White-handed or Lar Gibbon

Hylobates lar
(Linnaeus, 1771)

Photo by Tim Knight

Identification

Gibbons are the smallest of the apes. Most species weigh about 5 kg, although Hylobates hoolock weigh slightly more (6 to 8 kg), and the siamang Hylobates syndactylus is twice as large (10 to 12 kg). Gibbons are prototypical brachiators, with long arms and hands and flexible forelimb joints. Scientists are still debating whether these morphological features originally evolved for efficient food collection (i.e. hang-feeding and climbing on small branches; Fleagle 1976) or for efficient travel between food sources (Temerin and Cant 1983). The small size and suspensor locomotion of gibbons permit them to move more easily and directly through the rain forest canopy than sympatric orangutans and macaques (Temerin and Cant 1983; Grand 1984).

Geographic Range

Gibbons are restricted to tropical evergreen and less seasonal parts of semievergreen rain forests in Asia. H. agilis occur in Sumatra, West Malaysia, southwestern Borneo, and southern Thailand; concolor in Vietnam, Laos, Cambodia, and southern China; hoolock in Asam, Bangladesh, and Burman; klossii in the Mentawai Islands west of Sumatra; lar in Thailand, peninsular Malaysia, and northern Sumatra; moloch in western Java; muelleri in all of Borneo but the southwestern corner; pileatus in southeastern Thailand and western Cambodia; and syndactylus in Sumatra and peninsular Malaysia. The siamang is sympatric with lar and agilis. All other species are allopatric, except for slight overlap between lar and pileatus in Thailand, between lar and agilis in West Malaysia, and betweenagilis and muelleri in Borneo.

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Natural History

Gibbons are frugivores. They eat mostly ripe, sugar-rich, juicy fruits (Gittins and Raemaekers 1980) and also large quantities of figs (Ficus fruits). Fruit comprises about 60% and leaves 30% of the diet for most of the smaller gibbons, but monthly proportions of fruit sometimes exceed 90% or fall below 30%. The larger siamang (H. syndactylus) is more folivorous, klossi and pileatus may be more frugivorous, and klossii apparently obtains protein primarily from insects instead of young leaves (Whitten 1984).

Gibbons have fixed home ranges that average about 34 hectares, with exclusive portions (territories) averaging 75% of this area. Siamang home ranges are smaller than those of sympatric lar groups, but similar in size to those of many other gibbon species.

Gibbons are active from 8 to 10 hours a day on average. Activity usually starts at dawn and stops well before sunset. Adult males and offspring become active sooner and often stay active later than females. Singing and the most intense feeding occurs in the morning, but compared to most other primates, gibbons show little change in activity over the day. Gibbons spend most of their day foraging in the main canopy (ca. 20 to 35 m high in primary forest). Emergent trees are used primarily to rest, sleep, and sing. Little time is spent in the lower canopy, and this mostly to visit small food trees (Gittins and Raemaekers 1980).

Reproduction

Male and female gibbons mature at similar rates and appear to pair at 8 to 10 years of age. Females first menstruate at around 8 years of age (Carpenter 1940). They do not have sexual swellings; however, Carpenter and Chivers both describe changes, presumably associated with ovulation, in the color and turgidity of female genitalia. Females cycle for only a few months (ca. 5), at intervals of 2 or more years between births (Ellefson 1974), and copulations during these periods of sexual activity occur at a rate of less than once a day.

Social Organization

Gibbons are invaribly monogamous and territorial, and defend their territories through regular loud morning songs and occasional encounters with neighbors and intruders. Outside these intergroup activities, gibbon families lead a relatively subdued social life. In addition to the adult pair, a gibbon group potentially includes one infant (0to 2 or 2.5 yr), one juvenile (2 to 4 yr; defined by locomotion independent of the mother), one adolescent (4 to 6 yr; not yet adult sized), and one subadult (6+ yr; fully grown, but unmated). Mean group size is usually about four. A mated pair produces an average of five to six offspring over a reproductive lifetime of 10 to 20 years (Carpenter 1940; Tilson 1981). No stable groups have been observed with more than one female carrying an infant.

