Until very recently, there have only been two known species of orangutans, named after the two islands they live on. The Sumatran orangutan can be exclusively found on Sumatra which is a large island in western Indonesia. Their habitat is restricted to the north of the island, namely the Aceh province. The Bornean orangutan lives on Borneo, on both, the Indonesian and on the Malaysian parts of the island.
For a long time, it has been believed that the two species were, in fact, one as they are very similar in their appearance and their behavior. One of the only obvious differences is the longer facial hair of the Sumatran orangutans. Primatologists have observed that they also have closer social bonds and their offspring lives with the mother for a longer time while the Bornean orangutans become independent a bit earlier. Male adults in Borneo have been known to wander from the treetops more often than their Sumatran relatives and they will occasionally travel on the ground on all fours.
In 2017, Orangutan researchers have found a third species, which is quite a sensation as there are only six species of living great apes in total. The group of about 800 individuals that has lived isolated from other orangutans in the north of Sumatra has been named Batang Toru orangutans (Pongo tapanuliensis) after the region in which they were now discovered.