Social Structure
Unusual in primates, orangutans are primarily solitary (live alone).
Social behavior varies between Bornean and Sumatran orangutans in that Sumatran orangutans have access to more fruit and sharing opportunities, leading to greater sociability.
Female & Young Social Structure
Adult females are less solitary than males and have even been described as semi—social. Females often live with their offspring, staying in almost constant physical contact with them for the first two years of their lives. Young orangutans will travel and sleep with their mothers until they reach five to seven years of age. Even as juveniles, female offspring range near their mothers.
Although primarily solitary in nature, there is growing support that orangutans live in loose (widely-spaced) communities. Because female offspring generally establish home ranges near their mothers, these loose communities consist of clusters of related females and an adult male with whom they all prefer to mate.
Young orangutans are highly social, establishing bonds with same gender and age mates. As they approach adolescence (seven to ten years of age), males move off on their own and females will often establish home ranges near their mothers.
Male Social Structure
Adult males are the most solitary, engaging in threatening displays upon meeting other males. They share company with females only for reproductive purposes.
Home Ranges
Orangutans are not territorial. Neither gender excludes others from home ranges that are continuously used. Males are however intolerant of each other and remain separated as much as possible when traveling in the same area.
Adult male orangutans develop a home range that consists of as many sexually responsive females as possible. Once pregnant, females are sexually uninterested for several years, forcing dominant males to move to other areas that include receptive females.
Bornean Orangutans
Bornean orangutans' home ranges overlap and are not defended.
Bornean females, usually with offspring, have home ranges between 0.5 to 5.0 km2 (0.2 to 2.0 mi2) in size.
Bornean males consistently have two to three times larger home ranges than females, ranging in size from 1.0 to 15 km2 (0.4 to 6.0 mi2).
Sumatran Orangutans
Sumatran orangutans' home ranges are large, stable and overlapping. Individuals living in rugged, mountainous terrain have smaller home ranges than those who live in swampy, flat terrain.
Sumatran females living in rugged environments have home ranges around 3 km2 (1.2 mi2) in size. Females living in swamp environments have home ranges around 8.5 km2 (3.4 mi2) in size.
Sumatran males living in rugged environments have home ranges around 8 km2 (3.2 mi2) in size. Males living in swamp environments have home ranges around 25 km2 (10 mi2) in size.
Social Behavior
Males are generally intolerant of each other and will avoid each other in overlapping home ranges. If two males meet, a confrontation is likely to occur. Threatening displays often include staring, inflating their throat pouches, producing long call vocalizations, and shaking branches.
Females are more tolerant of each other than males and will occasionally spend time together feeding. If both females have young, the offspring will play together during their mothers' feeding time.
Communication
Vocal Communication
One of the best known orangutan vocalizations is the long call produced from adult males. This vocalization is composed of a series of grumblings and bellowings, which can last over four minutes in duration. It is thought the call serves several functions.
- Territorial call — to identify the male's location.
- Attraction call — to attract receptive females/ repel rival males.
- Community call — a social signal to coordinate seasonal movements for food resources.
The fast call is used as a threat. It is similar in sound to the long call but with a faster pace and shorter duration.
A kiss-squeak vocalization is made when orangutans become annoyed. It is produced by pursing their lips together and making a smacking sound.
Visual Communication
Orangutans, like most great apes, are highly visual in nature and use a variety of visual expressions to communicate.
Visual communication may involve the whole body; posture, movement, facial expressions, and external objects such as tree branches are often used in visual communication.
- Playful — Relaxed open mouth teeth covered.
- Threatening — "Males" — Inflation of throat sac posture exaggeration for a larger, more formidable appearance. Males and females will also bare their teeth in an open-mouth threat.
- Warning — Shaking tree branches in expressive ways to ward off intruders
- Calming communication — Sideways glances.
Olfactory Communication
Adult males produce a musk-scent that is marked throughout their home range as an attractant for receptive females.
Individual Behavior
Tool Use
Orangutans use sticks to help knock fruit down from trees and as visual deterrents when threatened (shaking and/or throwing them).
Leaves have been used as a sponge to obtain water from streams and to wipe unwanted substances from their hair.
Bornean orangutans have been observed taking shelter from the sun or rain by holding twigs or leaves over their heads.
Sumatran orangutans have been observed making leaf gloves or cushions to protect themselves when consuming thorny foods.
Nest Building
Orangutans construct nests in trees just before sunset each night. Most often located near the last food source visited for the day, the nests are formed by bending and weaving small branches.
Nests are built 18 to 28 m (60-90 ft.) up in the trees. Branches used in nest construction are usually tested by swinging on them to ensure its strength can withstand their weight for up to 14 hours of sleep.
Nests can take up to 30 minutes to build.
Brachiation
Brachiation refers to the manner in which orangutans move through trees. Using their long fingers to hook over a branch, they swing forward grasping the next branch with the other hand. In this manner, orangutans traverse the treetops hand over hand.
Orangutans climb trees as high as 45 m (150 ft.) and often do not touch the ground for three weeks at a time.
Orangutans do not readily go into open water but have been documented wading in waist-deep water to attain food sources.
