A recent study suggests that the size of primate’s brains may have grown larger and more complex due to a diet that was filled with fruit. The researchers conducting the study analyzed the brain sizes and diets of over 140 primate species spanning apes, monkeys, lemurs, and lories and found that those who ate fruit instead of leaves had 25 percent more brain tissue, even when controlling for body size and species relatedness.
For example, in the case of spider monkeys and howler monkeys. They both live in the rainforest of South America in groups of about 10. However, where howler monkeys leisurely chomp on trumpet tree leaves all day, spider monkeys venture out in small groups just after sunrise to scavenge for passion fruit and other ripe pieces.
Despite their similar environments and social setting, spider monkeys have bigger brains than howlers. Primates, like the baboon, may have evolved larger, more complex brains over generations of seeking out fruit rather than sticking to low-calorie leaves.
Alex DeCasien, a doctoral candidate at New York University and the lead author of the study claims, “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.”
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,” DeCasien said. Moreover, so, a diet of fruit may, in turn, have led to the evolution of the more significant 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 study found. The researchers speculated that this could be because primates need more cognitive power to hunt and consume things like frogs, birds, and insects compared to the brain power required to eat leaves.
DeCasien said that 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 may be easy to capture,” she says.
The findings challenge a long-held scientific hypothesis that the size of social groups among primates is the most significant 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 a more multifaceted social structure, are associated with larger brains. Scientists have used this idea, called the social brain hypothesis, to explain why humans and other specific primates including chimpanzees and bonobos have larger brains compared to other primates.
Scientists also think that the eating of cooked meat has given our bodies the extra energy to fuel the building of our more significant brains. Overall, diet does appear to be the deciding factor of brain size and social complexity.
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