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Orangutans represent the only Asian species of great apes whose ancestor split from the line with the African great ape species 12-15 million years ago. They exclusively live in restricted areas on the islands Sumatra and Borneo and the populations on the two different islands. According to Grove (2001) they represent two distinct species: the Sumatran orangutan (Pongo pygmaeus abelii) and the Bornean orangutan (Pongo pygmaeus pygmaeus) with the latter split into three subspecies that are morphologically distinguishable.

Young orangutan running with arms raised.
Image by Dorothee Classen

Both species differ in morphological features such as length and color of the hair, shape of the male's cheek pads, the size and shape of the throat pouch and body build. However, the classification into two species is still debated given the high degree of variability of morphological features but also social characteristics within each species.

Orangutans are characterized by a strong sexual dimorphism. With a weight between 60—80 kg, males are twice as heavy as females, (Rowe 1996; Schulz, 1956). In addition, there are two different sexually mature male morphs (which is sometimes referred to as bimaturism, Bennett, 1998) that are distinguished by both morphological features and behavioral traits (Delgado & van Schaik, 2000). Fully developed or flanged males develop a number of secondary sexual characteristics, such as wide cheek pads, long hair and a large laryngeal sac (Knott, 1999; Rodman, 1988). Unflanged males are about the same size as adult females (Galdikas, 1985) and do not show these secondary sexual features, but they are sexually fully mature and able to sire offspring (Rijksen & Meijaard, 1999).

Current population estimates refer to 23,500 orangutans in Borneo and 3,500 in Sumatra.


Distribution and ecology

Orangutans are mainly restricted to lowland tropical forest close to rivers or swamps between 200 and 400 m altitude. They are characterized by an almost exclusively arboreal lifestyle in the canopy of the tropical rain forest. However, adult Bornean male orangutans sometimes travel on the ground since large terrestrial predators such as tigers are absent on this island.

Orangutan grazing in some bushes.
Images by Dorothee Classen

Orangutan lying on the ground.The orangutan density varies between 2-7 individuals per km2 close to rivers and less than one individual per km2 in higher elevations/altitudes. Orangutans are a frugivorous species feeding mainly on a wide range of different species of fruits, supplemented by leaves and animal protein, such as insects or eggs.

They spend most of their daily activity with feeding (45.9%) and resting (39.2%). Beside traveling (11.1%) and nest building (1%), only less than 2% are spent with social activities, such as mating, vocalizing or socializing.


Social structure and behaviour

The social structure of orangutans is described as an individual-based fission-fusion system highly variable over space and time. There are three basic social units that include a) adult females with their offspring, b) adolescent and/ or subadult individuals of both sexes, and c) solitary, adult males. The majority of adult females live as residents in a particular area for a longer period lasting up to several years.

Mother with baby orangutan
Image by Dorothee Classen

Orangutan food sharingThe size of female ranges varies between 0.6 and 1 km2, whereas adult males can cover ranges larger then 10 km2. However, residents represent the minority of an orangutan population. The majority are commuters that are characterized by a nomadic existence and are regularly present in a certain area for several weeks or months each year over several years.

In addition, there are few wanderers that are only seen infrequently or once. Since the ranges of different individuals can overlap, different social units can aggregate to form temporary associations or social groups during feeding in the same fruit tree or traveling. Whereas adult flanged males inhabit a certain range, unflanged males travel widely to enforce copulations with females. Their later ascent to a higher socio-sexual status appears to be associated with the establishment of long-lasting sexual relationships.