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. Their arms are even longer compared to the legs than white-cheeked, and their hands and feet are broader. 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 they are usually bipedal. 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 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.
The life of a siamang follows a daily pattern or routine. They wake at sunrise and perform their morning concert, and then set 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 return to their sleeping place.
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 receives more grooming than it gives. An adult male grooms a female and sub-adult males. In the breeding season, he focuses more time on the female. Just as in the white-cheeked gibbons the adult female is the dominant animal in the group.