Compared to leaf-eaters, primates who ate fruit had around 25 percent more brain tissue.
Primate brains may have grown larger and more complex thanks to a fruit-filled diet, a new study suggests.
The researchers analyzed the brain sizes and diets of over 140 primate species spanning apes, monkeys, lemurs and lorises and found that those who munched on fruit instead of leaves had 25 percent more brain tissue, even when controlling for body size and species relatedness. Take spider monkeys and howler monkeys, for example. They both live in the rain forests of South America in groups of about 10. But where howler monkeys leisurely munch on trumpet tree leaves all day, spider monkeys venture out in small groups shortly after sunrise to forage for passion fruit and other ripe morsels. Despite their similar environments and social setting, spider monkeys have bigger brains than howlers.
“If you are foraging on harder-to-access food, like fruit instead of leaves, then you need to have all the cognitive strategies to deal with that,” says Alex DeCasien, a doctoral candidate at New York University and lead-author on the study. Fruit can vary from season to season, be tucked away in hard-to-reach nooks, and require skill and strength to crack into, smarter primates could be more apt to scope it out and reap its nutritious rewards. “All of that is so much more complicated than just grabbing a leaf and eating it,” she says. And so, a diet of fruit may in turn have led to the evolution of the bigger brains over generations, she adds.
Monkeys and apes who incorporated animal proteins into their diets also had slightly larger brains than the leaf eaters, the Nature Ecology and Evolution study found. Again, the researchers speculate this could be because primates need more cognitive power to hunt and consume things like frogs, birds, and insects compared to the brain power needed to eat leaves. But DeCasien says she and her colleagues were surprised to find that these omnivores have significantly smaller brains than fruit-eaters. They suspect it could be because many of these omnivores, like lemurs and lorises, eat insects. “[Insects] may be abundant like leaves and might be easy to capture,” she says.
The findings challenge a long-held scientific hypothesis that the size of social groups among primates is the biggest determinant of brain size. The bigger the social group, the more complex the social interactions, leading to the evolution of larger brains with more computing power, the theory suggests.
Previous studies have shown that larger groups of primates with more complex social structures are correlated with larger brains. In fact, scientists have used that idea – called the social brain hypothesis – to explain why humans and certain other primates like chimpanzees and bonobos have bigger brains than other primate species. (Now, diet is thought to have played a big role in making human brains bigger than any of our primate cousin’s. As we’ve reported before, scientists think eating cooked meat gave our bodies some extra energy to fuel the building of bigger brains.)
But the authors of the new study compared body size, diet, and social lives (factors like whether they were solitary or lived in pairs, monogamous or polygynous, and the size of their groups) of these various primate species to their average brain sizes. Overall, diet appeared to be a more consistent predictor of brain size for a species than social complexity — brain sized increased with fruit eating more consistently than with greater number of social connections.