Utrecht University biologists have observed orangutans in Indonesia using tree leaves to deceive or scare off predators. By holding the leaves to their mouths, the apes can produce a lower tone of warning sounds, which make them seem larger than they actually are. The research results were published online in the scientific journal Proceedings of the Royal Society B on 5 August.
When orangutans see a leopard or a python, they press their lips together to produce a sound to warn the predator that it is being watched. The field studies conducted by Utrecht University behavioural biologist Madeleine Hardus and her colleagues demonstrate that some orangutans have learned how to alter these sounds by using leaves from trees. When in grave danger, the apes remove the leaves from a branch and hold them in front of their mouths with their hands, which lowers the pitch of the sound produced by a factor of four. The orangutans probably think this will scare off predators because a lower tone is usually indicative of a larger animal.
Deliberate use of sounds
It was always assumed that the sounds produced by apes were nothing more than a reflex. However, Hardus’ research shows that orangutans actually do exercise some control over the sounds they make. This is consistent with the notion that the leaf technique was discovered and remained part of the orangutan population’s behavioural repertoire because the animals learned it from each other. As a result, the research results provide a fresh perspective on the cultural behaviour of apes, as well as on human evolution. According to Hardus, ‘For years, the ability of anthropoids to communicate was underestimated. This study will open new doors for research into the evolution of language.’
Publication
Tool use in wild orangutans modifies sound production: a functionally deceptive innovation?, Madeleine Hardus et al., Proceedings of the Royal Society B.
Images and recordings
Photo above: Orangutan holding leaves to its mouth.
Video and audio recordings available on request.
More information
Robert Kerst, Faculty of Science Information Officer, +31 (0)30 253 1422,
r.kerst@uu.nl.
If there is no response, please contact Roy Keeris, Utrecht University Press Officer, +31 (0)30 253 2411,
r.b.keeris@uu.nl.