The orangutan, Asia's "wild man of the forests", could disappear in just 20 years, a campaign group believes.
WWF, the global environment network, says in the last century the number of apes fell by 91% in Borneo and Sumatra.
Globally, it says, there were thought to be somewhere between 45,000 and 60,000 orangutans as recently as 1987.
But by 2001 that number had fallen by virtually half, to an estimated 25,000- 30,000 of the animals, more than half of them living outside protected areas.
The apes, restricted to the islands of Borneo and Sumatra, are regarded as two species, the Bornean (Pongo pygmaeus) and the Sumatran orangutan (Pongo abelii).
Hesitant breeders
The Sumatran animal is classified as critically endangered, with possibly no more than 9,000 specimens.
Across their range they are at risk because of the fragmentation and destruction of the forests.
WWF says: "This is caused by commercial logging, and clearance for oil palm plantations and agriculture. Almost 80% of all forests in Malaysia and Indonesia have now been logged.
"The apes are also threatened by hunting and poaching for the bushmeat and pet trade, and by forest fires.
"Over 60% of orangutans are living outside reserves, and this catastrophic decline will continue until conservation efforts are scaled up to tackle habitat loss and poaching on privately-owned land."
Female apes become fertile around the age of 12, and can live for up to 40 years. But they have the slowest reproductive cycle of all the great apes, averaging eight years between births.
Domestic demand
Stuart Chapman, head of the species programme at WWF-UK, told BBC News Online: "A mother can probably bear four or five young and rear them successfully in her entire lifetime.
"One study suggested the orangutan could tolerate a loss in numbers of about 2% annually. But this loss of about 50% in just 15 years is completely unsustainable, hence the urgency of the conservation work.
"And for every orang that is caught and traded, we estimate five or six more die and are never found."
WWF says the international trade has declined sharply, because of Taiwan's improved enforcement of its import laws, but there is still demand in Indonesia for the animals as pets.
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