June 8, 2009

Chimps Use Memory To Map Most Productive Fruit Trees

New research finds that chimpanzees remember the precise location of their favorite fruit trees.

In fact, their spatial memory is so exact that they are able to locate a single tree among more than 12,000 others within an entire forest, primatologists have found. Furthermore, the chimps also remember how productive each tree is, and decide to travel further to eat from those trees they know will provide the most fruit.

Scientists say such an ability may have helped steer the evolution of sophisticated primate brains.

Emmanuelle Normand and Christophe Boesch of the Max Planck Institute for Evolutionary Anthropology in Germany and Simone Ban of the University of Cocody in Abidjan, Cote d'Ivoire studied 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," Normand told BBC News.

To determine how the chimps accomplished this, Normand's team mapped the location of 12,499 individual trees growing near a group of chimpanzees living in the Tai National Park in the Cote d'Ivoire. They then used GPS to map the precise position of each tree.

The scientists also identified 17 species of fruit trees that the chimps routinely fed from, and determined how often each of the trees belonging to these 17 species would be in fruit each month. Using that data, the researchers were able to determine the probability that a chimp randomly walking around the forest might encounter a fruit tree from which to feed.

The researchers found that the chimps did not seek out 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 chimps navigated toward the trees by their sense of smell.

Rather, the chimps were found to be targeting certain trees and walking directly to them. For instance, they visited one particular fruit tree, Pouteria aningueri, more than any other, despite it being one of the rarest trees in their home range, the scientists found.

The chimps also walked much shorter distances to each fruit tree than would be expected by random chance, confirming that they were directly seeking out these trees.

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

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

Surprisingly, as well as recalling the location of their favorite trees, the chimps also remembered the time in which each tree would be producing the most fruit. They would then often travel further to reach these more productive trees rather than make a shorter trip to a less productive tree.

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

"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."

Interestingly, female chimps traveled shorter distances to eat than their male counterparts.  Although the researchers aren't sure why, they speculate it is either because females better recall the locations of trees, or because males compete with one another by traveling more widely throughout their territory.

In at least one aspect, it is not surprising that chimps have developed such remarkable abilities to navigate their home range, Normand said.

One concept, referred to as the 'ecological hypothesis', says that the need to remember and locate food resources might have spurred the evolution of primate brains. It says that a preference for eating fruit, or frugivory, would select for intelligence compared with 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," Normand explained.

Chimpanzees live in larger territories and are highly frugivorous than monkeys, suggesting that the development of superior abilities to navigate to fruit trees might have been a significant driver in the evolution of ape intelligence.

The research was published in the journal Animal Cognition. The full report can be viewed at http://www.springerlink.com/content/h3kp01462p63x796/fulltext.pdf .


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