Siamangs are found on the island of Sumatra and on the Malay Peninsula in lowland, submontane, and montane forests. They are diurnal and arboreal.
Siamangs have relatively smaller ranges than white-cheeked gibbons, about 60 acres (0.24 km2). 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 their 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 slightly larger than other gibbons, 29 to 35 inches tall (74 to 89 cm) and about 23 pounds (10 kg). Males, females, and infants have a long, shaggy black coat all over their body, except for some 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 be as large as a grapefruit. The arm length may reach 2.5 times the length of the body. There is 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. In the trees, they move by acrobatic hand-over-hand swinging through the branches, a type of locomotion known as brachiating. When moving slowly, they swing much like a pendulum as they grab one branch before grabbing the next, so that the body is freely projected through the air. Flights of 25 to 32 feet (8 to 10 m) have been witnessed. The heavier and larger siamangs, however, travel slower than the white-cheeked gibbons.
Like the white-cheeked gibbons, siamangs live in small groups typically composed of an adult pair and their offspring. Although siamang groups are most often monogamous and groups are normally highly cohesive, polyandry may also occur, with a single female and two, typically unrelated males.
Siamangs are notable for having more coordination and contact during daily activities. Siamang groups usually forages for food as a unit. Siamangs are highly 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.
The life of a siamang follows a routine. They wake at sunrise and perform their morning concert, and then set out in search of food.
Like most primates, one of the most important social activities of a siamang is grooming. Grooming is a display of dominance; the more dominant receives more grooming than it gives.
Siamangs’ gestation period is approximately 6.5-7.5 months. A single offspring is born every two or three years. Offspring cling to their mother’s belly constantly for the first three to four months. The male may begin to carry an infant at the time of weaning. The infant is weaned by two years old and reaches maturity by the age of six or seven. Siamangs are unique among the gibbon family because males typically become the primary caregiver for infants during their second year of life. Shorter interbirth intervals are possible for siamangs because the males’ carrying of infants reduce female energy output.