Captive orangutans who had never seen stone tools could figure out how to use them to strike or cut things, but they couldn’t figure out how to make them.
February 16, 2022
Captive orangutans who had never seen stone tools spontaneously picked up stones and used them as hammers. An individual also used a sharp stone as a cutting tool.
The discovery suggests that even orangutans, which live in trees and rarely encounter stones, can find ways to use them. But there are limits: animals have never learned to make their own raw stone tools, even after learning to do so.
“We didn’t find that orangutans could combine these behaviors,” says Alba Motes Rodrigo, who led the research at the University of Tübingen in Germany.
Motes Rodrigo and his colleagues studied two male orangutans living at the Kristiansand Zoo and Amusement Park in Norway. The monkeys received a box containing a piece of fruit, sealed with a rope. They also received a concrete hammer and a piece of rock. In theory, the orangutans could have used the hammer to knock sharp flakes off the rock, then use the flakes to cut the rope and reach the fruit. But they didn’t.
The orangutans were hitting things with the hammer, though. “We found percussion, which is interesting because orangutans in the wild very rarely interact with stones,” says Motes Rodrigo. Orangutans could have been playing, she suggests.
In a follow-up experiment, Motes Rodrigo gave orangutans a shard of flint sharpened into an axe-like tool. “One person, after trying to open the boxes in different ways, got this sharp stone ax that I made myself and used it to open the box and get the reward,” she said. “They can spontaneously, without any training, recognize that a sharp stone can be used to cut and use it as such.”
In a similar vein, in another experiment, Motes Rodrigo gave orangutans sharp stones and then rewarded them with grapes if they returned them. This was intended to encourage orangutans to mentally associate sharp stones with rewards. Then the orangutan who had cut the rope now picked up a blunt piece of rock and “started banging it”. It knocked off a few sharp flakes, so in a sense the orangutan had made some sharp tools. However, he did nothing with them, suggesting that he still didn’t understand what he had done.
In a final experiment, Motes Rodrigo showed three other orangutans how to make stone tools. But despite repeated demonstrations, they never succeeded – even though one of them started hitting the block of rock with the hammer. “Obviously, mere observation was not enough for that,” she says. “There’s something else missing there.”
Orangutans may not be able to understand the spatial relationships involved in shaping the tools, or may not be able to think far enough ahead, she speculates.
Journal reference: PLoS OneDOI: 10.1371/journal.pone.0263343
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