A leading theory for why primates have bigger brains than other mammals has been debunked.

The “fruit-diet theory” contended that foraging ability and intelligence go hand in hand in a sort of fruit-foraging feedback loop. Essentially, it posited that animals with larger brains can find fruit easily, then eating that fruit fuels brain growth, which, in turn boosts foraging ability.

But, a study in Proceedings of the Royal Society B that compared larger-brained primates fruit foraging with smaller-brained mammals showed no difference in success. “We’re not finding support for this idea that finding fruit is driving the increase in brain size, and that applies to primates,” says Ben Hirsch of the Smithsonian Tropical Research Institute, an author of the paper. “I think it applies very much to humans, and it might apply to other species as well.”

Natural Laboratory

Creating the conditions for the study required a bit of cleverness. Fruit-finding tests in labs would be unnatural. And watching animals forage for multiple food types in disparate environments would make for unclear comparisons.

So a team of scientists from the Smithsonian and the Max Planck Institute of Animal Behavior exploited a natural phenomenon that occurs in the rainforest on Barro Colorado Island in Panama. For three months every year, fruit-eating mammals are forced to feed on one tree species, Dipteryx oleifera.

Once they identified what was essentially a natural laboratory, the scientists used drones, GPS tracking, accelerometers, and other techniques to follow the foraging of four mammals: two large-brained primates (spider monkeys and white-faced capuchins) and two smaller-brained raccoon relatives (white-nosed coatis and kinkajous). GPS sensors showed the paths that animals took to the fruit-bearing trees, while accelerometers confirmed that an animal was active and potentially feeding during a tree visit.

The scientists then calculated route efficiency as the daily amount of time eating from Dipteryx trees divided by how far they travelled to get them. According to the fruit-diet hypothesis, the big-brained capuchins and spider monkeys should find more efficient routes than the smaller-brained mammals. But there was no significant difference.

Read More: What Are the Smartest Primates?

Breaking Assumptions

The scientists also noticed some other assumption-breaking patterns. For instance, one would think animals with larger “hunting grounds” would travel a larger daily distance seeking food. That was not the case, says Hirsch. “We thought there would be a link between how big a home range is and how much an animal travels a day and we didn’t see that.”

If brain size isn’t a factor in foraging, what else could be driving intelligence? Hirsch hypothesizes that the size or development of specialized parts of the brain — like the forebrain or areas devoted to visual processing might play a role. But there’s no evidence to prove these hypotheses yet.

“Something else is going on, something else is driving this increase in intelligence in some species, and what that is, we don’t know,” says Hirsch. “But we’re pretty sure we eliminated one of the leading hypotheses.”

Read More: 5 of the World’s Most Intelligent Animals

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Before joining Discover Magazine, Paul spent over 20 years as a science journalist, specializing in U.S. life science policy and global scientific career issues. He began his career in newspapers, but switched to scientific magazines. His work has appeared in publications including Science News, Science, Nature, and Scientific American.