Fact Sheet: Hoolock (Hoolock hoolock)
Other names: Hoolock gibbon, White-browed gibbon
Traditionally, the hoolock has been considered being
a member of the genus Hylobates and the monotypic representant of a distinct
subgenus (e.g. Geissmann, 1995; Rowe, 1996; Marshall & Sugardjito, 1986). 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 (Brandon-Jones et al., 2004; Geissmann, 2002),
and the traditional scientific name of the hoolock changes from "Hylobates
hoolock" to Hoolock hoolock (Mootnick & Groves, 2005).
The hoolock occurs in Bangladesh, Burma, NE. India (Assam, Arunachal Pradesh, Manipur, Meghalaya, Mizoram, Nagaland, Tripura) and SW. China (W. Yunnan) (Geissmann, 1995; Groves, 1967). Geographically, the hoolock's distribution area extends west to the Brahmaputra River and east to the Salween River. Of all gibbon species, its range extends the farthest north and west.
The hoolock is found in several types of habitats: tropical evergreen forest, the wetter tropical semi-evergreen forests, sub-tropical monsoon evergreen broadleaf forests, and sub-tropical evergreen broadleaf hill or mountain forests. The species appears to be less common in deciduous forest and scrub forest, and absent from mangrove (Choudhury, 1996a; Gittins & Tilson, 1984; Lan, 1994). It occurs at altitudes of 80-1500 m (Choudhury, 1996a; Mukherjee, 1986).
Ecology and diet
Like other gibbons, the hoolock is an arboreal and a diurnal species, and like other gibbons, it prefers the upper canopy of the forest, and sleeps and rests in emergent trees (Leighton, 1987). Like other gibbons, hoolocks eat mostly fruits (51-89%), with the main supplement being leaves (6-32%); in addition, hoolocks also consume smaller quantities of flowers and insects (Alfred, 1992; Feeroz & Islam, 1992; Gittins & Tilson, 1984; Islam & Feeroz, 1992; Tilson, 1979). Mukherjee (1986) found lower amounts of fruits (30-40%) and higher amounts of leaves (40-60%) in the diet of hoolocks in Tripura (India). Among fruits, figs appear to be to most important food item and make up about 60% of the fruits consumed by hoolocks (Alfred, 1992) and about 38% of their total diet (Feeroz & Islam, 1992; Islam & Feeroz, 1992).
Reproduction and ontogeny
Morphology and anatomy
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.
When their habitat is fragmented and food trees are isolated, gibbons are forced to descend from trees to cross clearings, as already observed by McCann (1933).
The pelage of adult males is black with distinct
white eyebrows. The male has a long genital tassel. In the eastern subspecies, the
white brow streaks are well separated with no white hairs between, and both the chin
beard and the genital tassel tend to be of buff or whitish color; in the western
subspecies, the brow streaks are close together, and the chin beard and the genital
tassel are usually black with no or only a faint grizzling (Groves, 1967).
Group size and social structure
Hoolock live in small, monogamous family groups.
Typical groups consist of an adult pair with 0-4 immature offspring. Average group
size ranges from 2.7 to 4 (2-6) (Alfred, 1992; Choudhury, 1990, 1991, 1996a; Feeroz
& Islam, 1992; Gittins and Tilson, 1984; Mukherjee, 1992; Siddiqi, 1986; Tilson,
1979). Young gibbons leave their natal group when they become adult. One untypical
group which included two adult females (probably sisters) was unstable and ended
up with one female permanently leaving the group (Ahsan, 1995a).
On average, a group covers a day range of about 600-1200 m (300-1600 m) (Feeroz & Islam, 1992; Mukherjee, 1986). Territories are defended from intrusion by other gibbons by loud morning songs and by actively chasing intruders off of the territory.
Like other gibbons, hoolock gibbons will go out
of their way to avoid water and may drown if they fall into deep water (Candler,
1903), but at least one captive infant was observed to swim (McCann, cited in the
editor's comment to Parsons, 1940).
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,
Hoolocks exhibit a sternal glandular field as well as a less well defined glandular concentration in the inguinal area, which may play a role in olfactory communication (Geissmann, 1993; Geissmann & Hulftegger, 1994).
Social grooming is the most commonly observed social behavior in hoolocks (Alfred, 1992) and probably plays a role in reinforcing the bonds between group members.
There is some evidence for self-recognition in the mirror (Ujhelyi et al., 2000).
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
Status and conservation
IUCN Category of threat (Hilton-Taylor, 2000): endangered
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