“Astonishing behaviour.” Filmmakers left a robot orangutan in the Borneo rainforest. The footage helped solve a mystery that puzzled scientists

The intelligence of chimpanzees has been frequently studied, such as their ability to use tools, medicines and even ‘toilet paper’. However, while orangutans have been subject to less scrutiny, they perform just as well as chimps in cognitive ability tests.

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So to learn more about these incredible animals, a team of researchers and filmmakers teamed up for the BBC TV series Spy in the Wild to film a group of them using spy cameras in the Borneo rainforest.

In the footage, an orangutan mother crosses a river to reach an old research post to obtain a bar of soap left behind by humans. Then, she shares it with a friend.

• Researchers spent 455 days and nights watching orangutans sleep – what they observed was very human

Around 40 years ago, rescued orangutans released here learned how to use soap by observing people washing in the river.

However, the orangutans that ‘Spy Orang’ films were born wild – indicating that this behaviour has spread among the wild population.   

Why the wild population continues this has been unclear. But then, Spy Orang captures a female doing something extraordinary. And it might just explain this behaviour.