Wild orangutans in Borneo hold leaves to their mouths to make their voices sound deeper than they actually are, a new study shows.
The apes employ the leaf trick when they are threatened by predators, according to scientists observing them.
By holding leaves to their mouths, the orangutans lower the frequency of the sounds they produce.
This is used to ward off predators, giving them the impression the apes are a bigger target.
The international team made the discovery while observing distress calls made by the orangutans.
The apes make the sounds in response to approaches by snakes, clouded leopards, tigers or humans.
These distress calls are known as "kiss squeaks" because they involve a sharp intake of breath through pursed lips, producing a sound similar to that made during a kiss.
But by using the leaves to modify the sound that comes out, orangutans deceive predators into thinking the calls are being made by a bigger animal.
Co-author Madeleine Hardus, from the University of Utrecht in the Netherlands, told BBC News: "This study clearly indicates that the abilities of great ape communication have been traditionally undervalued and that there may be traces of language precursors in our closest relatives, the great apes."
She added that the findings suggest that primate calling behaviour is not purely based on instinct, but instead is socially learned.
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