Geographic Distribution and Habitat
The siamang is found among a wide distribution of habitats, ranging from the mountain forests of the Malay Peninsula, to the rainforests, monsoon forests, and lowland forests of Sumatra (the westernmost island of Indonesia). They can be found at altitudes up to 2.4 miles (3,800 m). They have relatively small ranges of about 60 acres (24.3 hectares), of which about 60% is defended as group territory. Territorial boundaries are difficult to determine, because confrontations between groups are rare, and their loud calls seem to create more space between groups. Siamangs are arboreal, meaning they spend most of their time in the trees.
The siamang is the only species in the genus Symphalangus. It has two subspecies: the Sumatran siamang (S. s. syndactylus) and the Malaysian siamang (S. s. continentis).
Size, Weight, and Lifespan
The siamang is the largest gibbon in the world, reaching 3.3 feet (1 m) in height, and weighing up to 31 pounds (14 kg). Siamangs exhibit a slight degree of sexual dimorphism (physical differences between males and females), with males being slightly heavier than females—females weigh on average about 23 pounds (10.5 kg), and males about 26 pounds (12 kg) on average. Their lifespan is 25–40 years in the wild.
Siamangs have long, shaggy, black coats covering their slender bodies, with pale hair around the mouth and chin. Their grayish or pinkish, hair-free throat sac, called the gular sac, inflates before and during vocalizations. In males especially, the sac can inflate to the size of the siamang’s head enabling him to emit the loudest calls of all gibbon species.
Their arms—which they use to acrobatically swing from tree to tree (known as brachiating)—are longer than their legs. Like humans, they have four long fingers and a smaller opposable thumb on their hands, as well as five digits on their feet. Unlike humans, however, their big toe is opposable, too. They also have webbing between their second and third toes, the use of which is unknown.
Siamangs have long canine teeth and, because they are apes, lack a tail.
Siamangs are omnivores. The majority of their diet consists of fruit and leaves. They also eat flowers, insects, spiders, small vertebrates, and bird eggs. Siamangs suspend themselves by one arm when feeding. The siamang eats at least 160 plant species, ranging from vines to woody plants. Siamangs prefer ripe fruit compared to unripe, and young leaves rather than old. When eating large flowers, they only eat the petals, whereas when consuming smaller flowers, all parts are eaten.
The Sumatran siamang is more frugivorous (fruit-eating) than the Malaysian siamang, with 60% of its diet consisting of fruit. A major food source of siamangs is figs.
Behavior and Lifestyle
Siamangs are diurnal, meaning they are most active during daylight hours. When moving by foot along a branch or on the ground, they gallop slightly on their two legs, stretching their arms over their head to maintain balance. Most of their days are spent traveling and foraging in family groups, covering a range of slightly more than a half mile (1 km) each day, and roaming within about 33 feet (10 m) of each other.
It usually takes around five hours for a siamang to eat its fill. After eight to ten hours of daytime activity, they find a place to take a break or sleep. Sleeping is typically done at designated sleep trees, and siamangs sleep sitting upright in the fork of the tallest part of tree, usually alone, but sometimes huddled together. Siamangs do not make nests.
Like most primates, one of the most important social activities (and displays of dominance) that siamangs participate in is grooming. Adults groom on average 15 minutes per day, and the more dominant individuals receive more grooming than they give. Typically, an adult male grooms both the females and sub-adult males; however, in the breeding season, they will focus more on grooming the former than the latter.
Thanks to their size, maneuverability, and security in the trees, siamangs have no natural predators.
The siamang’s throat sac can inflate to the size of a grapefruit.
The siamang’s call is one of the loudest in the animal kingdom. Their songs can reach up to 113 decibels, which is roughly equivalent to a chainsaw.
Siamang groups consist of an adult breeding pair and two or three offspring two to three years apart in age who have yet to leave the group. Group territories are around 60 acres (24 hectares) in size. The offspring of a breeding pair will leave their natal group upon reaching sexual maturity at around six to seven years of age.
