The Gunung Leuser National Park covers 8,000sq km of northern Sumatra. It's home to about one-fifth of the island's estimated 5,000-7,000 population of orang-utan, not to mention tigers, elephants, rhinos, clouded leopards, sun bears, tapirs and about 500 species of birds. Thankfully this wildlife population wasn't badly affected by last summer's forest fires, which were predominantly in the south - and, on an island the size of Spain, that's some distance away. Down in the south it was a different story, though, with the already endangered orang-utans having to flee their burning habitat, only to be popped in the cooking pots of starving villagers whose crops were also being destroyed.
This makes the work of the Orang-utan Rehabilitation Station at Bukit Lawang, on the edge of Gunung Leuser's protected jungles, all the more important. Here orphaned orang-utans are being reintroduced into the wild. This is no straightforward task, as an infant orang-utan will spend between five and ten years with its mother, learning how to climb, make nests to sleep in, and how to tell a durian fruit from something that might be deadly poisonous. These skills have to be taught instead by the staff at the station, while also weaning the apes off human contact.
Bukit Lawang is a tiny and laid-back riverside resort, surrounded by rubber and cocoa plantations. It takes about two minutes to walk round the centre, and 15 to walk up the path beside the Bohorok River to the rehabilitation station. Visitors are permitted across the river twice a day to watch the semi-wild orang-utans being fed; only about six people at a time can cross the river in a wobbly dug-out canoe hauled over on a pulley system.
"There's talk of building a bridge," said a station volunteer, Tanya Kemprud, "but I hope that never comes about. You need to make a balance between the needs of the orang-utan and the need to raise public awareness and raise money. A bridge will mean this becomes a tourist attraction, not a wildlife project. It will also mean that cats and dogs could then cross the river, bringing diseases."
When they arrive at the centre the orang-utans are quarantined for at least six months before being released into the jungle near the feeding station, where they may stay for a few days or even several months until they gain their full independence. They are deliberately given a monotonous diet of bananas, to encourage them to find food for themselves. Outside the quarantine area Tanya gives the handful of visitors an essential introductory chat.
"If an orang-utan approaches you when you go to the feeding station, just back away slowly and don't be tempted to touch it. They can give you disease, and you can give them diseases, which we're more concerned about. It might appear hard to resist a hug from an orang-utan, but I would ask you to make the effort."
As we climb the steep and slippy jungle track up to the feeding station, black gibbons are whooping in the tree tops. Slowly the orang-utans emerge from the trees to approach the feeders who've brought them bananas and a drink. One baby, as cute as they come, hangs upside-down from a branch with one foot while peeling a banana and pirouetting. Later he descends to the feeding platform to torment an older male, who responds with a wallop, causing the baby to bash the wooden floor in a tantrum. I'm not surprised to hear Tanya say later that we share 99 per cent of our DNA with these creatures.
"They are so intelligent, too," she goes on, happy to answer questions when we return to the station. "One of the orang-utans recently learnt how to unlock its cage by banging the lock with a rock so that it sprang open. One two-year-old took a few seconds to learn how to unfasten my belt, and then how to fasten it again."
We're interrupted by a call from the canoe man, waiting to take the last of us back over the river. "One moment, please," Tanya asks. "I have no more moments," he says, so we leave.
Next morning we leave Bukit Lawang for a two-day trek in the jungle. All visitors to Gunung Leuser must be accompanied by a licensed guide, and ours is Nasib Suhardi, who combines trek-leading with looking after his rubber trees. We walk through a rubber plantation, where we stop to see how the tree is cut and the sap caught.
"You cut the trees at 6am," Nasib tells us. "The trees self-heal if they're cut properly. You cut them three times a week, all year round. If you cut them every day you don't get as much rubber. If a family has between one and two hectares of rubber trees they should provide just enough to live on. Not getting rich, but enough so as not to starve. It's small money but regular."
Beyond here is a small durian plantation. "Orang-utans and tigers both love them," Nasib explains, "and the fruits fall in the night so we have to harvest them at midnight, because if we left them till the morning the animals would eat them. Westerners say they smell like shit but we like them. Want to try one?"
Nasib makes it sound so tempting that I can scarcely refuse, despite knowing that many hotels ban them from the premises because of the pervasive odour. Thankfully this one is more raspberry sorbet than latrine.
Nasib seems to know every plant and animal, though here on the edge of the jungle it is more flora than fauna - apart from a few small snakes, a turtle and butterflies that looked big enough to carry off your day- pack. We have a break and soak our chins with passion fruit, pineapple and oranges, welcome in the jungle's humid heat. We listen to the sound of white-handed gibbons. Near by, a tree crashes down. Then we hear an orang-utan calling; a large adult male, says Nasib.
"What are the chances of seeing one?" I ask. "Well," says Nasib, "sometimes we do, but I'd say maybe about 20 per cent."
He shows us the peacock fern which is a cure for bee stings, and other leaves to rub on everything from bleeding wounds to nappy rash.
As we camp that night by the river, Nasib busies himself crushing red chilli to make a fresh sauce for our evening meal of soya bean cake, rice, sweet vegetables and spicy vegetables. All I can say is that he must have brought a lot of red chilli with him. Breakfast is noodle soup and fresh pineapple, fortifying us for the steep climb of several hundred feet out of the river valley, deep as an axe-cut in the earth.
Why do I do this, I find myself muttering while the sweat pours off me. My thighs are burning as if the chillis have just reached them. Why do I take adventure holidays when I could be lying on a beach?
Then I discover why. "Sssh," says Nasib. He's heard the sound of an orang- utan somewhere in the canopy up above us. We stop and crane our necks, about the only bit of me that's not aching - till Nasib spots the dark shadow next to the trunk. Eventually it moves along a branch and the darkness turns to rust-coloured fur, and the shadow becomes a huge male orang-utan. It rips a bit from the branch and throws it in our direction, telling us he knows that we're there and doesn't like it. Sometimes they pee on humans, too, although ours decides to defecate, which may be coincidental. Thankfully it misses.
Not wanting to disturb it further, we walk on through the jungle till Nasib stops again. More orang-utans. He seems as excited as we are to see two females, each carrying a youngster. We stand silently looking up, and the mothers chatter and throw a few broken branches down. They don't seem distressed, merely wary. Slowly they move on, swinging through the branches, babies clinging underneath. We move on, too, and stop in a peaceful clearing. I sit on a log, open a passion fruit and sink my teeth into its sweet frogspawn seeds. A half-litre of water later and I feel almost human again. They say there's no pleasure without pain, and there are few pleasures to equal the sight of an animal in the wild. Even if it does try to shit on you.
The author travelled to Sumatra courtesy of Travelbag Adventures. It offers a 17-day Longhouses and Rainforest tour, including a jungle trek at Bukit Lawang. Trips depart all year round and cost from pounds 1,175, including return flights, accommodation and some meals. Travelbag Adventures (tel: 01420 541007).
For information about, and donations to help, the orang-utan of Sumatra contact The Orang-utan Foundation, 7 Kent Terrace, London NWI 4RP (tel: 0171-724 2912).
Sumatra (Periplus, pounds 11.95) is an excellent read, but Indonesia (Lonely Planet, pounds 15.99) and The Indonesian Handbook (Moon, pounds 15.95) also cover Sumatra in some depth.
Sumatra is mainly Muslim, so eat and greet with your right hand, leaving the left for toilet duties. Outside of hotels, the toilets are squatties. It is considered rude to blow your nose in public.
Sumatra is on the equator, so is warm all year with few seasonal extremes. The rainy season is roughly from October to April, though that only means it is even wetter than the "dry" season.