Geographic Distribution and Habitat
The siamang is found in the mountain and lowland forests on Sumatra (the westernmost island of Indonesia), Malaysia, and small parts of Thailand. Despite being the largest of the “lesser apes”, siamang families cover only about 50 to 60 acres of territory, particularly areas rich in leaves and figs. Like all gibbons, they are arboreal acrobats who prefer to swing from branch to branch rather than walk on the ground.
Siamang habitats go through a dry season and a wet season with little change in temperature throughout the year. Temperatures range from 72°F to 95°F (22-35°C).
Size, Weight, and Lifespan
Siamangs are the largest gibbons in the world. Male siamangs have an average weight of 26.2 lbs (11.9 kg) while females generally weigh around 23.1 lbs (10.7 kg). Both genders can grow to be about 3 feet tall (90 cm).
Estimates vary, but siamangs can live somewhere between 25 and 40 years in the wild and can potentially live for over 40 years in captivity.
Physically adapted to living primarily or exclusively in trees, such as arboreal-dwelling (tree-dwelling) monkeys.
Originating or occurring naturally in a particular place; native.
Visit the Glossary for more definitions
All siamangs can be differentiated from other gibbons in two major ways: A naked throat sac (also called a gular sac) that can expand to the size of a siamang’s head when inflated prior to making calls; webbing between their second and third toes, the purpose of which is not yet known. This webbing provides the siamang with its scientific name, Symphalangus syndactylus, which translates from Ancient Greek to “together phalanx” + “united finger.”
The siamang’s body is perfectly adapted to a life of swinging from branch to branch (also known as brachiation). As with other gibbons, their arms are incredibly long with an arm span sometimes twice as long as body length. These proportions make walking along the forest floor awkward, but it allows them to move through the canopy with ease and grace. To help grip onto branches, siamang thumbs have evolved to be shorter and lower on the hand so that they do not get in the way. This trait is also observed in the hands of orangutans, spider monkeys, and, of course, other gibbons. Not only does the siamang’s expertise in the trees give them access to more food, but it also takes much less energy to swing from branch to branch than it does to walk on the ground.
Siamangs have dark black fur with gray or reddish hairs scattered around their mostly naked face. Their fur is relatively shaggy compared to other gibbons.
The siamang diet is mostly made up of fruit, leaves, insects, and flower petals. Fruits and leaves especially combine to form about 90% of their diets.
Fruit makes up over half of the diet of the Sumatran siamang, but only about 30% of the Malaysian siamang’s diet. They eat from over 160 different types of plants, usually changing their diet depending on the time of year and preferring young leaves to mature leaves. In many cases, they avoid mature leaves altogether.
Behavior and Lifestyle
Siamangs are diurnal, waking at dawn and retiring in the early evening with their family to the highest branches of a single tree where they sleep for the night. 34-44% of their time is dedicated to foraging for food, with an equal amount of time dedicated to resting. The rest of their day is spent traveling from one food patch to another and grooming each other.
There are no major behavioral differences between the two subspecies, although the Sumatran siamang spends less time foraging, thanks to its nutrient-rich, fruit-heavy diet.
Thanks to their size, agility, and safe location in the trees, siamangs have no known natural predator.
Siamangs live in small groups of 3 to 6 individuals. The family is made up of a monogamous mating pair of adult siamangs and their offspring who have yet to leave the family. Families are fiercely territorial and rarely venture out of their range.
Calls from siamangs most often serenade their forests in the mid-morning period between 9 and 10 a.m. Calls are generally used to establish territory, but mating pairs may also sing together to strengthen their bond. Siamang calls are louder than most gibbons, traveling upwards of over 1.2 miles (2 km).
A typical siamang song has two parts. First, the siamang sings into the gular sac, causing it to expand and let out a low moaning sound (almost like a “bloop”) that can resonate through thick forests. Then the siamang opens his mouth to make several loud, high-pitched cries in quick succession. This will go on for 10 to 20 minutes.
Nearly all siamangs are monogamous, with a mating pair producing a newborn every 2 or 3 years. The mother is pregnant for a period of 7 to 8 months. Once born, the baby siamang will cling to his mother for around 8 months. After that, the father takes on a more prominent role by holding onto the baby until he gains enough independence to travel on his own, usually at age 3. The siamang then enters the subadult phase, where he will stay with his family until he has fully matured into an adult between his 6th and 8th year, at which point the siamang will leave to find a mate and start a new family in a new territory (although there may always be a power struggle for their current territory).
Siamangs don’t necessarily have a set mating season, but they are more likely to mate between May and July, when food is at its most abundant, thus ensuring a more successful pregnancy. When food availability is at a constant, such as in a zoo, siamangs will mate year-round.
Siamangs are noted for their role in seed dispersal. When they eat the fruit of one tree, they defecate its seeds anywhere throughout their territory, leading to a healthier, more diverse tree population.
The siamang is listed as Endangered by International Union for Conservation of Nature (IUN, 2015), appearing on the IUCN’s Red List of Threatened Species. Their numbers are in decline. Their two biggest threats are habitat loss and illegal pet trade.
In addition to road building and illegal logging, the single biggest reason for habitat loss is the palm oil industry. Over 85% of the world’s palm oil is produced by oil palm trees planted in Indonesia and Malaysia. Consumer demand for palm oil, now found in half of all packaged food and cosmetics, has increased five-fold since 1990 and is expected to double in the next 10 years. Companies burn down the habitats of siamangs and other indigenous animals to make room for plantations. In addition to destroying the dwindling habitat of local wildlife, these controlled forest fires also emit more greenhouse gases into the atmosphere than the entire United States, and pollution related to the fires has been blamed for over 100,000 deaths in Southeast Asia in just the past 3 years.
International trade of siamangs is banned by the Convention on International Trade in Endangered Species (CITES). Several reserves and national parks exist where siamangs and their habitats are legally protected from logging and poaching. Most conservation organizations are focused on assisting local governments in gaining more data to help the species in the most efficient way possible. Because they occupy the same habitats, siamangs may also benefits from conservation efforts aimed to help other animals, such as orangutans and the Sumatran tiger.
In 2017, the European Union passed a resolution calling for tougher environmental regulations on the palm oil industries of Indonesia and Malaysia.
- Reichard U.H., Barelli C., Hirai H., Nowak M.G. (2016) The Evolution of Gibbons and Siamang. In: Reichard U., Hirai H., Barelli C. (eds) Evolution of Gibbons and Siamang. Developments in Primatology: Progress and Prospects. Springer, New York, NY
Written by Eric Starr, February 2018