Fossil evidence suggests the orangutan once ranged throughout most of south east and mainland Asia, encompassing both high and lowland areas in the wet and seasonal tropics of modern day Vietnam, northern India and southern China (van Schaik & Delgado, 2000). Today, the orangutan is the only great ape found in Asia, and is found on just two islands, Borneo and Sumatra. Situated in the Malay/ Indonesian archipelago and both part of the large Sunda continental shelf, Borneo and Sumatra are two of the world’s largest islands, and are isolated from each other by the South China Sea.

Until relatively recently, the orangutan was classified as one species, with two distinctly different, in appearance and behavior, sub species, the Bornean orangutan (Pongo pygmaeus pygameus) and the Sumatran orangutan (Pongo pygmaeus abelii). However, recent analysis of mitochondrial DNA has shown that the genetic difference between the two sub-species reaches values that comfortably separate species in other animals, including chimpanzees and gorillas, and, as such, the orangutan has been elevated to having two distinct species, the Sumatran orangutan (Pongo abelii) and the Bornean orangutan (Pongo pygmaeus) (Goossens et al, 2009).

Further studies have also shown that there are a number of sub-species of the Bornean orangutan, although the exact number is still disputed. Molecular data suggests the Bornean and Sumatran orangutans diverged between 2.7- 5 million years ago, and that the Bornean orangutan underwent a population expansion between 39,000-64,000 years ago, with the island’s rivers strongly determining migration routes and contributing to the genetic differences now seen in Bornean populations (Goossens et al, 2009).

There are currently two accepted sub-species, the Western Bornean orangutan (Pongo pygmaeus pygmaeus), which is found north of the Kapuas River in West Kalimantan in Indonesia, and in parts of west Sarawak in Malaysia, and the Southern Bornean orangutan (Pongo pygmaeus wurmbii) in south west Kalimantan, between the Kapuas and Barito rivers. A third population of orangutans south of the Mahakam River in East Kalimantan and in the Malaysian state of Sabah is either a third separate sub-species (pongo pygmaeus morio), or a separate population of the Western Bornean orangutan (Goossens et al, 2009).