The adult male and female of each group will vocalize in loud duets each morning and late afternoon for up to 30 minutes to mark their territory. These “songs” begin as deep, bell-like tones that escalate into shattering yells and culminate in high-pitched “laughter.” The adult pair sings in tune with each other, and often swings through the trees near the end of their song. The songs can be heard up to two miles (3.2 km) away through the forest. Confrontations between groups are rare, and if they occur, they involve the adult males performing high-speed chases through the trees, slapping and biting as they go.
Siamangs are a very social species, and use a variety of tactile and visual gestures, as well as actions and facial expressions to both communicate and increase the social bonds within their group. Grooming is used to maintain social bonds between group members, and loud songs are made by the adult breeding pair to maintain territorial boundaries between neighboring groups. Calls are more numerous when fruit is more abundant as opposed to when it is less available. Calls are also be used to strength the bond of the breeding pair.
The edge of a siamang’s home range, which can overlap that of another group, is often the location where calling is made. When calling, branch shaking, swinging, and moving around among the tree branches are also performed, which may be done to show neighboring groups their location.
Most siamangs are monogamous, with a breeding pair producing one offspring every two to three years. Sexual maturity (and thus, independence from the natal family group) is reached at six or seven years of age. Gestation lasts seven to eight months. Offspring cling to their mothers’ bellies constantly for their first three to four months of life, and are weaned from their mothers after two years. Fathers play a significant role in rearing offspring and may carry their infant upon weaning. A female siamang will rarely give birth to more than 10 offspring in her lifetime. There is no distinct breeding season for the siamang, but the peak of breeding activity occurs when fruit is most abundant.
As frugivores, siamangs disperse seeds through defecation as they travel across their territory. This helps to replenish their forest habitat and leads to a healthier, more diverse tree population. Siamangs can carry seeds while digesting, and defecate between 269 and 1,200 feet (81.9 and 3675.9 m) from the original source of the seeds.
The International Union for Conservation of Nature lists the siamang as Endangered (IUCN, 2015), appearing on the IUCN Red List of Threatened Species.
Living in fragmented forest habitats, siamangs face numerous threats to their survival. Infants are captured for the illegal pet trade throughout southeast Asia (which often involves the killing of adults), and their numbers have declined by 50% over the last 40 years as a result of their two biggest threats: the pet trade and habitat loss.
One of the most serious threats to siamang survival is the unsustainable practice of timber extraction. Habitat destruction and the subsequent degradation of the forest—either from commercial timber harvesting or converting the land to agriculture (particularly palm oil)—poses a critical threat. Other threats include residential and commercial development, mining and quarrying, road construction, and fires. Many of these threats also encroach onto national parks and “protected” forests. These threats impact both subspecies of siamang, and the siamangs’ numbers continue to decline.
The siamang is listed in Appendix I of the Convention on International Trade in Endangered Species (CITES), an international agreement between governments whose goal is to ensure that international trade in specimens of wild animals and plants does not threaten their survival.
There are eleven (small) protected areas within Sumatra and Thailand. However, even these areas are threatened with continued human activity and encroachment. Numerous organizations, such as the World Wildlife Fund (aka World Wide Fund for Nature) and the Association of Zoos and Aquariums are members of the Roundtable for Sustainable Palm Oil (RSPO). The RSPO creates standards for the production of sustainable palm oil, and certifies both qualified growers and processors. Around 20% of the world’s palm oil is certified as sustainable by the RSPO. By purchasing products that feature the RSPO logo as part of the packaging, companies using certified sustainable palm oil can be supported. In addition, recycling paper products (many of which come from trees cut down in the siamang’s habitat), as well as cans and bottles made from glass and aluminum (elements found in rainforest soil), can reduce the amount of digging performed in the siamang’s habitat. By performing these seemingly small steps, not only can the siamang be saved, but other endangered species found in the same habitat, such as Sumatran tigers, elephants, and rhinos, can be saved as well.
Ultimately, the key to conserving the siamang relies on the ability of local governments to uphold their national laws and regulations. Improving law enforcement, stopping illegal logging, curbing legal logging and forest conversion, implementing forest restoration projects, stopping road construction, and enhancing connectivity across fragmented landscapes are all key actions that are needed to save the siamang.
Written by Sienna Weinstein, December 2023