Gibbons: Family life
Because Gibbons are small, there is usually enough fruit on a tree to satiate several of them, so it is practical for them to travel together and they live in tightly knit families. A pair is accompanied by up to four of their offspring of varying ages. Every morning, the family sings in chorus. The male starts with one or two isolated and tentative hoots, others join in, the group launches into a ecstatic song and finally the female takes over with a rising peal that gets faster and faster and higher and higher until it becomes a trill of tonal purity that no human soprano could ever challenge.
The parallel with the indri of Madagascar is an obvious one. Because of different ancestral histories, one creature uses its fore limbs as its major propellant, the other its hind. Otherwise, the tropical rain forest in different parts of the world has produced creatures that are remarkably similar- families of singing, vegetarian gymnasts.