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Orangutans live in primary and secondary tropical rainforests of South East Asia on the islands of Sumatra and Borneo. Annual rainfall varies on average from 3000 mm (Sumatra), to 4300 mm (Borneo). Temperatures on both islands are quite comparable (Sumatra: 17°C - 34.2°C; Borneo: 18°C - 37.5°C) as is the humidity (in both islands is near 100 % year-round).

Their range within these ecosystems is quite varied from lowland swampy areas to mountainous areas (such in Sumatra), from peat swamps (Tanjung Puting National Park for example) to dipterocarp forests. Peat swamp forests are located immediately behind the coastline and extend inland along the lower reaches of the main river systems. Dipterocarp forests occur on dry land just above sea level to an altitude of about 900 metre. They are characterized by the fact that most of the largest trees in these forests belong to one plant family known as Dipterocarpaceae.

The highest densities of orangutans are found in areas which have a mixed habitat that provide high quantities of food throughout the year, such as lowland swamp forests. Peat swamps support medium-densities of orangutans, and dipterocarp forests, with their typical extreme variability in the abundance of fruit from season to season and year to year, only support low densities of orangutans.

Sumatra is comprised of lowland swamps and wide, mountainous regions, with elevations up to 1500m. The orangutans here are found primarily in the lowland dipterocarp, freshwater and peat swamp forests. Higher elevations have lower population densities of orangutans, as tree diversity is much lower. The dipterocarp forests of Sumatra are especially famous for their mast fruiting, which occurs every 2 to 10 years. Mast fruiting is a phenomenon in which a large numbers of trees fruit simultaneously, despite the absence of any seasonal change in temperature or rainfall. During this time, orangutans greatly exceed their daily caloric intake requirements and put on additional fat stores. This propensity to overeat and store fat reserves may be why captive orangutans often struggle with obesity. When mast fruiting does not occur during a year, there is still an annual fruit peak.

Borneo consists largely of peat swamp forests and dipterocarp forests. Orangutans on Borneo are not found in forests at elevations higher than 1000m. Nevertheless they still occupy the remaining forested habitat in Borneo.