Previous stuᴅɪᴇs 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. This idea states that it was the challenges of managing increasingly complex social relationships and interactions that mainly drove the development of larger brains, as opposed to the challenge of finding food. After all, being a social creature is a lot of work.
The researchers analyzed the brain sizes and ᴅɪᴇts 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 ᴅɪᴇt of fruit may in turn have led to the evolution of the bigger brains over generations, she adds.
That makes sense, because fruit is much more nutrient-dense source of food than foliage, says Katherine Milton, a physical anthropologist at the University of California at Berkeley who researches primate ᴅɪᴇtary ecology, and was not involved in this study. “Because highly folivorous [leaf-eating] primates are generally taking in less ready energy per unit time than highly frugivorous [fruit-eating] primates, one would think their brain size would correlate with this ᴅɪᴇtary difference,” Milton said via email.
The authors of this new study compared body size, ᴅɪᴇt, 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, ᴅɪᴇt 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.
Monkeys and apes who incorporated animal proteins into their ᴅɪᴇts 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.
Yet Robin Dunbar, an anthropologist and evolutionary psychologist at Oxford University who developed the social brain hypothesis, says that DeCasien’s study has a “serious flaw in [its] whole structure.” Dunbar says that ᴅɪᴇt is a constraint on brain size, while sociality serves as a cause for brain development. Comparing these to each other as if they are equivalent is incorrect, Dunbar says; in his opinion, they are complementary, not alternative explanations.
DeCasien agrees that ᴅɪᴇt and social lives are probably both at play here. “ᴅɪᴇt, social structures, cognitive abilities — they’re likely to have co-evolved together in primates,” DeCasien says. However, she is quick to note that these evolutionary trends take many generations and millions of years to manifest. So don’t go looking to eat more fruit because of the new findings – sure, they pack a nutritional punch, but it doesn’t mean they will make you and me any smarter.