Males, females and infants have long, shaggy black coats with pale hairs around the mouth and chin. Males have a longish tuft of hair in the genital region. Siamangs have a grayish or pinkish throat sac, which they inflate during vocalizations. The throat sac can become as large as a grapefruit. Their arms are longer than the legs of the white-cheeked gibbon, and their hands and feet are broader. The arm length may reach two and a half times the length of the body. The primates have slight webbing between their second and third toes. Both sexes have long canine teeth, opposable thumbs and a great toe that is deeply separated from the other toes. Like other primates, siamangs have a highly developed brain.

When on the ground, siamangs are usually bipedal, moving on their two legs. In the trees, they move by acrobatic hand-over-hand swinging through the branches, a process called brachiating. When moving slowly, they swing much like a pendulum as they grab one branch and release it before grabbing the next, so that the body is freely projected through the air. Flights of 25 to 32 feet (8 to 10 meters) have been witnessed. The heavier and larger siamangs, however, travel more slowly than the white-cheeked gibbons.

Siamangs are slightly larger than other gibbons at 29 to 35 inches (74 to 89 centimeters) tall and weighing about 23 pounds (10 kilograms).

Siamangs live in the mountains of the Malay Peninsula and Sumatra in rainforests and monsoon forests. They have relatively small ranges, about 60 acres (0.24 square kilometers). They seem to travel about half as much daily, which may be because they consume a greater percentage of a more common food, leaves.

They defend about 60 percent of their range as group territory. It is harder to determine boundaries for siamang groups as their loud calls seem to create more space between groups and confrontations are very rare. Siamangs are arboreal.

Vocalization is a major social investment. Males and females call together, even during the female great call. When vocalizing, the siamang can produce two different kinds of notes using its throat sac: a deep boom (when it sings into the sac with its mouth closed) and a loud "wow" (when it sings into the sac with its mouth opened).

The deep boom sound carries farther in the forests than the high-pitched wow sound. Siamang also make a bark-like vocalization. The set calls are repeated one after another. They start off slowly and increase in speed. Calls are often accompanied by behavioral acrobatics.

Siamangs eat fruit and new leaves and include a larger proportion of leafy matter in their diet than most other gibbons. Siamangs also eat a small amount of insects, bird eggs and small vertebrates. While eating, siamangs spend about 44 percent of their time eating fruit and 45 percent of their time eating leaves. During much of its feeding time, the siamang suspends itself by one arm.

At the Smithsonian's National Zoo, each group of animals is fed a mixture of kale, cabbage, green beans, carrots, apples, bananas, oranges, sweet potatoes and primate chow multiple times a day. Amounts depend on the makeup and ages of each group. Each animal receives a cooked egg once a week.

They are diurnal.

Like the white-cheeked gibbon, siamangs live in flexible social groups, typically comprised of an adult pair with offspring. Male siamangs exhibit more infant care for their offspring than do other gibbon species, taking over the majority of infant carrying during the infant's second year of life.

Siamangs are notable for having more coordination and contact during daily activities. The family usually forages for food as a unit. Siamangs are fiercely territorial and defend their territory with daily singing rituals. Boundary disputes, which are very rare, involve high-speed chases through the trees, slapping and biting as they go.

Siamangs follow a daily pattern or routine. They wake at sunrise and perform their morning concert before setting out in search of food. It usually takes a siamang about five hours to eat its fill. After eight to ten hours of activity, they identify a place to rest or sleep.

Like most primates, one of the most important social activities of a siamang is grooming. Adults groom on average 15 minutes per day. Grooming is a display of dominance. The more dominant animals receive more grooming than they give. An adult male grooms the females and sub-adult males. In the breeding season, adult males will focus more on the females.

The siamang gestation period is approximately seven and a half months. A single offspring is born every two or three years. Offspring cling to their mothers' bellies constantly for the first three to four months. Fathers may begin to carry an infant at the time of weaning after the infant reaches two years of age. Siamangs reach maturity by the age of six or seven. A female rarely gives birth to more than ten offspring in her lifetime.

Siamangs are endangered. Their numbers have declined by 50 percent over the past 40 years, primarily because of the illegal pet trade and habitat loss. Many adults are killed so humans can have an infant as a pet, even though this practice is illegal.

One of the most serious threats to siamang survival is the unsustainable practice of timber extraction in Indonesia and southeast Asia. Habitat destruction and the subsequent degradation, either from commercial timber harvesting or conversion of land to agriculture (particularly palm oil), poses a very serious threat to these arboreal apes. Moreover, the illegal pet trade is booming in Southeast Asia and infant apes are very popular pets. 

The Association of Zoos and Aquariums' Ape Taxon Advisory Group (AZA Ape TAG) has supported conservation initiatives such as anti-poaching patrols and law enforcement, additional research and support from zoos, improved management of protected areas and support of sanctuaries, and increased community involvement to help protect apes.