Primates, including humans, typically have larger brains than most other mammals.
Scientists have long thought that this difference in brain size is related to diet, specifically the consumption of fruit.
The idea is that primates evolved larger brains to help them find and remember where fruit is, making them more efficient foragers.
A new study from the Max Planck Institute of Animal Behavior and the Smithsonian Institute of Tropical Research has put this theory to the test for the first time.
Surprisingly, the results suggest that the fruit-diet theory might not hold water.
Researchers used drone imaging, GPS tracking, and detailed behavioral analyses to observe how four species of fruit-eating mammals solved the same foraging challenge in a Panamanian rainforest.
These species included two large-brained primates (spider monkeys and white-faced capuchins) and two smaller-brained mammals related to raccoons (white-nosed coatis and kinkajous).
The scientists expected that the larger-brained primates would be better at finding fruit. However, the results showed that the larger-brained primates did not find fruit more efficiently than the smaller-brained mammals.
The study, published on May 28 in the journal Proceedings of the Royal Society B, challenges the idea that larger brains are necessary for smarter foraging decisions.
According to the leading theory, primates evolved larger brains because their intelligence helped them find fruit more efficiently, which in turn provided more energy to fuel brain growth. Fruit is a valuable but unpredictable resource, requiring animals to find fruiting trees and remember when they will have ripe fruit.
Previous studies have supported this theory by showing a correlation between brain size and the amount of fruit in an animal’s diet.
However, the researchers from MPI-AB and STRI thought this theory needed more scrutiny. “The fruit-diet hypothesis had never been supported experimentally,” says Ben Hirsch, a research associate at STRI and the study’s first author.
To test the hypothesis, the team took advantage of a unique natural situation on Barro Colorado Island in Panama. For three months each year, fruit-eating mammals on the island feed almost exclusively on the fruit of the Dipteryx oleifera tree. This situation provided an opportunity to compare how efficiently different species solved the same foraging problem.
The researchers mapped the locations of all the Dipteryx trees on the island using drones. They then tracked individual animals with GPS sensors to see how they navigated to the trees. They also used accelerometers to confirm that the animals were active and potentially feeding during their visits to the trees.
The scientists calculated each animal’s foraging efficiency by dividing the amount of time spent actively foraging by the distance traveled. Contrary to the fruit-diet hypothesis, the larger-brained capuchins and spider monkeys did not show greater foraging efficiency than the smaller-brained coatis and kinkajous.
“We didn’t find any evidence that animals with larger brains made smarter foraging decisions,” says Meg Crofoot, the study’s senior author.
The study suggests that larger brains might not be solely about finding food more efficiently. Larger brains could be related to other factors, such as better memory, tool use, cultural behaviors, or social complexity.
While this study does not provide a definitive answer to why some species evolved larger brains, it challenges the long-held belief about diet and brain size, opening the door to exploring other possibilities.
