Wild orangutans in Borneo hold leaves to their mouths to make their voices sound deeper than they actually are, a new study shows.

一项新研究显示，波罗洲的野生星星将树叶贴近嘴边使它们的声音听起来比平常低沉。

The leaf trick gives an exaggerated impression of the ape's body size

The apes employ the leaf trick when they are threatened by predators¹（食肉动物）, according to scientists observing them.

By holding leaves to their mouths, the orangutans（猩猩） lower the frequency of the sounds they produce.

This is used to ward² off predators, giving them the impression the apes are a bigger target.

The international team made the discovery while observing distress³ calls made by the orangutans.

The apes make the sounds in response to approaches by snakes, clouded leopards⁴, tigers or humans.

These distress calls are known as "kiss squeaks⁵" because they involve a sharp intake⁶（引入口，通风口） of breath through pursed（皱起，使缩拢） 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（低估，看轻） and that there may be traces of language precursors⁷（先驱者，前导） in our closest relatives, the great apes."

She added that the findings suggest that primate⁸ calling behaviour is not purely⁹ based on instinct（本能）, but instead is socially learned.