Fruit foraging in primates may be key to large brain evolution

Foraging for fruit may have driven the evolution of large brains in primates, according to research attempting to unpick the mystery of our cerebral heftiness.

The finding appears to be a blow to a long-held theory that humans and other primates evolved big brains largely as a result of social pressures, with extra brain power needed to navigate and engage in complex social interactions. Instead the researchers say it supports the view that the evolution of larger brains is driven by diet.

“All of these things are co-evolving: brains are getting bigger, sociality is becoming more complex, diet quality is becoming better, but it is maybe a shift in that focus on what might have been relatively more important, or more consistent throughout [primate] evolution,” said Alex DeCasien, co-author of the research from New York University.

Writing in the journal Nature Ecology & Evolution, DeCasien and colleagues describe how they analysed the differences in brain size between more than 140 non-human primate species to unpick whether larger brains were linked to diet or to social factors. These factors included group size, mating habits and social system – for example, whether a species was solitary or lived in a system where males are surrounded by a harem of females.

After taking into account factors such as body size and the position of species on the evolutionary tree, the team found no evidence that greater sociality is linked to bigger brain size.

Instead, they found that big brains appear to be linked to diet. According to the study, primates that eat fruit have about 25% more brain tissue than leaf-eaters of the same body weight. Omnivores were also found to have larger brains than leaf-eaters, although there was no difference when compared to fruit eaters.

DeCasien says the results support the idea that fruit-eating provides more energy than leaf-eating, aiding brain growth. “[Fruit] is higher quality, it is more nutrient dense, it requires less digesting time, than the leaves,” she said.

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At the same time, DeCasien adds, foraging for fruit could be a driver for large brains since finding fruit in a forest, logging its location, knowing how to get into the fruit, and remembering when it is likely to be ripe, all take brain power. “That is much more demanding than eating leaves which are relatively abundant all around you,” said DeCasien. “That might allow, afterwards, then an increase in how complex your social interactions are.”

While the study looked only at non-human primates, experts believe that its findings could shed light on why our own species is endowed with a large brain.

“[We are the only primate that] is able to get lots of calories from meat really easily from cooking it and making it more digestible,” said Chris Venditti, an evolutionary biologist at the University of Reading who was not involved in the research. “So if diet is really that important it could be that that was important in our own brain evolution – that transition to being able to process food and eat meat and take on even more energy which gives us even more opportunity to grow larger brains.”

But, Venditti warns, the latest study has its drawbacks, not least that measures of group size might not reflect the degree to which individuals interact with each other, and that the team only looked at the overall relative brain size of different species, rather than the size of the neocortex – the area primarily involved in complex cognitive processes such as perception, reasoning and thought.

“Different brain regions can evolve independently of each other,” said Venditti. “If you look at the specific brain region involved in cognition itself it might be that there could be a relationship between [group size and brain size].”

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Robin Dunbar, professor of evolutionary psychology at the University of Oxford agrees, pointing out that neuroimaging studies have shown a link between the size of components of the neocortex and group size in humans and monkeys.

What’s more, he says, it is a mistake to assume that social group size and diet are two alternative explanations for the evolution of big brains, pointing out that one is a cause and the other a constraint.

“You cannot evolve a large brain to handle anything, social or otherwise, unless you change your diet to allow greater nutrient acquisition so as to grow a larger brain,” he said. “But that is not an explanation for why large brains evolved.”