Sumatra, Indonesia (CNN) -- A loud crack echoes throughout the canopy as two young orangutans come tumbling down, grasping at branches along the way to break their fall. They recover and sheepishly scamper back up.
This is lesson one of jungle school here in the forests of central Sumatra, one of the few places where orangutans are being successfully rehabilitated into the wild.
"They have to learn that their whole environment is completely different from the cage," says Peter Pratje of the Frankfurt Zoological Society. "They have to learn that branches and small trees -- the size of bars in the cage -- don't carry them any longer. They bend and break."
"During the first phase of this jungle training, they are very often falling out of the trees because they use rotten branches."
The two youngsters swing awkwardly between the trees. On the ground below, their trainers keep a watchful eye and try to coax them toward fruit trees. Learning to forage is another crucial lesson in survival.
Back in the massive enclosure, the orangutans undergo enrichment exercises to keep their minds occupied and prolong their feeding time.
Success here is critical. Scientists say the Sumatran orangutan will be the first great ape to go extinct.
"The orangutan is an extremely vulnerable species because they have a very slow breeding cycle. Usually an orangutan stays for around 7 to 9 years with its mother," Pratje says. "Besides natural mortality, if there is only a little increase in mortality over a longer time already it drives an orangutan population to extinction."
The numbers of Sumatran orangutans have already dwindled to around 6,000. The main reason for that is habitat destruction. Sumatra has lost 85 percent of its natural forest, mainly due to palm oil and pulp and paper companies, scientists say.
The sanctuary, a Frankfurt Zoological Project, is just outside the Bukit Tigapuluh National Park. The lowlands surrounding it are an ideal habitat for orangutans, and it's where those that have been released are choosing to build their habitats. According to the environmental group WWF, it's also home to the endangered Sumatran elephant and a quarter of the critically endangered Sumatran tigers left in the wild.
But the area, which is not currently protected, is being threatened by pulp and paper companies that want to see the region turned into plantations. So far the government has rejected logging permits, but unless this is declared a conservation area, Pratje and other conservationists fear that could change.
So far, more than 100 orangutans have been released here, with just over a dozen more in various stages of training. Pratje has lived here since he established the sanctuary seven years ago, dedicating his entire life to this project.
"It's like fighting for a member of my family," he says. "I love them because they are smart, smarter than other great apes."
But he says the fight is not just about the extinction of a species. The orangutan has become the ambassador for the threatened rainforest.
"If we sacrifice these forests, we may sacrifice our chances for getting medicine for important diseases," he says. "The problem is there is no second chance. If you shut down an ecosystem that is hundreds of years old, you can't regrow it any longer.
"So this is the last chance."