An untrained captive orangutan can complete two main steps in the sequence of using stone tools: hitting rocks together and cutting with a sharp stone. It is the conclusion of a study conducted by Alba Motes Rodrigo. from the University of Tübingen (Germany) and colleagues, published in the open access journal “PLOS ONE”.
The researchers tested tool making and use on two captive male orangutans (Pongo pygmaeus) from Kristiansand Zoo, Norway. They were not previously trained or exposed to demonstrations of target behaviors.
Each orangutan was provided with a concrete hammer, a prepared stone core, and puzzle boxes with bait that required them to cut a rope or silicone skin for a food reward. Each of the orangutans automatically hit the hammer against the walls and floor of the enclosure, but did not direct the blows to the core of the stone.
In a second experiment, the orangutans were also given sharp man-made flint flakes, which the orangutans used to cut through silicone skin, thus solving the mystery. This is the first demonstration of cutting behavior in an untrained or uncultured orangutan.
To see if the monkeys could learn the remaining steps by watching others, the researchers demonstrated how to tap a pulp to create a flint on three female orangutans at Twycross Zoo in the UK. After these performances, one of the females began using the hammer to strike the heart, striking toward the edge as shown.
This is the first study to report spontaneous stone tool use without close guidance in orangutans that were not induced by humans. The authors say their observations suggest that two key requirements for the emergence of stone tool use—beating with stone hammers and recognizing sharp stones as cutting tools—may have been present in our last common ancestor with orangutans, 13 million years ago.
“Our study is the first to indicate that untrained orangutans can spontaneously use sharp stones as cutting tools.” underline. “We also found that they easily engage in rocky percussion and that this activity sometimes dislodges sharp stone blocks.”
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