Hylobates is the sole genus of primates in the family Hylobatidae. The family, whose members are known as lesser apes, includes the gibbons (Hylobates spp.) and siamangs (H. syndactylus). Hylobates are called “lesser apes” due to their stature (5-12 kg) and height (about 3 feet), which are smaller than the great apes (orangutans, gorillas, chimpanzees, and bonobos: 31-175 kg). Hylobates species are divided into four subgenera, which are grouped together based on chromosomal numbers and coloration.
The first subgenus uses the same name as its genus, Hylobates, and is also referred to as the “lar group.” Lar gibbons include the silvery Javan gibbon (H. moloch), white-handed gibbon (H. lar), and Kloss’s gibbon (H. klossii), all found on various islands in Indonesia. The dark-handed or agile gibbon (H. agilis) occurs on Sumatra, Borneo, and the Malay Peninsula. The gray gibbon (H. muelleri) can be found on Borneo, while the pileated gibbon (H. pileatus) lives in Thailand and Cambodia. Lar gibbons have 44 chromosomes.
Bunopithecus, the second subgenus, comprises the hoolock or white-browed gibbon (H. hoolock), found in India, Burma, and China. Hoolock gibbons are sexually dichromatic; the males are black, and the females are tan. Females of this subgenus have whitish hairs around their entire face, while males have only white eyebrows. White-browed gibbons have 38 chromosomes.
Living members of the third subgenus, Nomascus, are the black gibbon (H. concolor) of China, Viet Nam, and Laos; the golden-cheeked gibbon (H. gabriellae) of Cambodia, Viet Nam, and Laos; and the Chinese white-cheeked gibbon (H. leucogenys) of China and Viet Nam. Members of the concolor group have 52 chromosomes.
The last subgenus, Symphalangus, also has 52 chromosomes. It is comprised of one species, the siamang (H. syndactylus), found in west Malaysia and Sumatra. Siamangs are distinguished from the other lesser apes by their larger body size (10.7-11.9 kg), throat sac, and webbing between their second and third toes.
Hylobates spp. are the acrobats of the forest, using their long arms and long, slender, and curved fingers to brachiate (swing) through the canopy. Gibbons are frugivorous, with most of their diet consisting of ripe fruit. They also eat young leaves and flowers. The diet of siamangs, however, consists of more leaves. Insects and arachnids may also comprise a small percentage of the lesser-ape diet.
Life expectancy for lesser apes is 10 to 20 years in the wild. They live in small, monogamous groups and mate year-round. Often when offspring reach adulthood (about 6-7 years), they are driven out of the group by the same-sex parent. Many usurp a small part of their parents’ home range and eventually begin their own social group. The vocalizations of hylobatids are well-known and can be heard up to 1 kilometer away. The calls are used for territorial defense, among other things.
Human activities such as deforestation and poaching have caused decreases in lesser-ape populations. All species of Hylobatidae are considered endangered, and the silvery Javan gibbon is critically endangered.
References:
- Chivers, D. J. (1974). The siamang in Malaya: A field study of a primate in tropical rain forest. Basel, Switzerland: S. Karger.
- Fleagle, J. G. (1999). Primate adaptation and evolution. San Diego, CA: Academic Press.
- Palombit, R. A. (1992). Pair bonds and monogamy in wild siamang (Hylobates syndactylus) and white-handed gibbon (Hylobates lar) in northern Sumatra. Davis: University of California Press.
- Preuschoft, H. (1985). The lesser apes: Evolutionary and behavioral biology. Edinburgh, Scotland: Edinburgh University Press.
- Rowe, N. (1996). The pictorial guide to the living primates. Charlestown, RI: Pogonias Press.
- 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. Coward, UK: McCann & Geoghegan.
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