A female gibbon has been observed enhancing her territorial song with a percussive noise.
Each time her song reached its natural climax, the gibbon slammed shut the door of her enclosure, using the loud noise it made to accentuate her call.
The gibbon used the door to create a single beat rather than a rhythm.
But her behaviour is yet another example of how smaller ape species are also capable of novel tool use, says the primatologist who witnessed it.
Thomas Geissmann is a leading expert on the conservation and behaviour of small apes, which comprise four genera of gibbon and siamang.
Yet while great apes, the gorillas, chimps and orang utans, are frequently observed to use tools both in captivity and in the wild, gibbons are rarely seen to do the same.
That was until Geissman observed a female white-handed gibbon (Hylobates lar) living with a male at Zoo Seeteufel in Studen, Switzerland.
In many gibbon species, the males call out using a series of short distinct noises that gradually become more complex. At regular intervals, females join in, singing long phrases known as 'great calls'.
The gibbons are thought to produce these sounds to defend their territory, and they can be heard from up to 2km away in natural forest.
No other gibbons lived at the zoo with the white-handed pair, but a group of siamangs did live in an enclosure nearby.
But despite the absence of other gibbons, the pair sang regularly.
However, Geissmann soon noticed that the female would exhibit a rather unique behaviour every other time she made her great call.
Just a few seconds before she started her great call, Geissmann reports in the 5th edition of the Gibbon Journal, the female would retreat into her sleeping box, singing as she went.
She then half shut the sliding door to the wooden box.
At the climax to her call, she would then slam the sliding door shut, and after it bounced back open again, she would jump out of the box, thrashing her arms and legs in a display.
"She would go into her little sleeping box made of wood, and always at the same point of her duet song she would jump out and smash shut the sliding door, which made a bang," says Geissman, who is based at the University of Zurich in Switzerland.
"I realised what I had seen was tool use."
The noise of the door slamming can be heard to coincide exactly with the climax of her great call.
More often than not when singing, the gibbon would enhance her song in this way, and she almost never slammed the door at other times.
In all his years studying gibbons in the wild and in captivity, Geissmann has never seen or heard anything like it.
Listen to the song with the door slamming here.
Full story here.