Siamang

About the siamang

The siamang is an arboreal, black-furred gibbon native to the tropical rainforests of Indonesia, Malaysia, and Thailand. It is the largest of the gibbons, able to grow to twice the size of other gibbon species, reaching about 1 meter (3.3 feet) in height and weighing up to 14 kg (31 lb). It is the only species in its genus, Symphalangus, and fossils attributed to siamangs date back to the Middle Pleistocene. Two subspecies are recognized: the nominate Sumatran siamang and the Malaysian siamang of the Malay Peninsula, and the species shares its range with two other gibbons, the agile gibbon and the lar gibbon.

Two features set the siamang apart from other gibbons. The second and third toes on each foot are partially joined by a membrane, which is where its scientific name, syndactylus, comes from — Greek for "with finger." More strikingly, both males and females have a large throat pouch, called a gular sac, that can inflate to the size of the siamang's own head and lets it produce loud, resonating calls. Its long, dense, shaggy hair is the darkest of any gibbon's, its arms are longer than its legs, and its face is mostly bare except for a thin mustache. Average length is about 90 cm, with the largest recorded individuals reaching 150 cm.

Siamangs live in the forest remnants of Sumatra and the Malay Peninsula, from lowland forest up to mountain forest and even rainforest, at altitudes as high as 3,800 meters. They travel in groups of up to six, four on average, holding a home range that averages 23 hectares — a notably smaller day range than the sympatric Hylobates gibbons they share the forest with. Their melodious calls typically ring out in the early morning, after the agile and lar gibbons have already called.

Behavior and diet

Fruit makes up a large share of the siamang's diet, with the Sumatran population especially frugivorous — fruit accounts for up to 60 percent of what it eats, much of it high-energy fruit like figs and durians. Field studies in Malaysian rainforests documented this selective frugivory alongside a fallback on leaves when fruit is scarce. Across its range the siamang eats at least 160 plant species, favoring ripe fruit over unripe and young leaves over old, and it prefers only the petals of large flowers while eating smaller flowers whole. Despite its heavy fruit intake, the siamang is the most folivorous of all gibbons, fitting a broader primate pattern in which larger species lean more on leaves. As it travels and digests, it disperses seeds through its droppings, sometimes carrying them hundreds of meters from where they were eaten, which helps forest regeneration.

A siamang group typically consists of a dominant adult male and female with their offspring, sometimes joined by a subadult that has not yet left home; subadults, especially females, generally depart between six and eight years of age. Gestation runs 6.2 to 7.9 months, and mothers care for infants through the first year of life, though siamang males take on more paternal care than is typical among gibbons, carrying the infant regularly from around eight months of age onward. Pairs are generally monogamous, though both monogamous and polyandrous groups occur in South Sumatra, and infants in monogamous groups tend to receive more overall male care.

Siamangs rest for more than half of their waking hours, interspersed with feeding, foraging, moving, and social time such as grooming, which is one of their most important bonding behaviors. They are highly social and territorial, using loud calls, along with tactile and visual gestures, to communicate within the family and to announce territory to neighboring groups. Mated pairs perform elaborate duets — described as booms, barks, ululating screams, and bitonal screams — that advertise their bond and help defend territory; newly paired siamangs sing more than established pairs. These calling bouts, often made from tall trees near feeding sites, peak in the late morning and are usually aimed at neighboring groups rather than at other members of the caller's own group.

Range and habitat

The siamang is found only in the forest remnants of Sumatra and the Malay Peninsula, occupying everything from lowland to mountain forest and rainforest at elevations up to 3,800 meters, though it favors lowland forest between 500 and 1,000 meters in Sumatra specifically. Bukit Barisan Selatan National Park, the third-largest protected area in Sumatra at 3,568 square kilometers, held an estimated 22,390 siamangs across roughly 2,570 square kilometers of remaining forest cover as of a 2002 census — a steep drop from an estimated Indonesian wild population of 360,000 in the 1980s. Group composition and density both shift with habitat disturbance: burnt and regrown forest supports smaller, sparser groups with more adults and subadults, while intact forest holds more infants and juveniles and lower infant mortality.

Conservation status

The siamang is listed as Endangered. Habitat loss is its principal threat, driven by palm oil plantations — more than 107,000 square kilometers of oil palm have been planted since 2002, replacing rainforest across Indonesia and Malaysia — as well as illegal logging, forest fires, agricultural encroachment such as coffee planting, and infrastructure development that fragments remaining forest. The illegal pet trade adds further pressure: poachers target infant siamangs and often kill protective mothers first, and many captured infants die during transport. The species is found in at least 11 protected areas across Indonesia, Malaysia, and Thailand, including Gunung Leuser and Way Kambas National Parks and the Hala Bala Wildlife Sanctuary.