Siamang gibbons are the largest of the gibbons and have a particularly large throat sac which enables their loud call.
They swing through treetops hand over hand - a movement that's called brachiation. With their ability to rotate their shoulder sockets and their long, strong arms and extra long fingers, you can see how they have been designed for this sort of movement.
Their distinctive and very loud call happens at dawn and dusk, which helps maintain bonds between pairs and as a territorial warning.
Siamangs are found in Malaysia and Sumatra - where illegal logging and the burning of rainforests and peat swamp forests to plant oil palm plantations threaten the survival of this precious species.