Gibbons and Siamangs

Species of gibbons and siamangs

Resources

Gibbons are 10-12 species of tropical forest apes in the family Hylobatidae of the Primate order, which also includes a wide variety of monkeys, the great apes, and humans. Gibbons lack a tail, they have a more-or-less upright posture, and they have a well-developed brain. However, gibbons are generally regarded as the least intelligent of the anthropoid apes.

The true gibbons are five to six species in the genus Hylobates, while the siamangs are two larger species of the genus Symphalangus. (However, some taxono-mists also classify the siamangs in the genus Hylobates, thereby treating them as large gibbons.) The three species of crested gibbons are currently classified in the genus Nomascus and the hoolock gibbon is the single species in the genus Bunopithecus. All of the gibbons occur in tropical forests of Southeast Asia, Malaysia, and Indonesia.

The gibbons are the smallest of the apes, being less than one meter tall, and weighing about 11-17.5 lb (5-8 kg). Siamangs weigh 17-29 lb (8-13 kg). Gibbons have a willowy body shape, and have much longer and more slender arms than the other anthropoid apes. In fact, the arms of gibbons are long enough to easily touch the ground as these animals walk. The hands and fingers are also elongate and slender, with a distinctively deep cleft between the thumb and the index finger. All of these are adaptations for the active life of these animals, which is mostly spent in the forest canopy.

Gibbons are highly arboreal animals—the name of their genus, Hylobates, is derived from Greek words for “dweller in the trees.” Gibbons are extremely agile, and are sometimes referred to as the most acrobatic of all the mammals. They can move swiftly and gracefully through the treetops by swinging hand over hand from branch to branch, a method of locomotion known as brachiation. Gibbons can also get about by leaping through the air until they manage to hook onto a secure branch, or land on a stable limb. These flying leaps, sometimes assisted using elastic branches, can cover a distance as great as 40 ft (12 m).

Gibbons do not often venture to the ground, but when they do, they are awkward walkers, typically holding their arms high to maintain their balance as they ambulate. Gibbons do not swim, and are in great danger of drowning if they ever fall into deep water.

Gibbons are typically colored in various hues of brown or black, with white body markings in some species. There is a great deal of variation in coloration of individuals within species, and even within the same family of gibbons. The young of most gibbons have white fur, and do not attain the darker, adult coloration until they are 2-5 years old, depending on the species.

Gibbons are highly vocal animals. Groups of gibbons (known as a “troop”) often make very loud hootings, barks, and hollers in the early morning, known as their noisy “dawn chorus.” These animals are also very vociferous during the day.

Gibbons become sexually mature at an age of 5-8 years. The gestation period is about seven months, and one baby is born. The young are weaned after about seven months, and until that time they are constantly carried by their mother. Gibbons have lived as long as 23 years in captivity.

The usual family group is a monogamous pair of an adult male and female, plus three or four prereproductive offspring. Sometimes, however, a mature male will live in a polygynous relationship with several mature females, with the young being raised communally. The family groups defend a territory, which can range in size from about 25 to more than 50 acres (10-20 hectares) in area.

Gibbons forage during much of the day. They typically sleep while sitting erect in dense vegetation at night. Gibbons are mostly herbivorous, eating a wide range of fruits and leaves. However, they also sometimes feed on bird eggs, nestlings, and other small animals. These agile creatures are known to capture flying birds, while leaping through the air, or while brachiating rapidly. Gibbons drink by dipping a hand into water, and then sucking the moisture from the fur.

The usual habitat of gibbons is tropical forest. This can range from lowland forest around sea level, to montane forest at an altitude as great as 7,900 ft (2,400 m). The siamang occurs as high as almost 9,850 ft (3,000 m) on the island of Sumatra, in Indonesia.

The most important natural predators of gibbons and siamangs are large cats such as the clouded leopard, large snakes such as pythons, and eagles.

