Pongo abelii
Sumatran orangutan

Sumatran orangutans inhabit the island of Sumatra, in Indonesia. These orangutans have been restricted to the northern tip of Sumatra in fragmented forest. Logging has severely limited the range of this species. (Rijksen, 1978; Singleton and van Schaik, 2002; WWF, 2007b)

Sumatran orangutans are found in primary tropical lowland forests, including mangrove, swamp forests, and riparian forests. They live almost completely in the trees, building nests in which they nap or sleep for the night. Preferred elevations are 200 to 400 m, the area in which their preferred fruiting trees occur, but Sumatran orangutans can be found up to 1,000 to 1,500 m. (Rijksen, Meijaard, and Van Schaik, 2003)

Sumatran orangutans are the largest non-human primates in Asia and the largest arboreal primates. They have long, fine red hair on their bodies and faces. Males have large cheek pads that are covered in a fine white hairs.The arm span, from finger tip to finger tip, is 2.25 m. The legs are small and weak compared to their muscular arms. There is sexual dimorphism between males and females. Female weights range from 30 to 50 kg and they can reach 1.3 m tall. Male weights range from 50 to 90 kg and reach a height of 1.8 m. Some old males may get too large to move around in trees easily and may have to resort to walking on the ground. (Maple, 1980; Rijksen, Meijaard, and Van Schaik, 2003; Sumatran Orangutan Society, 2007; WWF, 2007a)

Sumatran orangutans may be distinguished from Bornean orangutans (Pongo pygmaeus) by their longer fur, more slender build, white hairs on the face and groin, and long beards on both males and females, but molecular characters are considered most definitive. (Cocks, 2003)

Female life spans range from 44 to 53 years in the wild. There have been no reports documenting the onset of menopause and females seem to be capable of giving birth up to 51 to 53 years old. Male life spans are slightly longer, 47 to 58 years. Males are still considered healthy at these late ages by the tightness of their cheek pads and absence of bald spots. A captive female Sumatran orangutan lived to 55 years at the Miami Zoo. (Rijksen, 1978; Rijksen, Meijaard, and Van Schaik, 2003)

Wild orangutans are almost completely arboreal except for occasional forays on the ground. Exceptionally large males spend more time traveling on the ground, possibly because many trees cannot sustain their weight. Sumatran orangutans are active during the day and build new nests in trees each night in which to sleep. Nests are built with bent branches, sticks, and leaves. Young orangutans use brachiation extensively, but older, larger orangutans tend to use hand over hand motion. Sumatran orangutans are more social than Bornean orangutans (Pongo pygmaeus), spending more time in small groups. (Maple, 1980)

Grooming is a major source of social interaction in most primates, but there are few grooming techniques used by Sumatran orangutans, which are more solitary. Females occasionally scratch and preen each other. Social grooming has only been documented on the upper part of the body. Mother-offspring grooming has only been seen in zoos. In this case, most mothers have to hold down their offspring once the young are capable of independent locomotion. They also cut toenails and fingernails with their teeth. Most mother-offspring grooming is done by mouth; hands are rarely used. When grooming, they normally use one finger, moving it in one direction. The same technique is used for itching, but the whole hand or arm is used. When self-grooming, orangutans flip through their hair with their lips and mouth. They only attend to areas they can see and reach. (Maple, 1980)

Play is either non-social or social and has been recognized in adults and juveniles. Juveniles exhibit a surprisingly high amount of playful interactions, such as non-aggressive biting. Actual contact is not always necessary, sometimes simple body language is used as play. Compared to all other great apes, orangutans are capable of the most facial expression, due to their very flexible lips. Sumatran orangutans are exceptionally intelligent and capable of learning complex tasks and language. (Maple, 1980)

Home Range

Males and females share overlapping home ranges. Females range from 500 to 850 ha areas that overlap with each other. Male territories have a minimum range of 2,500 ha that will cover up to three female territories. (Fox, 2002; Maple, 1980; Singleton and van Schaik, 2002)

Male Sumatran orangutans are capable of long, exceptionally loud calls (called "long calls") that carry through forests for up to 1 km. The "long call" is made up of a series of sounds followed by a bellow. These calls help males claim territory, call to females, and keep out intruding male orangutans. Males have a large throat sac that lets them make these loud calls. They may also pull small trees and limbs down to add a crashing sound along with the call. Sumatran orangutans vocalize with grunts, grumbles, and squeaks when they meet each other, and young orangutans squeak, bark and scream. Both adults and young make a variety of sounds with their lips and throats, including sucking, burping, and grinding their teeth. (Maple, 1980)

Sumatran orangutan food choices vary seasonally. Most fruits are only available seasonally and within a limited range. Orangutans follow the fruiting season of local trees, feeding when they are ripe. Figs are one of the most important components of the Sumatran orangutan diet. During dry seasons, when fruit is less available, Sumatran orangutans will consume other vegetation. Fruit makes up about 60% of their diet, with the remainder being young leaves (~25%), flowers and bark (~10%), insects, mainly ants, termites, and crickets (~5%), and an occasional egg. (Sumatran Orangutan Society, 2007)

The primary predators currently of Sumatran orangutans are humans (Homo sapiens). Hunting of orangutans has decimated their populations. Natural predators of Sumatran orangutans are clouded leopards (Neofelis nebulosa) and Sumatran tigers (Panthera tigris sumatrae). These predator species are also under threat of extinction due to hunting by humans. (MacKinnon, 1974)

Sumatran orangutans play a critical role in the lowland rainforests of Sumatra and are considered a keystone species. As widely ranging fruit eaters, orangutans are important in dispersing seeds and maintaining diversity of rainforest woody plants. They also prune and aid in regenerating plant growth because they only choose to eat green leaves and stalks. (WWF, 2007a; WWF, 2007b)

There are no known negative affects of orangutans on humans. They are susceptible to many of the same diseases as humans, and thus can carry and transmit them as well, including tuberculosis, meliodosis, influenza, cholera, and intestinal parasites.

Sumatran orangutans are important in seed dispersal. The protected status of orangutans make them an umbrella species. As umbrella species, if orangutans are protected, so is the rainforest they inhabit and all of its associated biodiversity.

There is still an active illegal trade in orangutans as pets. (Thompson, 2007)

Critical orangutan habitat is rapidly being lost through illegal and legal logging in Sumatra. Their habitat has decreased over 80% in the last 20 years. Hunting orangutans for meat and killing adult females to obtain infants for the illegal pet trade has also caused an estimated decline in the orangutan population of 30 to 50% in the last 10 years. Uncontrolled forest fires have also harmed orangutan habitat. ("Sumatran Orangutan", 2006; Thompson, 2007; WWF, 2007a; WWF, 2007b)

Sumatran and Bornean orangutans were previously considered subspecies of Pongo pygmaeus. They were recently split into two species, Bornean orangutans (Pongo pygmaeus) and Sumatran orangutans (Pongo abelii).

In 2006, a subadult female that was captive-born was released into the wild from Perth Zoo in Jambi, Sumatra. This is the first attempt to release a captive-born orangutan into the wild. (Cocks and Bullo, 2008; Singleton, Wich, and Griffiths, 2007)

Fossil evidence suggests that Sumatran orangutans once occurred throughout Sumatra and the island of Java. (Wich et al., 2004)