What Really Made Primate Brains So Big?

The benefits of a big brain may seem obvious to us. But many creatures besides humans have thrived without them and continue to do just fine, thank you very much. After all, as Alex DeCasien, a graduate student in anthropology at New York University, "the brain is super expensive to grow and maintain.” For anthropologists, this poses an evolutionary mystery: What would drive a creature to invest so much energy into an organ that doesn't provide any immediate hunting, mating or defense benefits?

More than two decades ago, anthropologists began embracing the “social brain theory” as a possible answer. 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. But a new study by DeCasien takes aim at this dominant theory by suggesting a different catalyst for brain development: fruit.

In her research, DeCasien mainly focuses on differences between the sexes in primates. A few years ago, she was rifling through studies on brain size differences between the primate sexes, and ran into what she calls a "huge contradiction." She found different recent studies suggesting that either polygynandrous primates (those that mate with multiple partners) or monogamous primates (those that maintain long-term mating relationships with one partner) had larger brains than the other. Strangely, "they both claimed the support of the social brain hypothesis," DeCasien says.

Some of these scientists argued that managing multiple partners would tax the mind more and encourage the development of bigger brains, while others suggested that forging a lasting mating bond with one partner would require more mental capacity. "When reevaluating this obvious contradiction, I wanted to take into account those older ideas," DeCasien says. Using modern techniques and expanded data, DeCasien and her team did just in a study published this week in the journal Nature Ecology and Evolution.

Her conclusion: Those older ideas were correct—mostly.

Older studies on primate brain development drew on up to 43 primate species, DeCasien says. Modern datasets allowed her team to draw on brain size data from more than 140 species. They compared these brain sizes to the species' diets and factors based related to social development, such as group size and mating system. They were able to crunch those numbers with advanced statistical analysis techniques developed or refined in the last decade.

"We employed all the newest methods that are available," DeCasien says. (This is the reason "we have a ridiculous number of supplementary tables" in the study, she adds.)

After running those analyses, DeCasien's team concluded that diet, not social complexity, appeared to be most correlated with larger brain sizes among primates. Specifically, primates that fed mostly on fruits—such as chimpanzees or spider monkeys—appeared to have the largest brain sizes. (Notably, humans were excluded from the data because “we are an outlier with regard to brain size,” according to the study, a common practice in comparative brain size research.)

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 dietary 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 dietary difference," Milton said via email.

However, simply having access to those nutrients doesn't alone explain why primates feeding on fruit would evolve larger brains, DeCasien says. "Increasing your diet quality … allows you the possibility of increasing your brain size, but it still doesn't explain why it would be advantageous," DeCasien says—let alone advantageous enough to justify spending more resources on your brain than other parts of your body.

That advantage comes in the difficulty required to find good fruit, DeCasien says. As Milton wrote in a 1981 study that compared frugivorous spider monkeys to folivorous howler monkeys, “fruit is a more patchily distributed food resource in tropical forests than leaves […] Thus spider monkeys are faced with a far more complex problem than howlers with respect to locating their food sources since, in effect, they are dealing with a supplying area over 25 times as large.” The study noted that spider monkeys have a brain size over double that of howler monkeys on average, and speculated that the difficulties of learning to find and remember where fruits are located could have driven that growth.

"The cognitive complexity that is required to become more efficient at foraging for those things would also provide the selective pressure to increase brain size," DeCasien says.

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 diet 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.

"What they essentially claim is that improvements in diet drove the evolution of large brains so as to allow improvements in diet," Dunbar says. He adds that the researchers were incorrect in looking at total brain size when they should have focused on the size of the neocortex—the part of the brain involved in cognition, spatial reasoning and language—which Dunbar analyzed in his 1992 paper proposing the social brain hypothesis.

Dunbar also takes issue with the data used by DeCasien and her team showing fruit-eaters having larger brains than leaf-eaters, saying that they ignored research since 1992 disproving this claim.

Other researchers who study primate evolution were less critical. "The authors' results are very convincing," says Michael Schillaci, an evolutionary anthropologist at the University of Toronto and was not involved in the study. DeCasien’s work "provides very strong support for an ecological explanation for the evolution of brain size in primates, which is a hallmark of our order,” he says. Still, he says he is unsure whether fruit-eating specifically helped drive brain size evolution in humans, the descendants of primates, as well.

Durham University evolutionary anthropologist Robert Barton agrees with some of DeCasien's findings, specifically that the social brain hypothesis is likely not the main driver of brain development. However, Barton, who was not involved in this study and is currently working on a larger study of the same topic, says he would hesitate to pinpoint any specific type of food as driving brain development.

“Where we agree is that there is little or no evidence of a ‘social brain’ effect in these large data sets, and that there are more robust correlations with ecological variables,” Barton said via email.

For her part, DeCasien cautions that fruit and diet are not the sole drivers of brain size evolution among primates, just the dominant one from their analysis. Increasingly complex social demands certainly still played a major role in encouraging brain size growth, she says. "It's not that it's one and not the other," she says. "We definitely think that these things are all kind of coevolving together."

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Ben Panko is a staff writer for Smithsonian.com