Today orangutans are found only in the forests of Borneo and Sumatra. In Sumatra they are restricted to Gunung Leuser National Park and a few areas adjoining it. In Borneo they are more widely spread but their range is diminishing and now covers less than 1/3 of the island. Orangutans are divided into 2 subspecies, each on one island; a Sumatran (Pongo ablilii) and a Bornean (Pongo pygmeaus) (Anbel-Simons 2000; Kaplan & Rogers 2000; van Schaik 2004). Although both sub-species are very similar and can produce viable hybrids in captivity, there are some notable differences in appearance. Sumatran orangutans when compared to Bornean have lighter, denser and longer hair as well as lighter and narrower faces (van Schaik 2004). Differences can also be found among the two sub-species in terms of their social and cultural behavior (van Schaik et al 2003; van Schaik 2004; Strier 2003).

The forests of southeast Asia are the most rich and diverse in the world and orangutans are adapted to live in the wide range of forest found on Borneo and Sumatra ranging form lowland swamp forest to mountainous regions up to 1500m (Kaplan & Rogers 1994; 2000; Nowak 1999).

Temperatures and rainfall in the forests of Borneo and Sumatra tend to be very high creating high levels of humidity and a lush environment. Despite the lushness of the rainforest, the soil is actually very poor and rates of growth among the primary species of trees (i.e. dictocarps), is actually very slow. Patterns of fruiting and flowering in these forests is also very irregular and varied, a characteristic common to tropical rainforests Kaplan & Rogers 1994).

The flora and fauna of the rainforest are incredible and include a huga variety of plants including hundreds of species of orchids. Hundreds of species of small animals such as the flying fox, mouse deer and slow loris, as well as large animals like wild pigs and elephants can be found in these forests (Kaplan & Rogers 1994).

The islands are also home to many species of primates including gibbons, siamangs, proboscis and macaque monkeys (Kaplan & Rogers 1994; 2000; van Schaik 2004).