Gibbons are members of the order Primates, family Hylobatidae, and genus Hylobates. They are most notably known for their unique vocalizations and long arm spans, which can reach twice the length of their bodies. Slender and agile, gibbons usually loco-mote using their long arms in a movement known as brachiation. Their hands, equipped with long, curved, and slender digits, are used as hooks as they swing from tree branch to tree branch. Some of the smaller species appear to throw themselves from tree to tree, crossing gaps of 10 meters or more. While feeding, however, gibbons often use quadrumanous climbing.
Gibbons are considered to be a “lesser ape.” Like all apes, they are characterized by an absence of tails, broad chests, and an upright body posture. However, unlike the great apes (chimpanzees, bonobos, gorillas, and orangutans), they have smaller cranial capacities and body sizes. In addition, they are more numerically successful and specifically diverse than the great apes.
Currently, there are 11 species of gibbons, all found in Southeast Asia. The 11 species are generally divided into four subgenera, which are grouped together based on chromosomal numbers and coloration.
The first of the four subgenera has the same name as the genus, Hylobates, and is also known as the “lar group.” Lar gibbons have 44 chromosomes and variable pelage colors. Species in this group include the white-handed gibbons (Hylobates lar), silvery Javan gibbons (H. moloch), agile or dark-handed gibbons (H. agilis), Mueller’s Bornean gray gibbons (H. muelleri), pileated or capped gibbons (H. pileatus), and Kloss’s gibbons (H. klossii).
The second group comprises the hoolock or white-browed gibbon (H. hoolock), subgenus Bunopithecus. White-browed gibbons have 38 chromosomes and are sexually dichromatic. Females are more tan colored than males, which are black. Also, females have whitish hairs encircling their faces, while males have only white eyebrows.
The concolor group, subgenus Nomascus, has 52 chromosomes and is sexually dichromatic. Males of the black gibbon (H. concolor), golden-cheeked gibbon (H. gabriellae), and Chinese white-cheeked gibbon (H. leucogenys) tend to be black, while females are buff.
The fourth group is the siamangs, the largest of the gibbons, belonging to the subgenus, Symphalangus. Like the Nomascus group, siamangs have 52 chromosomes but are not sexually dichromatic. Unlike the gibbons, however, siamangs have webbing between their second and third toes. Also, males are slightly larger than females.
Gibbons mainly live in primary moist forest and spend virtually all of their time in the canopy. The bulk of their diet consists of ripe fruits but may also include leaves, flowers, and small invertebrates. Gibbons live in single, monogamous family units. Monogamy in gibbons is thought to have resulted from the need to spend large amounts of time rearing offspring. With no male to help defend a territory, females may not be able to obtain enough resources to conceive and/or rear the slow developing young. Offspring can stay with their parents for up to 10 years. At this point, they leave the family group and “bud off” (i.e., usurp a part of the parents’ home range) and look for a mate of their own.
Gibbon vocalizations are well-known. Most pairs sing duets in the mornings and evenings, with their calls carrying for over 1 kilometer. The vocalizations are used for territorial defense, maintenance of the pair bond, and general communication. Gibbon species can be distinguished by their calls.
Due to their propensity to spend their lives in the forest canopy, gibbons are extremely sensitive to habitat loss due to logging and agriculture. In fact, of the 11 species, three are considered to be vulnerable, two endangered, and two critically endangered.
References:
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- Fleagle, J. G. (1999). Primate adaptation and evolution. San Diego, CA: Academic Press.
- Preuschoft, H. (1985). The lesser apes: Evolutionary and behavioral biology. Edinburgh: Edinburgh University Press.
- Rowe, N. (1996). The pictorial guide to the living primates. Charlestown, RI: Pogonias Press.
- Rumbaugh, D. M. (1972). Evolution, ecology, behavior, and captive maintenance (gibbon and siamang). Basel, Switzerland: S. Karger.
- Rumbaugh, D. M. (1974). Gibbon and siamang: Natural history, social behavior, reproduction vocalizations, prehension (Progress in Biochemical Pharmacology). Basel, Switzerland: S. Karger.
- Teleki, G. (1979). Aerial apes: Gibbons of Asia. New York: Coward, McCann & Geoghegan.
- Whitten, T. (1982). The gibbons of Siberut. London: J. M. Dent.
