Two decades, and counting: The old man of the forest is facing a quick demise

The old man of the woods has two decades left. The orang-utan, one of the four great apes, is disappearing at such a rate that it is likely to be extinct in the wild in 20 years, the World Wide Fund for Nature (WWF) reports today.

Deforestation and hunting are taking such a toll that, within two decades, the orang-utan is likely to vanish from the wild in the only two places it still lives - the island of Sumatra, which is part of Indonesia, and the island of Borneo, which is divided between Indonesia, Malaysia and Brunei.

Figures from a recent conference in Kota Kinabalu, the capital of the eastern Malaysian state of Sabah, show that whereas in 1987 there were between 45,000 and 60,000 orang-utans in the world, by 2001 that number had fallen to between 25,000 and 30,000. According to WWF, such a rate of decline, about 50 per cent in 14 years,is likely to annihilate the animal in the wild in 20 years.

All four of the great apes - the orang-utan, the gorilla, the chimpanzee and the pygmy chimpanzee - are increasingly endangered. But the long-limbed, reddish-brown "man of the forest" - the literal translation of its name in Malay - is particularly vulnerable.

It is the only great ape that is a solitary animal, a fact that has been linked to its slow rate of reproduction. The orang-utan reaches sexual maturity at the age of 12 and can live until 40. But for the female orang-utan, there can be an interval of up to eight years between births. In a lifetime she may produce no more than four or five offspring. This means that the orang-utan's dwindling population struggles to rebuild its numbers.

Island species such as the orang-utan are also vulnerable because they cannot top up their numbers from elsewhere. Stuart Chapman, the head of the WWF-UK species programme, said: "The biology of the orang-utan simply does not allow it to sustain the spectacular rates of decline we are now seeing. A female tiger will produce three to four cubs a year and come into season again in the same year if its cubs are killed. The orang-utan is very different and its biology makes it completely unable to adapt to big reductions in its numbers."

Over the past 100 years, WWF believes that Sumatra and Borneo have lost 91 per cent of their orang-utans. But the decline has increased in recent decades because of the fragmentation and destruction of the animal's natural forest habitat. This has been caused by commercial logging and the clearing of land for oil palm plantations and agriculture. According to WWF, almost 80 per cent of forests in Malaysia and Indonesia have been intensively logged.

The orang-utan is also threatened by hunters and poachers working for the bush meat and pet trades, as well as by the forest fires that have ravaged Kalimantan, the Indonesian part of Borneo, in recent years. The fires, which in 1997-1998 spread a plume of smog over seven countries and affected 70 million people, are thought to have killed more than 1,000 orang-utans.

The fragmentation of the orang-utan's habitat is another problem. To survive, the animal needs to be able to move between protected areas, but the corridors between each one are usually through private land, and the tolerance of private landowners is low, said Mr Chapman. He said: "[Orang-utans] are vegetarian animals, primarily feeding on fruit and leaves, but they will raid orchards and oil palm plantations and are killed."

WWF and its associates in Borneo are testing different mitigation techniques to help orang-utans survive, including non-lethal ways of keeping them away from private land. Mr Chapman's main aim is to persuade landowners to be more tolerant of the animals. Another is to contain them within protected areas - 60 per cent of the population live outside protection boundaries.

The orang-utan is the largest of the tree-dwelling mammals and spends most of its time climbing slowly through the trees (its lack of a tail means it cannot jump). Its arms span up to 8ft, enabling it to swing from branch to branch.

The male orang-utan grows to about 5ft and can weigh up to 200lb. A dominant male orang-utan moves across a large territory mating with several females. Each mother spends years rearing her infant. The orang-utan used to be found throughout South-east Asia, but even its remaining habitat may not be home for much longer.

The orang-utan is arboreal, which means that it spends most of its time in the trees. It only comes down when it judges that a branch may not be able to bear its weight. Once on the ground, it moves on all fours. Remaining above ground protects it from its natural enemies, the chief of which is the tiger. It spends its nights as well as days in the trees, constructing sleep nests from branches each evening, which are roofed over to protect it from rain. Even water is obtained above ground. The orang-utan finds tree holes where rain has collected, dips its hand in and licks the water off.

Because of its size and the effort involved in travelling large distances through trees, it does not travel very far each day and it uses its intelligence and knowledge of the forest to find food. It is thought that an orang-utan can recognise up to 200 edible plants.

To destroy the forest is to destroy the animal's whole world.