Distribution and Habitat
Siamangs are found in the mountains of the Malay Peninsula and Sumatra Indonesia in rain forests and monsoon 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. 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.
Like the white-cheeked gibbons, siamangs live in small, monogamous families composed of a mated pair and up to four young. 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.
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.
Siamang Vocalization is a major social investment. In siamangs, males and females call together, even during the female great call. The siamang has an amazing throat sac. When it vocalizes, it can produce two different kinds of notes using this 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 (because lower sounds have less steep waves). There is also a bark-like vocalization. The set calls are repeated, one after another, starting off slowly and increasing in speed. Calls are often accompanied by behavioral acrobatics.
Reproduction and Development
Siamangs’ gestation period is approximately 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. A female rarely gives birth to more than ten offspring in her lifetime.
Diet in the Wild
Siamangs eat fruit and new leaves, and they include a larger proportion of leafy matter in their diet than most other gibbons. They also eat a small amount of insects, bird eggs and small vertebrates. Fruit eating occupies about 44 percent of siamangs’ eating time, and leaf eating occupies about 45 percent of their eating time. During much of their feeding time they are suspended by one arm.