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Fact Sheet: Hoolock (Hoolock hoolock)

Thomas Geissmann
Anthropological Instiute
University Zürich-Irchel
Winterthurerstrasse 190
CH-8057 Zürich
21 August 2001

Male hoolock

Female hoolock

Subadult male (left) and adult female (right) of the hoolock (Hoolock hoolock)


Other names: Hoolock gibbon, White-browed gibbon
Scientific name: Hoolock hoolock


Kingdom: Animalia (animals)
Phylum: Chordata (chordates)
Class: Mammalia (mammals)
Order: Primates (primates)
Family: Hylobatidae (small apes)
Genus: Hoolock (hoolock gibbons)
Species: hoolock (hoolock)

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).

Two subspecies of the hoolock are known: the western hoolock (H. hoolock hoolock) and the eastern hoolock (H. hoolock leuconedys). The Chindwin River in Burma forms the common boundary between the two subspecies (Groves, 1967).


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

  • Menstrual cycles have an average duration of 28 days (Matthews, 1946).
  • Mating appears to occur more often in March-May (Feeroz & Islam, 1992). Extra-pair copulations are very rare (Das, cited in Srivastava, 1999; Feeroz, 1996).
  • Gibbons give birth to single offspring. Birth typically occurs from November to February (Alfred, 1992; Alfred & Sati, 1990; McCann, 1933, Tilson, 1979). 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; Matthews, 1946).
  • 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, but hoolocks have rarely survived in captivity for more than a few years.


  • Head and body length: about 60-90 cm (24-35 inches) (estimate)
  • Body weight: about 7 kg (range: 6.0-8.5 kg or 13.2-18.7 lbs) (Geissmann, 1993)
  • Males and females are of about the same body size.

Morphology and anatomy

  • 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.
  • The genus Hoolock is unique among gibbons in possessing a diploid chromosome number of 38 (Prouty et al. 1983; Liu et al., 1987). In other gibbon genera the number is 50 (Symphalangus), 52 (Nomascus) and 44 (Hylobates), respectively.
  • Female hoolocks appear to possess a small throat sac (Kanagasuntheram, 1954) which may amplify the calls or some of its frequency bands.


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).

The pelage coloration of adult females strongly contrasts with that of adult males (sexual dichromatism). Females have a beige or gray-buffy pelage. The cheeks and ventral area are often darker brown. In addition to the white eyebrows, the female also exhibits a thin whitish margin around the black facial area and a white stripe below the eyes and across the ridge of the nose (Geissmann, 1995).

When a hoolock is born, its coat is a milky white color (Alfred, 1992; Alfred & Sati, 1990). During the second half of the first year, the color changes to black within a few months; only the brow band remain white. The gibbons then resemble adult males in their fur coloration. Females turn to a beige 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

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).

Like other gibbons, hoolocks are territorial. Each family group occupies a home range of about 14-55 hectares (Alfred & Sati, 1990; Feeroz, 1996; Feeroz & Islam, 1992; Gittins & Tilson, 1984, Tilson, 1979). One solitary female covered an area of about 73 ha which overlapped nearly 15% with the home ranges of three neighboring groups (Feeroz, 1996). Much larger home ranges of 300-400 ha were reported by Mukherjee (1982), whereas particularly small home ranges were found by Kakati (1999).

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.

Behavioral characteristics

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).

Hoolocks bask in the morning sun, especially during the cold winter season (Choudhury, 1996a; Tilson, 1979). High, leafless trees situated at the center of the territory are favored for this behavior.
Like other gibbons, hoolock groups appear to have favorite arboreal pathways across the canopy of their territory which they use more frequently than others (Feeroz & Islam, 1992; Islam & Feeroz, 1992).

Hoolocks exhibit strong seasonal fluctuations in their day range and activity budget. In the winter season, hoolocks spend more time feeding, less time traveling, their songs start later, they produce fewer song bouts, and they retire to their sleeping trees earlier than in summer (Feeroz & Islam, 1992; Islam & Feeroz, 1992; Mukherjee, 1986). Mukherjee (1986) also mentioned seasonal fluctuations in diet composition, and to judge by the figure published in (Feeroz & Islam, 1992; Islam & Feeroz, 1992), hoolocks appear to eat a slightly higher proportion of fruits and fewer leaves in winter than in summer.

At night, gibbons sleep sitting up. The family group spends the night in one of several preferred "sleeping" trees of the territory (Feeroz & Islam, 1992). The whole group may sleep on one or distributed over two different trees.

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 hoolocks typically produce duet songs which consist of coordinated vocal interactions by both partners using sex-specific phrases (Choudhury, 1989; Geissmann, 1993, 1995; Gittins & Haimoff, 1984; Haimoff, 1985). In contrast to other gibbon species, however, no sex-specific note types are used during hoolock songs.

Duet song bouts had an average duration of 15-18 min (Feeroz & Islam, 1992; Gittins & Haimoff, 1984; Tilson, 1979).

Other family members may participate in the song bout. Solo song bouts are typically produced by unmated hoolocks only.

Most song bouts are produced from preferred calling trees, mostly situated near the territorial border (Feeroz & Islam, 1992).

Olfactory communication

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).

Tactile communication

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

Not available.

Status and conservation

IUCN Category of threat (Hilton-Taylor, 2000): endangered (criteria: A1cd)

The species is threatened by habitat loss and by hunting for food, for Oriental medicine and for the pet market (Ahsan, 1995b; Choudhury, 1990, 1991, 1996a; Mukherjee et al., 1992). Hunting for food is the single most important reason for the decline of the species in several regions. Slash-and-burn shifting cultivation (Jhum) is the main factor leading to the destruction and fragmentation of gibbon habitat, but conversion of tropical forest to teak plantations, betel-leaf (Piper betle) plantation, and encroachment of forest land for settlement (Choudhury, 1996a; Mukherjee et al., 1992). Several other traditional uses of forests for commercial purposes contribute to habitat degradation, these include introducing exotic tree species, extracting fuel wood, extracting tree bark, extracting timber, and livestock herbivory (Ahsan, 1995b).

In Bangladesh, NE India and SW China, much of their habitat is extremely fragmented (Alfred & Sati, 1990; Choudhury, 1996b; Lan, 1994). This makes hoolocks particularly vulnerable to hunting and predation. Most populations are very small and declining (Choudhury, 1996b; Mukherjee et al., 1992), and many local populations will probably go extinct in the near future (Alfred & Sati, 1990). Not much information is available on the status of the hoolock populations in Burma (Myanmar). There have been virtually no studies of this gibbon in Burma since the Vernay-Hopwood-Chindwin expedition in the mid-1930s. Recent information suggests that hoolocks have disappeared from many forested areas in Burma, but a substantial number of sightings have been reported along the lower Salween River, a region of intensive logging (Anonymous, 1990; Eudey, 1990; Marshall, 1990).


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