It is widely believed that primates, thanks to their large brains, are smarter than most mammals. But what led to the development of such big brains? Australian researchers tackled this question in a new study. Their main hypothesis is based on a feedback loop: smarter animals use their intelligence to find food more efficiently, which allows them to consume more calories and provide the energy their brains need for further foraging.
Researchers aimed to identify a correlation between brain size and the amount of high-energy food, such as fruits, in an animal’s diet. As Ben Hirsch, a scientist at James Cook University and the study’s lead author, explained, fruits present a complex puzzle for animals. Different types of fruits ripen at various times of the year and across different areas of an animal’s habitat. Species that seek a more diverse diet are more likely to develop larger brains, the scientists suggested. They hoped that species with bigger brains would be smarter and, therefore, more effective at finding food.
In their research, Ben Hirsch’s team was the first to test the brain-evolution hypothesis using data from the wild. Tracking fruit-eating animals in Panama posed a significant challenge, because measuring foraging efficiency in the wild is complex, Science Alert reported. The mammals studied traveled great distances, typically more than three kilometers a day, making it difficult to replicate their food-seeking behavior in a lab setting. The scientists took advantage of a natural occurrence when the intricate fruit puzzle condensed into a few species of ripe fruits over a three-month period. During this time, all fruit-eating mammals focused on the harvest, particularly from dipteryx trees (Dipteryx oleifera). These trees are enormous, reaching heights of 40 to 50 meters in some areas, and in summer they bloom with bright purple flowers. Mapping all the purple patches with drones was straightforward for the researchers. A few months later, the trees bore fruit.
Now, the team needed to assess how effectively animals with different brain sizes visited these trees. They selected two species of primates with large brains (spider monkeys and white-faced capuchins) and two non-primates—relatives of raccoons with smaller brains (white-nosed coatis and kinkajous).
Over two fruiting seasons, the scientists collected movement data from more than 40 animals, resulting in over 600,000 GPS locations. They then determined when the animals visited food sources and how long they stayed. Some of the animals had the unfortunate habit of napping in the trees. Fortunately, collars recorded their activity, allowing researchers to identify when any of the fruit gatherers dozed off. The team calculated route efficiency as the amount of daily time spent in dipteryx trees divided by the distance traveled.
So, are brainy primates really better foragers? If animals with larger brains use their intelligence to visit fruit trees more effectively, you would expect primates with bigger brains to have more efficient foraging routes. However, the researchers found no such evidence. In both species of monkeys, the routes were no more efficient than those of the two non-primates. That was a serious blow to the fruit-diet hypothesis for brain evolution. If the bigger-brained species were truly better foragers, they would be able to meet their nutritional needs more quickly. At the start of the study, the scientists thought monkeys would navigate their routes more skillfully during the first few hours of the day after waking hungry. But that did not happen. Their results were no better than those of the non-primates.
So, what’s the point of having large brains? If the evolution of brain size did not help primates plan more efficient foraging routes, why did brain size increase? The team suggests the answer may lie in memory. If species with larger brains have better episodic memory, they might be able to return to familiar trees for food. Alternatively, the brain might have grown to help primates navigate the complexities of social life. The results of the study were published in the journal Proceedings of the Royal Society B.
