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The siamang (, ; Symphalangus syndactylus) is an endangered arboreal, black-furred gibbon native to the forests of Indonesia, Malaysia, and Thailand. The largest of the gibbons, the siamang can be twice the size of other gibbons, reaching in height, and weighing up to . It is the only species in the genus Symphalangus. Fossils of siamangs date back to the Middle Pleistocene. Otherwise, the Malaysian individuals are only a population. The siamang occurs sympatrically with other gibbons; its two ranges are entirely within the combined ranges of the agile gibbon and the lar gibbon.

The siamang can live to around 40 years in captivity. While the illegal pet trade takes a toll on wild populations, the principal threat to the siamang is habitat loss in both Indonesia and Malaysia. The palm oil production industry is clearing large swaths of forest, reducing the habitat of the siamang, along with those of other species, such as the Sumatran tiger.

Description

thumb|A close-up of adult male's head

The siamang has long, dense, shaggy hair, which is the darkest shade of all gibbons. The ape's long, gangling arms are longer than its legs. The average length of a siamang is 90 cm; the largest they have ever grown is 150 cm. The face of this large gibbon is mostly hairless, apart from a thin mustache.

Distribution and habitat

The siamang inhabits the forest remnants of Sumatra Island and the Malay Peninsula, and is widely distributed from lowland forest to mountain forest—even rainforest—and can be found at altitudes up to 3800 m. It lives in groups of up to six individuals (four individuals on average) with an average home range of 23 hectares. Their day ranges are substantially smaller than those of sympatric Hylobates species, often less than 1 km. The siamang eats at least 160 species of plants, from vines to woody plants. Its major food source is figs (Ficus spp.). The siamang prefers to eat ripe rather than unripe fruit, and young rather than old leaves. It eats flowers and a few animals, mostly insects. When the siamang eats large flowers, it eats only the corollae (petals), but it eats all parts of smaller flowers, with the small fruit collected in its hand before being consumed. When it eats big and hard seeds or seeds with sharp edges, it peels out the fruit flesh and throws away the seed.

Seeding

As a frugivorous animal, the siamang disperses seeds through defecation as it travels across its territory. The siamang can carry seed while digesting, and defecate between from the seed resource, which supports the forest's regeneration and succession.

Demography and population

A group of siamangs normally consists of an adult dominant male, an adult dominant female, with offspring, infants, and sometimes a subadult. The subadult usually leaves the group after attaining the age of 6–8 years; subadult females tend to leave the group earlier than subadult males. Siamang gestation period is between 6.2 and 7.9 months; after the infant is born, the mother takes care of the infant for the first year of its life. Siamang males tend to offer more paternal care than do other members of the family Hylobatidae, taking up a major role in carrying an infant after it is about 8 months old. Both monogamous and polyandrous groups, though, are found in South Sumatra. They are also territorial, and interact with other family groups by making loud calls to let other groups know where their territory is. The calls may be asynchronous, where they are not directed at a particular neighbouring group, or simultaneous group calls may take place across the territory boundary. Males are known to chase one another across the boundary.

Role of calling

thumb|Siamang calling with throat sac inflated

thumb|Siamang, Sumatra, 2022

The siamang starts its day by calling in the early morning; it calls less after midday, with the peak of the calls around 9:00 to 10:00 am. Most of the siamang's calls are directed to its neighbours rather than to those inside its home range. This means the siamang's calling is in response to disturbances and to defend its territory. Calls in the late morning typically happen when it meets or sees another siamang group. The edge of the siamang's home range, which may overlap another, is often the place where calling is made. Counter (co-response) calling occasionally happens near the border or in the overlap area. Calls are numerous when fruit is more abundant rather than when it is less available. Branch shaking, swinging, and moving around the tree crowns accompany the calling. This movement might be to show the other groups where they are.

The siamang prefers calling in the living, tall, and big trees, possibly where another group is easy to see. Besides that, such trees can support siamang movement. Calling trees are usually near feeding trees, but sometimes they call in the feeding trees.

Mated pairs produce loud, well-patterned calling bouts, which are referred to as duetting. These calls advertise the presence and status of a mated pair. Siamang duetting differs from other species because it has a particularly complex vocal structure. Four distinct classes of vocalizations have been documented: booms, barks, ululating screams, and bitonal screams. Females typically produce long barks and males generally produce bitonal screams, but both sexes have been known to produce all four classes of vocalizations. Unlike other gibbons in which vocalization is added by laterally expanded laryngeal sacs, those in siamangs fuse with each other and extend into the ventral area of the neck.

Threats and conservation

As an arboreal primate whose survival absolutely depends on the forest, the siamang faces population pressure due to habitat loss, poaching, and hunting.

Habitat loss

thumb|240px|Siamang, [[Tierpark Hellabrunn, Munich, Germany]]

A major threat to the siamang is habitat fragmentation due to plantations, forest fire, illegal logging, encroachment, and human development. Firstly, palm-oil plantations have removed large areas of the siamang's habitat in recent decades. Since 2002, 107,000&nbsp;km<sup>2</sup> of oil palm have been planted, which has replaced much rainforest in Indonesia and Malaysia, where the siamang originally lived. Secondly, in the second decade of the 21st century, forests in the Malay Peninsula have been destroyed due to illegal logging. Sixteen out of the 37 permanent forest reserves in Kelantan, in the Malay Peninsula, where most of the siamangs live, have been encroached upon by illegal loggers. Thirdly, forest encroachments change forest cover into cultivated land; for example, the rising price of coffee in 1998 encouraged people in Sumatra to replace the forest with coffee plantations. Fourthly, development in many areas requires infrastructure, such as roads, which now divide conservation areas and have caused forest fragmentation and edge effects.

Poaching and hunting

Unlike other parts of Asia, primates are not hunted for their meat in Indonesia. They are hunted for the illegal pet trade, with hunters preferring infant siamangs. Poachers often kill the mothers first, since siamang females are highly protective of their infants, and removing the infant without first killing the mother requires more effort. Most siamangs on the market are infants, which often die during transportation.

References

  • Siamang photos
  • Siamang songs
  • Gibbon Conservation Center
  • Primate Info Net Symphalangus syndactylus Factsheet
  • http://www.sandiegozoo.org/animalbytes/t-siamang.html