19 March 2015 Last updated at 09:12
By Victoria GillScience reporter, BBC News
Orangutans use their hands to alter their voices and make themselves sound bigger, say scientists.
The animals cup their mouths when they produce kiss squeaks - alarm calls that often signify a predator is nearby.
Researchers have now studied the acoustics of these "hand-modified kiss squeaks" and shown that the animals sound bigger and "more impressive" when they use their hands in the call.
The findings are published in the Journal of Experimental Biology.
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