An orangutan will produce an alarm call known as a "kiss squeak" when it encounters a predator like a snake or a human. The kiss squeak is produced by drawing a sharp intake of air through pursed lips (see this video for an example). Sometimes, though, an orangutan will take a branch, strip the leaves from it, hold the leaves in front of its mouth and then make the sound. Why?
Researchers studying the wild Bornean organutan (Pongo pygmaeus wurmbii) recorded kiss squeaks made by the animals near a research station. (Their study appears in the Proceedings of the Royal Society B.) They found that the leaves lowered the maximum frequency of the sound (i.e., made it deeper). Also, smaller orangutans were more likely to use the leaves.
The orangutans appear to be using the leaves to make themselves sound like they are bigger than they really are. The scientists say that this is the first case of an animal using a tool to manipulate sound.