Within their range, gibbons are sometimes kept as pets in villages. Some species of gibbons, especially the siamangs, are endangered by extensive losses of their natural habitat of tropical forests.

Species of gibbons and siamangs

The hoolock gibbon (Bunopithecus hoolock) occurs in Southeast Asia. Male hoolock gibbons are black, while the females are variable in color, ranging through black, gray, and brown, with a white band across the forehead.

The white-handed or lar gibbon (Hylobates lar), occurs in parts of mainland Southeast Asia, Malaya and Sumatra. The fur of this species has a basal color of black, brown, or yellow, but the upper surfaces of the hands and feet are white-colored, and the face is circled by white.

The dark-handed gibbon (Hylobates agilis) occurs on the Malayan Peninsula and Sumatra. The fur of this species varies from yellowish to dark-brown, and the upper surfaces of the hands and feet are always dark-colored.

The gray or Moloch gibbon (Hylobates moloch) occurs in Java and Borneo. The fur of this species is light or dark gray, and the face is black.

The black crested gibbon (Nomascus concolor) occurs in Southeast Asia, particularly Vietnam,

Myanmar (Laos), and Thailand. This is a dark-colored animal, with a distinctive, erect crest of long hair on the crown of the head, especially elongate in adult males. The black gibbon has a throat pouch, used to amplify its territorial noises, similar to the siamangs.

The siamang (Symphalangus syndactlyus) occurs in parts of Malaya and the island of Sumatra in Indonesia, while the dwarf siamang (S. klossi ) is native to the Mentawei Islands off the west coast of Sumatra. Siamangs are heavier than gibbons, typically weighing 17-29 lb (8-13 kg), with a body length as great as 36 in (90 cm), and an arm-spread of up to 5 ft (1.5 m). Siamangs have black fur, and a distinctive throat-pouch, which appears to amplify the booming and bellowing territorial noises of these animals. Siamangs are somewhat less agile than the true gibbons. They occur in montane and sub-montane forests between about 2,000-6,400 ft (600-2,800 m) in elevation. These animals defend their foraging range, and

KEY TERMS

Anthropoid apes— These consist of the gibbons, orangutan, chimpanzee, and gorilla, all of which lack a tail, have an upright posture, and a well-developed brain. Anthropoid apes are the closest living relatives of humans.

Brachiation— A method of arboreal locomotion involving hand-over-hand travelling, while holding onto branches. This is a characteristic locomotion of gibbons and some types of monkeys.

Monogamous— A breeding system in which a mature male and a mature female live as a faithful, mated pair.

Montane forests— Forests that occur relatively high on mountains, but below the open grasslands and tundra. Montane forests at low latitudes, for example in Southeast Asia, have a relatively cool and temperate climatic regime.

Polygynous— A breeding system in which a single male breeds with more than one mature female.

live in social groups consisting of an adult male and an adult female, plus any babies and sub-mature offspring that may be associated with the parents. These family groups defend a territory of about 25 acres (10 hectares) or more in area.

Resources

BOOKS

Else, J.G., and P.C. Lee, eds. Primate Ecology and Conservation. Cambridge: Cambridge University Press, 1987.

Fleagle, J.G. Primate Adaptation and Evolution. 2nd ed. San Diego: Academic Press, 1998.

Nowak, R.M., ed. Walker’s Mammals of the World. 6th ed. Baltimore, MD: Johns Hopkins University Press, 1999.

Smuts, B.B., et al. Primate Societies. Chicago: University of Chicago Press, 1987.

Stephens, M.E., and J.D. Paterson, eds. The Order Primates: An Introduction. New York, 1991.

Wilson, D.E., and D. Reeder. Mammal Species of the World. 2nd ed. Washington, DC: Smithsonian Institution Press, 1993.

Wolfheim, J. H. Primates of the World. Distribution, Abundance, and Conservation. Newark, NJ: Gordon and Breach Science Publications, 1983.

Bill Freedman