The largest of gibbon species, which are sometimes known as the ‘lesser apes’, Siamang gibbons are known for their graceful movement from tree to tree and impressive emotive calls. They sometimes grow to double in size compared to other gibbons. Siamang gibbons have shaggy black fur, except for a gray area around their chin and mouth. Infants are entirely black. Males and females are similar in appearance. Their large naked throat sac acts as a resonator for penetratingly loud, deep, and expressive sounds.
The Siamang lives in Sumatra, Peninsular Malaysia, as well as a small area in southern Peninsular Thailand. It inhabits the Barisan Mountains of the west-central region of Sumutra, and the mountains to the south of the Perak River in Peninsular Malaysia. These are primary and secondary areas of tropical rainforest, which receive as much as five meters of rain annually. Siamangs occur in lowland, hill, and upper regions of dipterocarp forest, spending most of their time within the mid-upper canopy.
Siamangs live in family groups of a mated pair and their offspring, up to three in number. Such a group lives in a stable home range, 15 to 35 hectares in size, most of which they defend as a territory. The parents mark their territory by singing a duet. If an intruder (including a human) enters their territory, a male will confront it, while the female usually retreats out of sight. These gibbons wake at sunrise and first perform their morning "concert". They then set out to find food. It generally takes siamangs around five hours to eat enough to be satisfied. After being active for around 8 to 10 hours, they return to their sleeping place. Grooming is one of the siamangs' most important social activities. Adults groom usually for 15 minutes a day. It is a show of dominance, with the more dominant individual receiving more grooming than it provides. An adult male will groom a female and males which are sub-adult. During the breeding season, he will spend more time grooming the female.
Siamangs are monogamous animals and create pairs which remain together for life. They do not breed seasonally, and they produce a single young every two to three years. Birth follows a gestation period of about 230 days and the infant clings to its mother until it is about 3 to 4 months old. They are nursed by their mothers until about the age of two years. Males assist with parental care by helping to defend their young and defending the territory, and sometimes they will groom, play with, or carry their young. Older siblings will also help with rearing younger siblings. Young siamangs leave their family group at about six years of age, then spending several years seeking a mate. They reach sexual maturity when they are eight or nine years old.
The main threats to this species are the fragmentation and loss of forest habitat. Logging, road development, hydroelectric schemes and conversion to agriculture, are destroying the siamang's habitat. Capture to be sold as pets is also a significant problem, and the siamang is one of the gibbon species most under threat in this illegal trade, where typically the mother is shot to procure her young for sale.
According to the IUCN Red List, the total number of Siamangs today is unknown but a specific population in the Bukit Barisan Selatan National Park, Sumatra, was estimated to be around 22,390 individuals. Overall siamangs’ numbers are decreasing today and they are classified as endangered (EN) on the list of threatened species.
Being frugivorous, Siamangs are important for seed dispersal within their ecosystem.