Hoolock gibbon is the common name for any of the arboreal, tailless, Asian apes belonging to the gibbon genus Hoolock, characterized by long limbs, thick and shaggy hair, a tuft of hair in the anogenital region, and males that are black with a white strip above the eyes. Also known as hoolocks and the white-browed gibbon, two extant species of hoolock gibbons currently are recognized: H. hoolock (western hoolock gibbon) and H. leuconedys (eastern hoolock gibbon), distinguished, among other characters, by H. hoolock having a white unibrow and H. leuconedys having two distinct white eyebrows. However, the recognition of two species, and the Hoolock genus itself, are recent developments, and in some taxonomies these two primate groups continue to be considered as subspecies within the one species of H. hoolock and traditionally the hoolock gibbons were placed at the subgenus level of the genus Hylobates.
As seed dispersers and part of forest food chains—consuming plant and some animal matter, and being consumed by large cats, birds of prey, and pythons—hoolock gibbons provide a valued function in their ecosystems. For humans, their unique form and behaviors, including aerobatic movement through the forest canopy, adds to the wonder of nature.
Despite these values, hoolock gibbons face an uncertain future. Loss of habitat and hunting for food, Oriental medicine, and for the pet market has led to their decline in several regions. In some regions, their habitat is very fragmented and further at risk from encroachment by humans, forest clearance for tea cultivation, the practice of jhuming (slash-and-burn cultivation), and forest degradation. Both species are considered Threatened according to the IUCN Red List, with the western hoolock considered Endangered and the eastern hoolock Vulnerable. The Western hoolock gibbons are now entirely dependent on human action for their survival.
The hoolock gibbons (genus Hoolock) are two primate species from the family of the gibbons, Hylobatidae, of the ape superfamily Hominoidea. Gibbons, which also are known as "lesser apes to distinguish them from members of the family Hominidae (great apes), generally have four recognized genera, Hylobates, ''Hoolock, Symphalangus, Nomascus.
However, the classification of gibbons has changed considerably over the years. Classically, all gibbons, including the hoolock gibbons, were classified in the genus Hylobates, as can be seen in the taxonomies of Groves (1997), Goodman (1999), Wilson and Reeder (1993), Nowark and Walker (1991), and Napier and Napier (1985). Goodman (1999) further separated the siamangs into their own genus, Symphalangus, but generally other taxonomies included the siamangs in the genus Hylobates. Bunopithecus was recognized as the hoolock subgenus distinct from other gibbon subgenera. However, molecular evidence indicated that the distance among the various subgenera was substantial and the subgenera were elevated to full genus status, making for the four currently recognized genera. Geissmann (2006a) reports that "recognition of the four groups as full genera is now widespread."
However, the type species for Bunopithecus is Bunopithecus sericus, an extinct gibbon or gibbon-like ape from Sichuan, China. Very recent investigations have shown that the hoolocks are not closely related to B. sericus and so have been placed in their own genus, Hoolock. The four extant genera (formerly subgenera) are recognized based on their diploid chromosome number: Hylobates (44), Hoolock (38), Nomascus (52), and Symphalangus (50) (Mootnick and Groves 2005; Geissmann 2006b).
The traditional taxonomy also recognized the hoolock as one species, first Hylobates hoolock and then Hoolock hoolock, with two subspecies (H. hoolock hoolock and H. hoolock leuconedys) (Geissman 2006c) These two suspecies generally are now raised to the species level, Hoolock hoolock (western hoolock) and Hoolock leuconedys (eastern hoolock) (Moontick and Groves 2005; Gron 2008). Many publications continue to list these two groups as distinct only at the subspecies level (Gron 2008).
Hoolock gibbons are tailless, like all other apes, but do possess a tuft of hair in the anogenital region. They have extremely long arms and relatively long legs, with elongated and hook shaped hands, and opposable thumbs, which are not used for swinging in the trees (Geissmann 2006c). They are unique among gibbons in having a diploid chromosome number of 38 (Geissmann 2006c). As with all gibbons, hoolocks have a ball and socket joint forming the wrist, connecting the hand with the forearm, and adaptation that greatly aids branchiation (swinging from tree limb to tree limb with their arms).
Hoolocks are the second largest of the gibbons, after the siamang. They reach a size of 60 to 90 centimeters and weigh 6 to 9 kilograms. The sexes are about the same size, but they differ considerably in coloration: males are black or blackish in color, with a white strip above their eyes. Females are lighter, ranging from pale and buff to a gray-brown fur, which is darker at the chest and neck. Both have black faces. White rings around the eyes and around the mouth of females give their face a mask-like appearance.
Morphological differences between the two species is most pronounced among males, with females harder to distinguish visually. In H. leuconedys males have two distinct white eyebrows and a white or silver genital tuft, about 7.5 centimeters (3.0 inches) long, while males of H. hoolock have a white unibrow and a black genital tuft, around 5 centimeters (2.0 inches) long (Gron 2008).
The range of the hoolocks is the most northwestern of all the gibbons, extending from Assam in North-East India, to Myanmar. Small populations (in each case few hundred animals) live also in the eastern Bangladesh and in southwest China. Hoolocks are the only apes in India and within China (Gron 2008).
The two species are separated by the Chindwin and Irrawaddy Rivers in Myanmar, with H. hoolock to the west of the watercourses and H. leuconedys to the east. H. leuconedys continues east as far as the Salween River in Myanmar (Gron 2008). H. hoolock continues west to the Brahmaputra River in India and Bangladesh (Geissmann 2006c).
Hoolock gibbons are found only in forest environments and dependent on a contiguous canopy. There are several different forest types they inhabit: Tropical evergreen forest, broad-leaved moist deciduous forest, sub-tropical broadleaf forest, and so forth. They prefer the upper canopy. Hoolock gibbons avoid water (Gron 2008; Geissman 2006c).
Like the other gibbons, hoolocks are diurnal and arboreal, brachiating through the trees with their long arms, as well as leaping, climbing, and jumping, and occasionally utilizing bipedal walking. They rarely come to the ground, doing so when trees are too far apart to leap. They typically feed while sitting or suspended in the trees. Most activity occurs between 6 and 20 meters (19.7-65.6 feet) above ground, including feeding, movement, resting, and calling, but social activities typically occur between 5 and 27 meters (16.4-88.6 feet). They sleep in the trees, primarily with the knees tucked into the chest and their arms around themselves, and they typically sleep in the taller trees and on slopes or hilltops (Gron 2008).
Hoolock gibbons are omnivorous, eating a wide variety of plant matter, including primarily fruits (and especially figs), as well as flowers, lianas, shoots, leaves, and nectar. They also consume invertebrates, such as insects, and birds eggs.
Gibbons are social animals and form pair bonds. Hoolock gibbons live together in monogamous pairs, as part of small family groups, which stake out a territory. The home range can vary from as small as 0.15 to 0.30 square kilometers (0.06-0.1 square miles), up to 3 to 4 square kilometers (1.2-1.5 square miles) (Gron 2008). Their calls serve to locate family members and ward off other gibbons from their territory. Hoolock gibbons call in the morning and usually in the earlier morning (Gron 2008).
Young hoolocks are born after a seven month gestation, with a milky white fur. After about six months, their fur turns black. After 8 to 9 years they are fully mature and their fur reaches its final coloration. Their life expectancy in the wild is about 25 years, and they have lived to be as old as about 40 years in captivity (Gron 2008).
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