why Are Primates Big-Brained? Food for Thought

“Complex foraging strategies, social structures, and cognitive abilities, are likely to have co-evolved throughout primate evolution,” adds Alex DeCasien, an NYU doctoral candidate and lead author of the study. “However, if the question is: ‘Which factor, diet or sociality, is more important when it comes to determining the brain size of primate species?’ then our new examination suggests that factor is diet.”

The social brain hypothesis sees social complexity as the primary driver of primate cognitive complexity, suggesting that social pressures ultimately led to the evolution of the large human brain. While some studies have shown positive relationships between relative brain size and group size, other studies which examined the effects of different social or mating systems have revealed highly conflicting results, raising questions about the strength of the social brain hypothesis.

Skulls of adult male spider monkey (Ateles paniscus; left) and howler monkey (Alouatta pigra; right). Skull collection: Center for the Study of Human Origins (CSHO). 

Photo credit: Megan Petersdorf.

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.

The researchers caution that the results do not reveal an association between brain size and fruit or protein consumption on a within-species level; rather, they note, they are evidence of the cognitive demands required by different species to obtain certain foods.

“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.”Contacts and sources:
James Devitt