For the first time ever, their songs have been recorded in Vietnam where they are almost extinct.
Hunting
The recordings are proof that at least eight of the rare apes still survive in the area. The monkeys have been disappearing because of hunting and loss of habitat.
And if something's not done urgently, we may never hear their songs again.
This is according to researchers from conservation group Fauna and Flora International (FFI) who took the recordings on the Vietnam-Chinese border.
The endangered Tonkin Snub-nosed monkey, is found in just one part of the country.
Protection
Fact File
Black crested Gibbon
Also known as Hainan Gibbons after the Chinese island where 14 live
30 of them left in the world
Males and females sing harmonies to mate and mark territory
Four other species need help in Vietnam
Although schemes to help villagers protect the monkeys have been successful, the smaller apes have been ignored by the rest of the world.
People have been more worried about protecting other ape species like orang-utans. But they say it's now time to help before the recordings are all we have left of the creatures.
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