Most monkeys love fruit, but langurs are leaf specialists. Their stomachs have several chambers - a bit like a cow's - so they can slowly break down tough leaves that most other monkeys couldn't digest. That means they can find food in places where fruit is scarce.
A langur's tail is almost as long as the rest of its body. It is not for grabbing things - it works more like a balance pole, helping the langur stay steady when leaping between branches. Watch a langur jump and the tail flicks out behind it, like a kite's tail.
Vietnam's Delacour's langurs are the rarest. They live on the steep limestone cliffs of the north, like the kind that make up Ha Long Bay. They are black with bright white 'trousers' that make them look as if they're wearing little shorts. Fewer than 250 are left, which is why they are so carefully protected.
Langurs live in family groups led by the females. The whole family helps look after the babies, and the babies start their lives a different colour from the adults - bright orange! As they grow up, their fur darkens to match the grown-ups.