Why Primates Evolved Large Brains: Tracking Fruit Eaters in Panama

Thanks to our comparatively large brains, humans and other primates are more intelligent than most mammals. Yet what causes certain species to evolve large brains in the first place?

The main explanation for the evolution of large primate brains centres on a feedback cycle: more intelligent animals can use their cognitive abilities to locate food more effectively, gaining additional calories that supply the energy needed to sustain a large brain. Evidence for this proposal has come from research showing a link between brain size and diet – particularly the proportion of fruit an animal eats.

Although fruit is energy-rich, it presents animals with a complex challenge. Various fruit species ripen at different points in the year, while also being dispersed across an animal’s home range. Animals required to locate this highly changeable food source could therefore be more likely to evolve large brains.

This argument rests on an important assumption: species with bigger brains are more intelligent and can consequently find food more efficiently. In a new study published today in Proceedings of the Royal Society B, we tested this brain-evolution hypothesis directly for the first time.

Monitoring fruit eaters in Panama

A central difficulty in testing the fruit-diet hypothesis is assessing foraging efficiency. The mammals we investigate cover considerable distances, typically travelling over three kilometres each day, so recreating realistic conditions in a laboratory is challenging.

Other scientists have experimentally altered the distribution of food for wild animals, but those animals required substantial training before they learnt to visit food sources created by people.

Our study made use of a natural event in Panama, when the usually complicated fruit challenge is reduced, for three months, to only a small number of ripe fruit species. In this period, every fruit-eating mammal must concentrate on a single tree species: Dipteryx oleifera.

Conveniently, Dipteryx trees are enormous, at times growing to 40–50 metres tall, and they produce vivid purple flowers during summer. During the flowering season, we used drones to map the island and locate clusters of purple flowers, enabling us to map almost every Dipteryx tree that would bear fruit several months afterwards.

This revealed the complete scale of the fruit challenge confronting our study animals, but we still had to establish how efficiently animals with varying brain sizes visited the trees. We selected two large-brained primates – spider monkeys and white-faced capuchins – alongside two smaller-brained relatives of raccoons: white-nosed coatis and kinkajous.

Across two fruiting seasons, we gathered movement data from over 40 individual animals, producing more than 600,000 GPS locations.

Next, we needed to determine when the animals visited Dipteryx trees and the length of each visit. This was complicated because, to identify precisely when an animal entered or left a fruit tree, we had to estimate its position between GPS readings recorded at four-minute intervals. Certain animals also inconveniently slept in Dipteryx trees. Fortunately, the collars measured animal activity, allowing us to distinguish sleeping from waking behaviour.

After resolving these issues, we measured route efficiency as the daily time spent active in Dipteryx trees divided by the distance travelled.

Do more intelligent foragers forage more effectively?

If animals with larger brains used intelligence to visit fruit trees more efficiently, the large-brained primates in our research should have taken more efficient foraging routes.

That was not our result.

Neither monkey species had routes that were more efficient than those of the two non-primates, seriously undermining the fruit-diet hypothesis for brain evolution. Were more intelligent species more efficient, they could potentially meet their nutritional requirements sooner and use the remainder of the day to relax.

If that were true, the monkeys should have selected routes more efficiently during the first hours after waking hungry. However, when we examined the initial 2–4 hours of each day, the outcome remained unchanged: the monkeys were no more efficient than the non-primates.

Why did large brains evolve?

If evolving large brains does not enable primates to plan more efficient foraging routes, then why did brain size grow in some species?

Memory may be relevant. Species with larger brains might possess stronger episodic memory, enabling them to optimise when they visit fruit trees and obtain more food. Early analyses of our data did not support this possibility, though more detailed research will be necessary to test it properly.

Intelligence could instead be associated with tool use, which may allow animals to obtain more nutrients from their surroundings. Of the four species we studied, only the white-faced capuchin monkey has been observed using tools, and it also has the largest brain relative to its body size.

Our research may also support the idea that brain size grew to manage the challenges of social-group life.

Large brains have developed in a range of vertebrates, including dolphins, parrots and crows, as well as invertebrates such as octopuses. Although our study cannot identify the precise causes of brain evolution across every one of these species, it has directly examined a key assumption in wild tropical mammals using a relatively non-invasive approach.

We have shown that the newest sensor technologies can be used to investigate major questions about animal evolution, psychology and behaviour in natural environments.

Ben Hirsch, Senior Lecturer in Zoology and Ecology, James Cook University

This article is republished from The Conversation under a Creative Commons licence. Read the original article.