Two of the purposes for breeding gibbons in captivity are to retain species and subspecies diversity and to create a viable gene pool, with the ultimate goal of releasing offspring into protected native habitat. Accurate identification of an individual gibbon's species may be complicated within some gibbon species by 1) the variety of coat colors, 2) the existence of different colors for the two sexes, 3) the occurrence of coat color changes at sexual maturity, and in all species by 4) the impact of malnutrition and housing (e.g. indoors only or in full sunlight) on coloration, 5) the ease with which gibbon species' vocalizations are confused, 6) the difficulty with which some gibbon subspecies are distinguished from each other, 7) and a dearth of information on the individual gibbon's origins, as may occur when a gibbon is confiscated. Given these problems, it is not surprising that breeding facilities may encounter difficulties in the identification of gibbons. For this paper, gibbons' specific and subspecific status was determined through the comparison of study skins housed in North American and Southeast Asian museums and live specimens housed at the International Center for Gibbons Studies and zoos.

Gibbons (Hylobates) are small, arboreal apes, inhabiting the tropical and semi-deciduous forests of Asia. They are distributed from Assam and Bangladesh in the northwest, eastward to Yunnan, Indochina, Thailand and the Malaysian Peninsula, and southward to the Indonesian and Malaysian islands. Adult gibbons weigh 5-15 kg and have long arms relative to their body size. Gibbons' pelage colors may vary within and between species, ranging from black to browns, grays, and buffs. With the exception of the siamang (Hylobates syndactylus), there is little sexual dimorphism in size, but some species do exhibit sex-specific color patterns.

The World Conservation Union (IUCN) considers all gibbon species threatened or endangered (Eudey, 1987; Groombridge, 1993). International captive breeding programs have been established to preserve the gibbon gene pool; unfortunately, some gibbon species have proved difficult to maintain in captivity. Factors contributing to the decline of some captive gibbons include hybridization, a monogamous mating system, few population founders from the rarer species, and behavioral abnormalities attendant with hand-rearing (Mootnick & Nadler, 1997).

Gibbon systematics traditionally have been controversial and confusing. One of the purposes of zoos or other captive facilities is to breed gibbons so that species and subspecies diversity is retained. But the accurate identification of an individual gibbon may be complicated by 1) the variety of coat colors within some gibbon species, 2) the existence of different colors for the two sexes in some gibbon species, 3) the occurrence of coat color changes at sexual maturity for some gibbon species, 4) the impact of malnutrition and housing (e.g. indoors only or in full sunlight) on coloration, 5) the ease with which some gibbon species' songs are confused, 6) the difficulty with which some gibbon subspecies are distinguished, 7) and a dearth of information on the individual gibbon's origins who were confiscated. Given these problems, it is not surprising that zoos and other breeding facilities may encounter difficulties in the identification of gibbons.

Methods

Specific and subspecific status was determined through the comparison of study skins housed in NY, C, W, H, B, and S. Additional criteria followed Groves (1972), Marshall and Sugardjito (1986), and Geissmann (1995). Vocalizations of live specimens were compared to Marshall & Marshall (1978) and Marshall & Sugardjito (1986). Data was collected between 1977 and 1997 at the International Center for Gibbon Studies and at the following zoos: SDZ, LAZ, SZG, ZNM, TSI, and KBR. Geographic distributions for each species are summarized in Table 1; sample body weights are provided in Table 2.

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Table 1 Distribution and Nomenclature of Extant Gibbons (Hylobates)

Subgenus Symphalangus (2n=50 chromosomes)

Subgenus Nomascus (2n=52 chromosomes)

Subgenus Bunopithecus (2n=38 chromosomes)

Subgenus Hylobates (2n=44 chromosomes)

Table 2 Sample Body Weights for Gibbons (Hylobates)

Subgenus Symphalangus

Subgenus Nomascus

Subgenus Bunopithecus

Subgenus Hylobates

*source: Ma, Wang, & Poirier, 1988