Humans and non-human primates are known for their intelligence, thanks to their large brains. However, the reason why some species have developed large brains has remained a mystery. The leading hypothesis suggests that primates evolved large brains due to a feedback loop: smarter animals use their intelligence to find food more efficiently, resulting in more calories to power a large brain. This idea is supported by studies that have found a correlation between brain size and diet, specifically the amount of fruit in an animal’s diet.
Fruit is a high-energy food but poses a challenge for animals as different fruit species ripen at different times and are spread throughout their habitat. Animals that need to find such variable food sources may be more likely to evolve large brains. The assumption here is that species with larger brains are more intelligent and can find food more efficiently. In a new study published in Proceedings of the Royal Society B, researchers directly tested this hypothesis of brain evolution for the first time.
Testing the fruit-diet hypothesis is challenging as measuring foraging efficiency is difficult. The mammals studied in this research travel long distances, making it hard to replicate realistic study conditions in a lab. Some researchers have manipulated food distribution in wild animals, but it required extensive training for the animals to learn to visit human-made food resources.
To overcome these challenges, the researchers took advantage of a natural phenomenon in Panama where the complex fruit puzzle shrinks to just a few species of ripe fruit over a three-month period. During this time, all fruit-eating mammals are forced to focus on one tree species: Dipteryx oleifera. The researchers mapped the island with drones during the flowering season and identified patches of purple flowers, mapping virtually every Dipteryx tree that produced fruit a few months later.
The researchers selected two large-brained primates (spider monkeys and white-faced capuchins) and two smaller-brained raccoon relatives (white-nosed coatis and kinkajous) to test how efficiently animals with different brain sizes visited these trees. Over two fruiting seasons, they collected movement data from over 40 individual animals, resulting in more than 600,000 GPS locations.
The researchers then had to determine when animals visited Dipteryx trees and for how long. This was a complex task as it required extrapolating their location between GPS fixes taken every four minutes. Some animals also slept in the trees, but the collars recorded their activity, allowing the researchers to determine when they were sleeping. Once these challenges were overcome, the researchers calculated route efficiency by dividing the daily amount of time spent active in Dipteryx trees by the distance traveled.
Contrary to expectations, the study found that the two monkey species did not have more efficient routes than the two non-primates, which challenges the fruit-diet hypothesis of brain evolution. If smarter species were more efficient, they would be able to satisfy their nutritional needs more quickly and have more time to relax. However, the monkeys were not more efficient than the non-primates, even in the first few hours of the day when they were hungry.
This raises the question of why primates evolved large brains if it doesn’t allow them to plan more efficient foraging routes. One possibility is that it is related to memory. Species with larger brains may have better episodic memory, allowing them to optimize the timing of fruit tree visits for maximum food intake. Another possibility is that intelligence is linked to tool use, which helps animals extract more nutrients from their environment. The white-faced capuchin monkey, the only species observed using tools in this study, also has the largest brain relative to body size.
The study also supports the hypothesis that large brains evolved to handle the complexities of living in social groups. While this research focused on wild tropical mammals, it demonstrates how sensor technologies can be used to test big hypotheses about the evolution, psychology, and behavior of animals in their natural environment.
