The orangutan population is in danger and seriously on the decline due to hunting, illegal trade and deforestation. Some say they may become extinct within the next decade.
Hope for their survival rests in a safe haven in Borneo at a sanctuary called Nyaru Menteng.
The orphaned and often injured orangutans are brought here and put through a rehabilitation program before being released back into the wild.
Lone Droscher-Nielsen, an orangutan enthusiast from Denmark, co-founded the project with the Bornean Orangutan Society (BOS) with the support of local forestry officials.
"It all started when I came here on holiday in 1993. I came back and stayed," Droscher-Nielsen told ABC News. A personal passion project, Droscher-Nielsen even welcomed orangutans into her own home -- to live with her while she cared for them.
Over seven years, she has had anywhere from 12 to 24 animal roommates. It wasn't until recently that she got her house back to herself.
Today what's left of the orangutan population exists only in the rainforests of Borneo and northern Sumatra in Indonesia. Orangutans have close to 97 percent of the same genetic makeup as humans and are arguably the most intelligent of the primates. The word "orangutan," derived from Malay and Indonesian, translates to "person of the forest."
These forest people spend most of their time hanging around in trees -- their arms may reach up to 6.5 feet -- significantly longer than their 4-5 foot bodies.
The sanctuary, an hour and a half flight from Jakarta, is nestled in a quiet, isolated location surrounded by lush tropical trees. It has 183 staff members for 641 orangutans, allowing for a ratio of a little over three orangutans per person.
On the first day of arrival, each orangutan is quarantined for one to two weeks. They receive a general health checkup, are treated for parasites and tested for tuberculosis and hepatitis A, B and C. Visitors to the sanctuary are advised to stay at least 25 feet away from the animals to protect both species.
The youngest orangutans, under 2 and a half years old, are taken to baby school. Some wear diapers. They are encouraged to climb trees and make nests.
The 2- and 3-year-olds have class every day where they learn how to be orangutans. They are led by the staff, some holding their hands as they walk, to forest school where they are encouraged to find food on their own and relearn the skills necessary to survive in the forests again.
They train their muscles to survive in the forest in an area specially designed to replicate tree branches and trunks with swinging ropes and tires. Their day ends back at the sanctuary for socialization time where they learn to mix with other orangutans.
The older residents of the sanctuary hang out on prerelease islands that staff visit daily to feed and check on their health. But for the most part they are left to their own devices as this is the last step in the rehabilitation process before they are allowed to go feral.
Droscher-Nielsen and BOS hope their hard work will pay off so the orangutans can return safely to their natural habitat. So far, they have been successful with 36 orangutans now living back in the wild.