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Fact Sheet: Yellow-Cheeked Crested Gibbon (Nomascus gabriellae)

Thomas Geissmann Anthropological Instiute University Zürich-Irchel Winterthurerstrasse 190 CH-8057 Zürich Switzerland E-mail: webmaster@gibbons.de 21 August 2001 Adult male (left) and adult female (right) of the yellow-cheeked crested gibbon (Nomascus gabriellae) Yellow-cheeked crested gibbon Other names: Buff-cheeked crested gibbon, red-cheeked crested gibbon Scientific name: Nomascus gabriellae Classification Kingdom: Animalia (animals) Phylum: Chordata (chordates) Class: Mammalia (mammals) Order: Primates (primates) Family: Hylobatidae (small apes) Genus: Nomascus (crested gibbons) Species: gabriellae (yellow-cheeked crested gibbon) Traditionally, all crested gibbons have been considered being members of a single species of the genus Hylobates (i.e. "H. concolor"), and this species was considered the only representant of the subgenus Nomascus. The yellow-cheeked crested gibbon has been considered being a subspecies ("H. concolor gabriellae") of that species (e.g. Groves, 1972; Marshall, & Sugardjito, 1986). More recently, vocal characteristics and other features suggested that yellow-cheeked crested gibbon should be recognized as a distinct species (Geissmann, 1995a; Geissmann et al., 2000). In addition, recent molecular evidence documented that the distance among gibbon subgenera was as large or larger than the distance between chimpanzees (Pan) and humans (Homo) (Roos & Geissmann, 2001). As a consequence of this finding, all four subgenera are now recognized as full genera, and the traditional scientific name of the yellow-cheeked crested gibbon changes from "Hylobates concolor gabriellae" to Nomascus gabriellae. No subspecies of the yellow-cheeked crested gibbon are known, but the affinities of the gibbon populations in the northern part of the distribution area unknown (Geissmann, 1995a; Geissmann et al., 2000). Range The yellow-cheeked crested gibbon occurs in southern Laos, southern Vietnam and eastern Cambodia (Geissmann, 1995a, Geissmann et al., 2000). Habitat This species is found in tropical evergreen forests. They are scarce between 1,500 and 2,000 m (Eames & Robson, 1993) and appear to prefer lowland forests (Dao Van Tien, 1983). Ecology and diet The yellow-cheeked crested gibbon remains virtually unstudied in the wild. Like other gibbons, it is an arboreal and a diurnal species. Like other gibbons, it probably prefers the upper canopy of the forest, and sleeps and rests in emergent trees (Leighton, 1987). It is mainly frugivorous, but will also consume leaves and insects. Reproduction and ontogeny Gibbons give birth to single offspring. The birth interval in gibbons is about 2-3 years. The gestation period in gibbons has a duration of about 7 months, i.e. about 183-225 days (Geissmann, 1991). Young gibbons are born with virtually hairless ventral parts and must rely on their mothers for warmth. Nursing lasts about two years. Young gibbons will stay with their parents until they are past adolescence. In captivity, gibbons usually attain sexual maturity at an age of about 6-8 years, but this appears to be highly variable (Geissmann, 1991). At maturity, offspring probably leave the natal group or may be chased off by their parents. Life span: in captivity probably up to 50 years Dimensions Head and body length: about 60-80 cm (24-31 inches) (estimate) Body weight: 7.0-7.4 kg (15.4-16.3 lbs) (Geissmann, unpublished) Males and females are of about the same body size. Morphology Gibbons have no tail. Gibbons have extremely long arms and relatively long legs. The hands are also elongated and hook shaped. The long arms and hands assist in suspensory locomotion. The thumbs are not elongated and are not used for swinging from branch to branch. They are opposable and are used for tactile probing and grooming. The body is small and typically held in an upright position. Locomotion Gibbons typically exhibit a highly specialized form of locomotion which is called brachiation. They swing below the branches suspended by their arms. Brachiation is an energetically advantageous mode of locomotion. It facilitates feeding in the fine branch niche. It allows for relatively high speeds in the canopy and for jumps of 10 meters or more (Fleagle, 1999). When moving on branches or on the ground, gibbons walk on two legs (bipedalism), often using their arms for balance. Coloration The pelage of adult males is black with small pale yellow or pale orange cheeks. Males also have a group of erect hairs (a crest) on the top of their heads, thus the name "crested gibbons" for the genus Nomascus. The pelage coloration of adult females strongly contrasts with that of adult males (sexual dichromatism). Females have a bright yellow or pale orange pelage with a black patch on the top of their heads. When a yellow-cheeked crested gibbon is born, its coat is a bright yellow color. During the second half of the first year, the color changes to black within a few months; only the cheek patches remain yellow. The gibbons then resemble adult males in their fur coloration. Females turn back to the yellow coloration about when attaining sexual maturity. The timing of the color changes is variable and again takes several months to completion. Group size and social structure Although yellow-cheeked crested gibbon are unstudied in the wild, they probably live in small, monogamous family groups. Typical groups probably consist of an adult pair with 0-4 immature offspring. Young gibbons leave their natal group when they become adult. Gibbons are territorial (Leighton, 1987). Each family group occupies an area of about 20-50 hectares, but the typical territory size of this species has not been reliably determined in the wild. Territories are defended