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 -- do n'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 ca n't regrow it any longer . `` So this is the last chance . ''
Peter Pratje says orangutans in Sumatra are threatened by pulp and paper companies . He 's working to introduce orangutans back into the wild . `` The orangutan is an extremely vulnerable species , '' Pratje says .