Why did primates develop such large brains? First study of its kind says it wasn’t for finding food

Thanks to our large brains, humans and non-human primates are more intelligent than most mammals. But why do some species develop large brains?

The main hypothesis for how primates evolved big brains involves a feedback loop: smarter animals use their intelligence to find food more efficiently, resulting in more calories, which provide the energy needed to power a big brain. This idea comes from studies that have established a correlation between brain size and diet – specifically, the amount of fruit in an animal’s diet.

Fruits are a very powerful food, but they create a complex puzzle for animals. Different species of fruits ripen at different times of the year and are distributed throughout an animal’s home range. Animals that need to find such variable food may be more likely to develop large brains.

A key hypothesis here is that species with larger brains are more intelligent and therefore can find food more efficiently. In a new study published today in Proceedings of the Royal Society B, we directly tested this hypothesis of brain evolution for the first time.

Tracking fruit eaters in Panama

A major problem with testing the fruit diet hypothesis is that it is difficult to measure foraging efficiency. The mammals we study travel long distances, typically more than three kilometers per day, making it difficult to replicate realistic study conditions in the laboratory.

Some researchers have experimentally manipulated food distribution in wild animals, but animals needed extensive training to learn to visit human-created food resources.

In our study, we took advantage of a natural phenomenon in Panama that occurs when the normally complex fruit puzzle is reduced to a few ripe fruit species over a three-month period. Meanwhile, all fruit-eating mammals are forced to concentrate on a single tree species: Dipteryx oleifera.

Luckily for us, Diptera the trees are huge, sometimes reaching 40 to 50 meters tall, and produce bright purple flowers in summer. We mapped the island with drones during the flowering season and identified patches of purple flowers, mapping virtually every Diptera which bore fruit a few months later.

This gave us the full scope of the fruit puzzle our study animals were facing, but we still needed to test how efficiently animals with different brain sizes visited these trees. We chose two large-brained primates (white-faced spider monkeys and capuchins) and two smaller-brained related raccoons (white-nosed coatis and kinkajous).

Over the course of two fruiting seasons, we collected data on the movements of more than 40 individual animals, resulting in more than 600,000 GPS locations.

We then had to determine when the animals visited us Diptera trees and for how long. This was a complex task, because to know exactly when our animals entered and exited the fruit trees, we had to extrapolate their position between GPS readings taken every four minutes. Some animals also had the bad habit of sleeping Diptera trees. Fortunately, our collars recorded the animals’ activity, allowing us to know when they were sleeping.

Once these challenges were resolved, we calculated the efficiency of the routes as the daily time spent active in Diptera trees, divided by the distance traveled.

Do smarter foragers feed smarter?

If animals with larger brains used their intelligence to visit fruit trees more efficiently, we would expect the large-brained primates studied in our study to have more efficient foraging routes.

That’s not what we found.

The two monkey species had no more efficient pathways than the two non-primates, which seriously challenges the hypothesis of brain evolution based on a fruit diet. If smarter species were more efficient, they might be able to meet their nutritional needs more quickly and then spend the rest of the day relaxing.

If this were the case, we would have expected the monkeys to move more efficiently in the early hours of the day after waking up hungry. Looking at these first 2 to 4 hours of the day, we found the same result: the monkeys were no more efficient than the non-primates.

Why these big brains, then?

So if the evolution of these large brains doesn’t allow primates to plan more efficient foraging routes, why has brain size increased in some species?

Maybe it’s memory related. If species with larger brains have better episodic memory, they might be able to optimize when they visit fruit trees to get more food. Preliminary analyzes of our dataset did not support this explanation, but we will need more detailed studies to test this hypothesis.

Intelligence could be linked to the use of tools, which could help an animal extract more nutrients from its environment. Of the four species studied, the white-faced capuchin monkey is the only one that has been observed using tools, and it also has the largest brain (relative to body size).

Our study could also support the hypothesis that brain size would increase to manage the complexities of living in a social group.

Large brains evolved in an assortment of vertebrates (dolphins, parrots, crows) and invertebrates (octopuses). Although our study cannot determine the exact drivers of brain evolution in all of these species, we directly tested a key hypothesis in wild tropical mammals, in a relatively non-invasive way.

We have demonstrated that using the latest sensor technologies, we can test big hypotheses about the evolution, psychology and behavior of animals in their natural environment.