from intrusion by other gibbons by loud morning songs and by actively chasing intruders off of the territory. At night, gibbons sleep sitting up. The family group spends the night in one of several preferred "sleeping" trees of the territory. Vocal communication Gibbon groups produce loud, stereotyped song bouts in the early morning. Songs probably serve to defend resources such as territories, food trees, partners, but may also help to attract potential mates. Gibbon songs include species specific characteristics which are inherited and not learned (Geissmann, 1993). Mated yellow-cheeked crested gibbons typically produce duet songs which consist of coordinated vocal interactions by both partners using sex-specific phrases (Geissmann, 1993, 1995a; Geissmann et al., 2000). Other family members may participate in the song bout (Merker & Cox, 1999). Solo song bouts (Goustard, 1976) are typically produced by unmated yellow-cheeked crested gibbons only. Olfactory communication Crested gibbons exhibit extended fields of skin glands situated in the axillary, sternal and inguinal areas of the body. The glands produce a reddish secretion and are particularly active under hot temperatures and when the animals are exited. It has been speculated that the glands may play a role in olfactory communication (Geissmann, 1993; Geissmann & Hulftegger, 1994). The glandular secretion also influences the amount of red visible in the yellowish female pelage coloration. Tactile communication Social grooming probably plays a role in reinforcing the bonds between group members. Psychology There is some evidence for self-recognition in the mirror (Ujhelyi et al., 2000). Predators Adult gibbons typically live in the crown region of the forest where they have no natural predators except man. In the lower stories of the forest, leopards, clouded leopards, and pythons may be potential predators of gibbons. Wild population estimates Not available. Status and conservation IUCN Category of threat (Hilton-Taylor, 2000): vulnerable (criteria: A1cd +2cd) The species is threatened by habitat loss (from development and logging) and by hunting for food, for Oriental medicine and for the pet market. Extensive military activities have also had a detrimental effect on the species mainly through habitat destruction (Duckworth et al., 1995, 1999; Eames & Robson, 1993; Geissmann, 1995b; Geissmann at al., 2000). This species is not efficiently protected anywhere, at present, not even in nature reserves and national parks (Geissmann, 1995b; Geissmann at al., 2000). References Dao Van Tien (1983). On the North Indochinese gibbons (Hylobates concolor) (Primates: Hylobatidae) in North Vietnam. Journal of Human Evolution 12: 367-372. Duckworth, J.W., Timmins, R.J., Anderson, G.Q.A., Thewlis, R.M., Nemeth, E., Evans, T.D., Dvorak, M. & Cozza, K.E.A. (1995). Notes on the status and conservation of the gibbon Hylobates (Nomascus) gabriellae in Laos. Tropical Biodiversity 3: 15-27. Duckworth, J.W., Timmins, R.J., Khounboline, K., Salter, R.E. & Davidson, P. (1999). Large mammals. In Duckworth, J.W., Salter, R.E. & Khounboline, K. (compilers), Wildlife in Lao PDR: 1999 status report, IUCN - The World Conservation Union / Wildlife Conservation Society / Centre for Protected Areas and Watershed Management, Vientiane, pp. 161-220. Eames, J.C. & Robson, C.R. (1993). Threatened primates in southern Vietnam. Oryx 27: 146-154. Fleagle, J.G. (1999). Primate adaptation and evolution, second edition. Academic Press, San Diego & London. Geissmann, T. (1991). Reassessment of age of sexual maturity in gibbons (Hylobates spp.). American Journal of Primatology 23: 11-22. Geissmann, T. (1993). Evolution of communication in gibbons (Hylobatidae), Ph.D. thesis, Anthropological Institute, Philosoph. Faculty II, Zürich University. Geissmann, T. (1995a). Gibbon systematics and species identification. International Zoo News 42: 467-501. Geissmann, T. (1995b). The yellow-cheeked gibbon (Hylobates gabriellae) in Nam Bai Cat Tien (southern Vietnam) revisited. Primates 36: 447-455. Geissmann, T. & Hulftegger, A.M. (1994). Olfactory communication in gibbons? In Roeder, J.J., Thierry, B., Anderson, J.R. & Herrenschmidt, N. (eds.), Current primatology, vol. 2: Social development, learning and behaviour, Université Louis Pasteur, Strasbourg, pp. 199-206. Geissmann, T., Nguyen Xuan Dang, Lormée, N. & Momberg, F. (2000). Vietnam primate conservation status review 2000 - Part 1: Gibbons (English edition), Fauna & Flora International, Indochina Programme, Hanoi. Goustard, M. (1976). The vocalizations of Hylobates. In Rumbaugh, D.M. (ed.), Gibbon and siamang, vol. 4, Karger, Basel and New York, pp. 135-166. Groves, C.P. (1972). Systematics and phylogeny of gibbons. In Rumbaugh, D.M. (ed.), Gibbon and siamang, vol. 1, Karger, Basel and New York, pp. 1-89. Hilton-Taylor, C. (compiler) (2000). 2000 IUCN Red List of threatened species. IUCN, Gland, Switzerland and Cambridge, UK. Leighton, D.R. (1987). Gibbons: Territoriality and monogamy. In Smuts, B.B., Cheney, D.L., Seyfarth, R.M., Wrangham, R.W., and Struhsaker, T.T. (eds.), Primate societies, University of Chicago Press, Chicago and London, pp. 135-145. Marshall, J.T. & Sugardjito, J. (1986). Gibbon systematics. In Swindler, D.R. & Erwin, J. (eds.), Comparative primate biology, vol. 1: Systematics, evolution, and anatomy, Alan R. Liss, New York, pp. 137-185. Merker, B. & Cox, C. (1999). Development of the female great call in Hylobates gabriellae: A case study. Folia Primatologica 70: 97-106. Ujhelyi, M., Merker, B., Buk, P. & Geissmann, T. (2000). Observations on the behavior of gibbons (Hylobates leucogenys, H. gabriellae, and H. lar) in the presence of mirrors. Journal of Comparative Psychology 114: 253-262.

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