The Bornean orangutan is the world’s largest mammal that lives in a tree and is also one of the world’s greatest apes. It has shaggy, long, dark red-brown hair. Its facial color ranges from red to pink to black. Their long arms are useful for reaching fruits and swinging from branch to branch. Their hands and feet are scoop-like and have a powerful grip for grasping branches. Adult males occur in two forms, which are flanged or unflanged. The flanged males are larger and on both sides of their face they have fleshy ‘flanges’, or cheek pads.
Bornean orangutans rarely come down to the ground out of the trees where they live. They spend most of the daylight hours sitting in the canopy and eating, before building a nest to sleep in at night, by folding leafy branches. Small groups of females sometimes travel with their babies when seeking food, but adult males usually are solitary, though they may have occasional social connections. 6 or more in a group of Bornean orangutans is rare but this is found at times of mass fruiting, when groups of trees suddenly fruit all at the same time. Seasonal and daily movements change often and are influenced by how available the fruit is. Bornean orangutans use a number of different methods to move from one place to another. Brachiation (arm swinging) is seen only in young orangutans, older orangutans walking on all fours, or occasionally on two limbs.
Bornean orangutans are frugivorous, eating forest fruits, shoots, leaves, insects, sap, vines, birds’ eggs, spiders’ webs, fungi, flowers, bark, and sometimes nutrient rich soils.
Bornean orangutans are polygynous. A dominant flanged male has an established territory that encompasses the territories of a number of females. Females within a particular male’s territory mate with him to produce his offspring. These animals breed year round. After gestation of up to 9 months, a female bears a single infant, which will cling to its mother's fur, being completely dependent on her until about the age of 10. Despite infants being weaned at the age of about three, offspring continue to accompany their mother in order to learn about what food to eat, where to look for it and for safety. Finally leaving her when at least 8 years of age to establish a territory of their own, young females tend to stay close to their mother, while males may roam the forest for quite some time before finally finding their own patch. Females give birth at around 14–15 years old.
The major threat to Bornean orangutan is habitat loss due to deforestation and wild fire. The other great threat is illegal hunting by local people who often unaware that orangutans are protected by law. Several thousand orangutans are killed every year for meat consumption.
According to the IUCN Red List, the total Bornean orangutan population is between 45,000 and 69,000 individuals. This species, numbers are decreasing today and it is classified as endangered (EN).
Bornean orangutans have a vital role to play in seed dispersal, especially for larger seeds which smaller animals are not able to disperse. In fact, due to their essential role in seed dispersal, they are called "gardeners of the forest".