Orangutans are found in Indonesia on the islands of Sumatra and Borneo.  These apes were once widespread throughout mainland Asia during the Pleistocene (3).  Orangutans prefer a habitat that exists along waterways and in lowlands because of their high dependence on fruit.  Biologists have described their habitats as inaccessible, swamp logged, leech-laden tropical forests (5).  The islands of Borneo and Sumatra have vast mountain ranges and Orangutans are rarely seen in elevations that exceed 500 meters.  An Orangutan's normal habitat is the tropical rainforest that is dominated by trees of the family Dipterocarpaceae, which interestingly enough, is also becoming endangered in parts of the world.

In Sumatra, Orangutans are entirely arboreal because of large predators (such as tigers, leopards) patrol the forest floor.  Borneo contains fewer large predators so males will travel Terrestrially and feed on the ground, but females will remain in the trees because of their smaller size.  In traveling through trees, Orangutans have shown intelligence by using common routes and showing the ability to back track to their original route if need be.  Orangutans are slow and deliberate animals and their movement in the trees reflects this.  It has also been seen that Orangutans can occupy peat swamp forest in addition to tropical forest (2).  Their habitat mainly reflects areas where the greatest majority of food is located and areas that will give them the most protection. 

Map of Orangutan Distribution in Sumatra & Borneo

Courtesy of Wikipedia