Female gibbons weigh nearly the same as males and have similar-sized canines. H. concolor, hoolock, and pileatus have sexually dimorphic coat colors, and all species but hoolock have sexually dimorphic vocal repertoires (Chiver 1977; Haimoff 1984). Males and females without sex-specific coloration are hard to distinguish by sight if the female is not carring an infant or if, as is often the case, the observer cannot get a close and clear enough view to spot enlarged nipples or parapenal hair tufts.

Sexual monomorphism in gibbons is correlated with a comparatively high degree of behavioral and social equality between adult males and females. Partners in long established pairs normally interact in a relaxed, tolerant, and well-coordinated manner. In contrast to many species of primates, female gibbons are often "codominant" (Carpenter 1940) with males.

Conservation Status

Hylobates lar: CITES - Appendix I; U.S. ESA - Endangered.

Gibbon populations are currently threatened most by loss of habitat to cultivation and commercial logging. Gibbon densities may decline minimally and temporarily where few trees are extracted from a forest (Johns 1981), but in most timber operations, logging is intensive and appears to increase forest destruction caused by agricultural encroachment, flooding, wind starms, and forest fires. Their future depends on the maintenance of the forest reserves to which they are not primarily or entirely restricted.

References:
Carpenter, C. R. 1940. A field study in Siam of the behavior and social relations of the gibbon (Hylobates lar). comp. Psychol. Monogr. 6:1-212.

Chiver, D. J. 1977. The lesser apes. In Primate conservation, ed. H. R. H. Prince Rainier III and G. H. Bourne. New York: Academic Press.

Ellefson, J. O. 1974. A natural history of white-handed gibbons in the Malayan peninsula. In gibbon and siamang. vol. 3, ed. D. M. Rumbaugh. basel: S. Karger.

Fleagle, J. G. 1976. Locomotion and posture of the Malayan siamang and implications for hominoid evolution. Folia Primatol. 26:245-69.

Grand, T. I. 1984. Motion economy within the canopy: Four strategies for mobility. In Adaptations for foraging in nonhuman primates: contributions to an organismal biology of prosimians, monkeys, and apes, ed. P. S. Rodman and J. G. H. Cant. New York: Columbia University Press.

Gittins, S. P., and Raemaekers, J. J. 1980. Siamang, lar, and agile gibbons. In Malayan forest primates: Ten year's study in tropical rain forest, ed. D. J. Chivers. New York: Plenum Press.

Haimoff, E. H. 1984. Acoustical and organizational features of the songs of gibbons. In the lessser apes: Evolutionary and behavioral biology, ed. H. Preuschoft, D. J. chivers, W. Brockelman, and N. Creel. Edinburgh: Edinburgh University Press.

Johns, A. D. 1981. The effects of selective logging on the social structure of resident primates. Malaysian Appl. Biol. 10:221-26.

Leighton, D. R. 1986. Gibbons: Territoriality and Monogamy. Pp. 135-145 in: Smuts, B. B.; Cheney, D. L.; Seyfarth, R. M.; Wrangham, R. W.; Struhsaker, T . T. (eds.), Primate Societies. The University of Chicago Press, Chicago.

Temerin, L. A. and Cant, J. H. 1983. The evolutionary divergence of Old World monkeys and apes. Am. Nat. 122:335-51.

Tilson, R. L. 1981. Family formation strategies of Kloss's gibbons. Folia Primatol. 35:259-87.

Whitten, A. J. 1984. Ecological comparisons between Kloss gibbons and other small gibbons. In the lesser apes: Evolutionary and behavioral biology, ed. H. Preuschoft, D. J. Chivers, W. Brockelman, and N. Creel. Edinburgh: Edinburgh University Press.

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