Gibbons are sometimes called 'lesser apes' - cousins of bigger apes like gorillas and orangutans, but smaller and lighter. The cao vit gibbon weighs about as much as a cat. Its arms are so long that when it stands upright, its fingertips reach below its feet.
Those long arms are perfect for swinging. A gibbon can fling itself between branches at over 50 km/h - faster than most cars on a country lane. It can leap up to 12 metres from one tree to another, which is the length of a large classroom.
Each morning, gibbon families sing together. The males and females have different parts of the song, and the older youngsters join in too. The whole family duet can last 10 to 20 minutes and can be heard a kilometre away. The point is partly to say 'this is our patch of forest' and partly just because they like singing.
The cao vit gibbon was once thought to have disappeared completely. Then, in 2002, a small group was found in a Vietnamese forest. Today there are around 130 of them. Scientists in Vietnam and China are working together to protect the trees they need, so the morning songs keep going.