Gunung Leuser is the last forest on earth where orangutans, tigers, rhinoceroses and elephants still live side by side, and the animal it reliably shows you is the orangutan. Most of the world's surviving Sumatran orangutans depend on this rainforest in northern Sumatra, and you meet them here on foot: no vehicle, no hide, no feeding platform. That way of meeting a great ape has almost vanished from the rest of Asia. Walks start from Bukit Lawang, a village on the park's eastern edge, and last anything from three hours to three days. A guide is required, and nothing about the meeting is arranged in advance: your guide listens for a branch that swings out of rhythm, or for fruit skins falling through the leaves. When an orangutan appears it is usually in the canopy above you, often a female with an infant holding on to her side, feeding steadily and paying you little attention. Some of the animals near the village are used to people, so visitors stay seven metres back and nobody feeds them.
The forest holds more than the orangutans. Siamangs live here too, the largest of the gibbons, and at dawn pairs of them sing booming duets that carry for kilometres before the calling stops by mid-morning. Thomas's leaf monkeys live in this corner of Sumatra and nowhere else, and you see them crossing the branches overhead, each with a spiky white crest. The longer walks go hours past the point where day visitors turn back, ending at simple camps of tarpaulin and bamboo beside the Bohorok river. Most people return to the village down that same river, floating on inner tubes tied into a raft with a guide steering at each end. The walking is hot, steep and slippery in every month, and the drier season from May to September is what makes it easier. The other three animals stay out of reach. Tigers, rhinos and elephants keep to parts of the forest no visitor sees, so the orangutan is both the reason you come and the only one of the four you will meet.