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Chimps mentally map fruit trees
Matt Walker
Editor, Earth News

A female chimpanzee in the dense Taļ forest, Ivory Coast
Where next?

Chimpanzees remember the exact location of all their favourite fruit trees.

Their spatial memory is so precise that they can find a single tree among more than 12,000 others within a patch of forest, primatologists have found.

More than that, the chimps also recall how productive each tree is, and decide to travel farther to eat from those they know will yield the most fruit.

Acquiring such an ability might have helped drive the evolution of sophisticated primate brains.

Emmanuelle Normand and Christophe Boesch of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany teamed up with Simone Ban of the University of Cocody in Abidjan, Ivory Coast, to investigate the spatial memory of chimpanzees in the wild.

"We were amazed by the apparent easiness by which chimpanzees discover highly productive fruit trees. Or how after being separated from other group members for hours or days, they could join each other silently at a large fruit tree, like if they would have had an appointment at this place," says Normand.

We think it's fair to assume that chimpanzees can remember the exact location of probably thousands of trees
Primatologist Emmanuelle Normand

To find out how they do it, Normand's team first mapped the location of 12,499 individual trees growing within the home range of a group of chimpanzees living in the Tai National Park in Ivory Coast. They identified each tree and used GPS to map its precise position.

The team also identified 17 species of fruit tree that the chimps regularly fed from, and worked out how often each individual tree belonging to these 17 species would be in fruit each month. From that, the researchers could determine how likely it would be that a chimp randomly walking around the forest might bump into a fruit tree that it could feed from.

The team found that the chimps didn't visit the most abundant fruit species most frequently, as would be expected if they were navigating without using spatial memory. They also excluded the possibility that the chimpanzees navigated toward the trees by smell.

Chimpanzees walking in the Taļ forest, with mother carrying her baby on her back
It's off to eat we go

Instead, they targeted certain trees and walked directly to them. For example, the apes visited one fruit tree, Pouteria aningueri, more than any other, despite it being one of the rarest trees in their home range, the team report in Animal Cognition.

The chimps also travelled much shorter distances to each fruit tree than would be expected by chance, confirming that they travel directly to the trees.

"We think it is fair to assume that chimpanzees can remember the exact location of probably thousands of trees," says Normand.

Of two females closely tracked, one ate from 391 separate trees, averaging 14 trees per day, while the other ate from 506 trees, averaging 18 trees per day. On average, each chimp revisited each tree once every five-and-a-half days.

Remarkably, as well as remembering the location of their favourite trees, the chimps also recalled when each tree would be in season, producing the most fruit. They would then often walk further to reach these more bountiful trees rather than make a shorter journey to a less productive one.

"Across all seasons, it seems that they have preferred tree species," says Normand.

A male chimpanzee eating some leaves on an inselberg
A male chimp has to make do with eating leaves not fruit

"Like when it is the coula nuts season, chimpanzees crack nuts using tools for hours during a day. Or when it is the Sacoglottis fruits season, then the chimpanzees stay hours digging their fruit wadge in the water to press a maximum of juice from those fruits."

Intriguingly, female chimpanzees travelled shorter distances to eat than males. The researchers don't know why, but speculate that it is either because females better remember the locations of trees, or because males simply compete with one another by ranging more widely through their territory.

In one respect, it is not surprising that chimpanzees have developed an outstanding ability to navigate their home range, says Normand.

One idea, known as the "ecological hypothesis" proposes that the need to remember and find food resources, such as fruit trees, could have driven the evolution of primate brains. In particular, it says that a preference for fruit eating, or frugivory, would select for intelligence compared to leaf-eating, or foliovory.

"That's because the distribution of fruits is more scattered, less predictable and fruits can be more difficult to manipulate than leaves, the nut cracking by Ta chimpanzees being an extreme example," says Normand.

Compared to monkeys, chimpanzees live in larger territories and are highly frugivorous, suggesting that developing an outstanding ability to navigate to fruit trees could have a key driver in the evolution of ape intelligence.

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