Species Hylobates spp. (9 spp.)
PHYSICAL DESCRIPTION: Gibbons are the smallest apes, weighing approx. 5-6 kg. Siamangs are about twice as large, weighing approx. 10-12 kg. There are 5-9 species of Gibbons & Siamangs.
GEOGRAPHIC LOCATION: Southern Asia (Borneo, Malaysia, Thailand, S. China, Viet Nam, Cambodia, Burma and Assam).
ECOLOGY: Gibbons and Siamangs are brachiators in middle canopy rainforest. Siamangs are sympatric with other Gibbons, utilizing different feeding patterns. Home ranges are about 40 hectares. Groups travel about 1 km per day, although there is considerable variation. Some species can travel from one side of their territory to the other in 10 minutes or less. Normally the home range is covered in about 3-4 days. Gibbons are active for about 10-12 hours a day, mostly spent foraging and resting. There is little social activity such as play. Gibbons are threatened by logging of the rainforest.
DIET: Frugivores and, to a lesser extent, folivores.
REPRODUCTION: There is little evidence for birth seasonality. Gibbons are sexually mature at about 8-10 years of age. Mating is very infrequent. Little estrus swelling.
SOCIAL ORGANIZATION: Gibbons are monogamous and territorial. Territories are maintained by loud vocalized singing. Groups are composed of a mated pair + infant (0-2), juvenile (2-4), adolescent (4-6), subadult (6+) and an occasional post-reproductive grandparent. Mated pairs produce an average of 5-6 offspring during their reproductive lifespan. There is little or no sexual dimorphism in canine size or body wt. There is little or no sexual dominance. Males are active fathers, frequently grooming and handling infants.
TERRITORIALITY: It is unclear whether Gibbons defend mates or space (probably both). Males sing just before sunrise, and mated pairs sing duets during the morning. Playback experiments indicate that females respond to calls by calling, whereas males silently approach the singing. Intergroup encounters usually involve males, who call back and forth or sit and stare at each other. In some species about 50% of the males had evidence of fighting (broken canines, scars, wounds). Females did not have these indicators of fighting. Widowed females with offspring sometimes mimic the duet role of their dead partner, apparently to communicate to neighboring gibbons that the territory is still occupied by a mated pair.
MONOGAMY: There are two non-exclusive hypotheses for the existence of monogamy among Gibbons: (1) Females are dispersed because of food distribution (small, long lasting patches) and the lack of predation pressure, and (2) male parental care is very important for offspring success. Concolor Gibbon may have infrequent polygyny.
Leighton, D.R. 1986. Gibbons: Territoriality and monogamy. In: Primate Societies, ed. Barb Smuts et al. Chicago: University of Chicago Press.