Gunung Leuser is a rainforest in the mountains of northern Sumatra, and it is the last ground on Earth that orangutans, tigers, rhinos and elephants still share. The orangutans are the ones you walk in to meet, from the village of Bukit Lawang at the forest's edge. You find them the way your guide does, by ear before eye: a branch that swings out of rhythm, fruit skins dropping through the leaves. Then the pointing up. A red-haired ape is moving through the canopy overhead, often a female with an infant at her side, feeding and paying you very little attention. Many of them are only half wild, descendants of a rehabilitation centre opened here in 1973 to return confiscated pets to the forest, and that is why they come near. The same trails turn up Thomas's leaf monkey, slim and spiky-crested, a primate that lives in northern Sumatra and no other place.
Out on the valley trail before the light is properly up, the sound comes off both slopes at once. It is the siamang, the largest of the gibbons, which fills a grey throat sac nearly as big as its head and booms a duet with its mate that carries for kilometres. You hear a pair long before you can place them, and by mid-morning the calling has stopped for the day. The Bohorok runs past the village clear and cold straight off the mountains, and floating down it on a rubber tube is how everyone washes the day off, the current doing the work while the forest slides past on both banks. That river has another face. In November 2003 it came down the valley at night carrying logs cut illegally upstream and killed 239 people, and the village was rebuilt on higher ground. Two- and three-day treks go hours past where the day walkers turn back, to a camp of tarpaulin and bamboo on a bend of the same water, with dinner cooked over a fire and eaten off banana leaves. Nothing up there wakes you with light. The gibbons start before it.