Gibbon

• Facts
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• Conservation
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• SSP Status

Like all “lesser apes”, Siamangs do not have a tail (unlike monkeys, which do have tails), they have an upright body and a broad chest. They tend to hang below branches rather than balance on top of them and rely more on vision than on smell. Both the male and female Siamang are black furred with a throat sac. A throat sac is present to amplify sound. They have long anterior limbs and a deeply detached grasping thumb and toe. Both male and female have large canine teeth, which are used primarily for defense. Gibbons also develop calluses on their hind quarters. These are thought to aid with comfort when sitting. Fingers are long and slightly hooked, and are used to grip objects with impressive force. The thumb is opposable and shorter than other primates, this helps them to grasp branches without wrapping their thumb underneath. This means they do not have thumb to index capability. Their feet have an opposable big toe capable of holding and carrying objects. Siamangs also don't have a clavicle bone, which allows for 360° arm movement. This helps them swing hand over hand through the trees. This method of movement is called “brachiation”. Gibbons are the only true brachiators of the primates. In order to swing quickly gibbons will pull their legs close to their trunk and let their body swing through like a pendulum, then they let go with one hand before the other hand has found a grip allowing their body to project freely through space. Spaces between trees are negotiated this way. Observers have documented the distance of some flights to be between 25 to 30 feet. Gibbons are also incredible acrobats. They can change quickly from a two-handed swing to a two-legged run, jump, spin around the tree truck it grasps, swing in another direction, fly far through the air and land on a higher branch, then sit quietly, huddled up, as though never been in motion.

Siamangs are the largest of all the gibbon species. The males are larger than the females. They usually weigh approximately 9–11 kg (20–25 lbs). Their arm span is longer than the height of their bodies. This allows for greater mobility and helps them to reach to the most nourishing parts of the trees.

Gibbon Facts

Range

Malay Peninsula, Sumatra

Habitat

Siamang territories can overlap other gibbons with little conflict, unlike other gibbons whose territories do not overlap. Like other gibbons they prefer the forested region of their range. Their territory is determined by food supply—fruit should be available year round.

Diet

The siamang diet is similar to that of other gibbons, consisting of pulpy fruits, young leaves, flowers, and insects. However the siamangs will consume more leaves in their diet than most other gibbons. Leaves consist of 50% of a siamangs diet, whereas the diet of other gibbons consist of only 30% leaves. Gibbons are not very possessive about food within groups. They rarely quarrel over the choicest morsels. If one gibbon finds something especially good, he may allow other family members to eat the choice pieces or take part of what he is holding. When drinking a gibbon dips the back of its hand into the water raises his hand above his head licks off the drops.

Behavior

Gibbons spend between 3 to 4 hours of the day feeding. The Siamang social structure is monogamous, 1 male and 1 female with offspring in a family group. Siamangs have a greater cohesion of social interactions than most other gibbon species. One of the few important social interactions between adults is grooming. The next most important is playing between the juvenile members of the group. Adult pairs will also sing, either to advertise the establishment of a territory, to warn off other family groups, or to attract a mate. Siamangs have an inflated throat sac that amplifies the sound to such a degree that it can be heard up to two miles away. Elaborate singing can take place every day or once every five days depending on territorial conditions. The singing behavior follows a strict format. The song will start with an introductory sequence while the singers “warm-up” followed by duet of the male and female engaging in a “great call”. Siamangs are particularly good at singing duets. A “great call” sounds like a series of alternating booms and barks that starts slowly and quietly, but then accelerates into a deafening call at the climax. After the female’s “great call" the male will sing a coda at the end. Pairs can be identified by their particular song.

Reproduction

Gibbons usually produce offspring only once every 2–3 years, so a family group usually will consist of 2 juveniles, but sometimes they may include as many as four. Copulation usually occurs dorso-ventral with the female crouched on a branch and the male suspended behind her, although occasionally copulation will occur while the gibbons are facing each other. The female will gestate for 7–8 months before giving birth to one live young. The first 0 to 3 months the mother pulls up to form a sort of lap to support the infant while hanging or swinging from branches. The newborn hangs on to the mother’s belly constantly, even through the most daring acrobatics. The young’s fingers do not just grip bunches of hair, they literally entwine into the fur. At 3 to 4 months the offspring will detach itself more and more from the mother and at 4 to 5 months babies begin to swing hand-over-hand. At 6 months they may start moving 2 legs and at 7 months they will begin to play with siblings and with the father. Their playing consists of gymnastics, chasing, and wrestling. Eventually contact with the mother gradually diminishes to cuddling at rest periods and at night. Weaning is begun at about a year and a half. The male will participate in the care for the juvenile somewhere between 8 months and 1 year and will continue to participate in the raising of the juvenile until he achieves independence at around 3 years of age.

Lifespan

Approximately 35 years.

Conservation Status

The forest people of southeast Asia have a high respect for gibbons because of their resemblance to man and usually do not hunt them. Foreign influences, however, have lead to increased hunting by outsiders. Continual habitat destruction also threatens the stability of this species. Human encroachment, the building of new roads, and slash and burn clear-cutting of areas for agriculture are all examples. Slash and burn clear cutting will after 1–2 years cause the nutrients in the soil to be lost and washed away and will also pollute the surrounding waters. Forests are cleared for firewood, paper, lumber, and agriculture. Fragmentation of habitat creates small islands of gibbons populations, which in turn creates the potential of inbreeding among isolated groups. Gibbons are not adapted for crossing open country. Current reforestation efforts have not proven adequate because the gibbons disappear from an area once it has been cut down, never to return. Also the trees that have been planted are intended to yield valuable timber for humans, but do not provide the type of habitat and food sources needed to support gibbons. Oil operations in tropical rainforests have a severe environmental impact, including water and air pollution, soil erosion, sedimentation, and disturbance of wildlife and habitats. Making roads and pipelines induces colonization and extensive deforestation. The Vietnam War devastated many areas in the South Pacific where gibbons inhabit; military debris, destruction of forests, and the effects of herbicide chemicals such as Agent Orange. Gibbons are often harvested for the pet trade which is illegal under CITES Appendix 1. Often the mother and father are killed in order to capture the young gibbons. Nine out of ten animals die during illegal smuggling and the removal of one adult female per year from a population of less than a 100 will double the risk of extinction. As of April 1995, there are approximately 150 Siamangs in North America zoological facilities. Because of this high number, Siamangs have a lower conservation priority according to the American Zoo and Aquarium Association. Their captive population will be allowed to decline to as low as 80 breeding individuals. These individuals will be managed for education and study with limited breeding.

Gibbon Conservation Issues

General statement

Gibbons are the least studied apes and they are also highly endangered. Recently, a subspecies of white-handed gibbon found only in China was declared extinct. Habitat loss, due to deforestation, has been a primary factor in this, and perhaps other, extinctions of gibbon subspecies. Because their survival is so intimately tied to the existence of pristine primary forest, habitat destruction and human encroachment pose severe threats to their survival.

Deforestation

Bushmeat

Pets

Political unrest

Online resources

• Gibbon Conservation Alliance
• Gibbon Conservation Center
• Gibbon Network
• International Primate Protection League

AZA Gibbon SSP Status

The Gibbon Species Survival Plan (SSP) provides oversight for all gibbons residing in AZA zoos. SSP members advise zoos on nutrition, veterinary care, husbandry, and breeding plans. Maternal-infant care is also of special interest to the SSP team. Three gibbon species are currently managed by the SSP: white-handed gibbon, white-cheeked gibbon, and siamang. To learn more, visit the Gibbon SSP at: www.brookfieldzoo.org/pgpages/pagegen.260.aspx