Siamang Gibbons

Siamangs are a tailless, arboreal, black furred gibbon native to the forests of Malaysia, Thailand, and Sumatra. They are the largest of the lesser apes and can be twice the size of other gibbons, reaching 3 feet (1 meter) in height, and weighing from 24 up to 50 pounds (11 to 23) kilos.

Siamang with inflated throat pouch while shouting.

The Siamang is distinctive for 2 reasons — 2 fingers on each hand are fused together and the large “gular sac” (found in both male and female of the species), which is a throat pouch that can be inflated to the size of its head, allowing the Siamang to make loud resonating calls or songs.

They live in groups of up to 6 individuals with a home range 23 hectares on average. The Siamang’s melodious choir singing breaks the forest’s silence in the early morning after the Agile Gibbon or Lar Gibbon’s calls. The Siamang in Sumatra and the Malay Peninsula are similar in appearance, but there are some differences in behavior between the 2 populations. A group of Siamang normally consist of an adult dominant male, an adult dominant female, with offspring, infant and sometimes a sub-adult. The sub-adult usually leaves the group after the age 6 to 8 years, with the sub-adult females generally leaving the group earlier than males. A study in relation to effect of habitat disturbance on the Siamang found that group composition is varied in age-sex structure between intact forest and post-burnt forest due to lack of food resources and trees for living.

The post-burnt population was more adult and sub-adults than the intact population. Post-burnt groups contain fewer infants, small juveniles and large juveniles compared to intact forest groups. Infant survival rates in post-burnt groups are lower than in intact forests. The number of individuals in intact forests is higher than in post-burnt forests.

Inflated throat pouch

These creatures typically rest for more than 50% of their waking period from dawn to dusk, followed by feeding, moving, foraging and social activities. They take more rest during midday, taking time to groom each other or play. Feeding behaviors, foraging, and moving are most often in the morning and after resting time. Siamang in southern Sumatra do less foraging than the those in other places because they eat more fruit and therefore consume more nutrition, which results in less time needed for looking for food. They will sometimes spend all of the day in one large fruiting tree, only moving out when they want to rest and then returning again to the fruiting trees.

They start their day by calling in the early morning and call less after midday, with the peak of their calls around 9:00 am to 10:00 am. Most of the Siamang’s calls are directed to their neighbors rather than to inside its home range in response to disturbances and to defend their territory. Calls in the late morning typically happen when they meet or see another Siamang group.

Siamangs prefer calling in the living, high and large trees which can support their movement. Calling trees are usually near feeding trees but they will sometimes call in the feeding trees. The Siamang can live up to over 30 years in captivity. Siamangs are part of the European Endangered Species Program (EEP) because they are under threat from habitat destruction in their native countries.

As an arboreal primate, they absolutely depend on the forest for existence, needing trees for their living. They are currently facing a population decrease due to habitat loss, poaching and hunting. While the illegal pet trade takes a toll on wild populations, the principal threat to the Siamang is habitat loss in both Malaysia and Sumatra. Palm oil production is clearing large swathes of forest, reducing their habitat.

Forest fires have destroyed 2,000,000 hectares of rainforest in the last 2 decades and palm oil plantations have replaced over 10,000,000 hectares of rainforest in Indonesia and Malaysia. They are poached and hunted for the illegal pet trade, mostly for infant Siamang. Poachers kill the mothers because mother Siamang are highly protective of their infants, making it very difficult to remove the infant without first killing the mother. Despite the fact that most Siamangs on the market are infants, many infants die during transportation. In the 1980’s, the Indonesian population of the Siamang in the wild was estimated to be 360,000 individuals. A 2002 census revealed a number of 22,390 Siamangs in Bukit Barisan Selatan National Park (BBSNP) — the third largest protected area (3,568 sq kilometers) in Sumatra, of which approximately 2,570 square kilometers remain under forest cover.