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Hoolock gibbon

Conservation status:
Endangered (H. hoolock), Vulnerable (H. leuconedys)

Life span: 30 years (captive)
Total population: <35000 (H. hoolock)
Regions: India, Bangladesh, Burma, China
Gestation: 6 to 8 months
Height: 81.2 cm (M & F)
Weight: 6.9 kg (M), 6.1 kg (F)

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Suborder: Haplorrhini
Infraorder: Simiiformes
Superfamily: Hominoidea
Family: Hylobatidae
Genus: Hoolock
Species: H. hoolock, H. leuconedys

Other names: hoolock gibbon, white-browed gibbon; tooboung, myouk umaigyall (Arakanese); hoolock (French); uluk (Hindi); gibón hulock (Spanish); holockgibbon, holok, hulock (Swedish); wu-wa (Thai); H. hoolock: western hoolock; H. leuconedys: eastern hoolock.

The taxonomy of the hoolock gibbon has recently been significantly revised. Hoolock gibbons have been placed within their own genus, Hoolock, as well as split into two discrete species, the western hoolock gibbon (H. hoolock) and the eastern hoolock (H. leuconedys) (Mootnick & Groves 2005). However, in many publications, the two species are only considered distinct at the subspecific level. Further, the vast majority of the available published literature is on the western hoolock, but due to presumable similarities between the species, both are included here.


H. leuconedys
Hoolock leuconedys
Photo: Gabriela Skollar/Gibbon Conserv Ctr

The hoolock gibbon is tailless like all other apes (Groves 1972; Choudhury 1991). However, the species possesses a tuft of hair in the anogenital region (Marshall & Sugardjito 1986; Choudhury 1991; Mootnick 2006). Sexual dichromatism is seen between males and females but sexual dimorphism is not pronounced (Groves 1972; Jenkins 1990; Choudhury 1991; Alfred 1992). Both sexes have thick and shaggy hair with long limbs (Roonwal & Mohnot 1977; Choudhury 1991). Males are black or blackish, with a white strip above their eyes which curves up at either end (Choudhury 1991; Das et al. 2006; Mootnick 2006). The face of both sexes is black (Choudhury 1991). In contrast to the males, adult females are lighter, usually ranging from buff and pale to various shades of tan with some brown, grey and yellowish coloration (Roonwal & Mohnot 1977; Choudhury 1991; Islam & Feeroz 1992; Mootnick 2006).

Between the species, there are some morphological differences, predominantly between males with females significantly harder to distinguish visually (Jenkins 1990). In H. hoolock, males are black, with a white unibrow (mono-brow) and a black genital tuft (around 5 cm (2.0 in) long) (Groves 1972; Jenkins 1990; Mootnick 2006). H. leuconedys males have two distinct white eyebrows and a silver or white genital tuft (around 7.5 cm (3.0 in)) (Groves 1972; Jenkins 1990; Das et al. 2006; Mootnick 2006).

The average height of a hoolock gibbon is 81.2 cm (32.0 in) (A. Mootnick pers comm.). Females weigh around 6.1 kg (13.4 lb) while males weigh around 6.9 kg (15.2 lb) (Leutenegger & Cheverud 1982).

H. leuconedys
Hoolock leuconedys
Photo: Gabriela Skollar/Gibbon Conserv Ctr

Hoolock gibbons are almost entirely arboreal, coming to the ground only in exceptional circumstances. Movement is primarily through brachiation (70-80% of movement), leaping, climbing and jumping (16-25%) and acrobatics or bipedal walking (4-5%) (Alfred 1992; Islam & Feeroz 1992; Sati & Alfred 2002). When moving bipedally they usually travel for less than a minute and they will move quadrupedally on rare occasions (Islam & Feeroz 1992). Any bipedal movement that does occur is typically unsteady and awkward (Candler 1904; Alfred 1992). Such bipedality can occur both arboreally or terrestrially with and without the animal using its arms for support. Terrestrial bipedality usually only occurs when trees are too far apart to leap between (Jindal & Sharma 1984; Alfred 1992). Hoolock gibbons feed while either sitting or suspended from a support, depending on the food (Sati & Alfred 2002). During the morning in the winter, hoolock gibbons will "sunbathe" in high branches exposing their backs to the sun for several minutes at a time (Tilson 1979; Choudhury 1996). In general, most of the daily activities occur between 6 and 20 m (19.7 and 65.6 ft) above ground level (feeding, movement, resting and calling) except social activities, which normally occur between 5 and 27m (16.4 and 88.6 m) (Hasan et al. 2007).

Hoolock gibbons have lived to be as old as an estimated 40 years in a captive setting (Weigl 2005).


Hoolock hoolock | Hoolock leuconedys

In general, starting in the east and moving roughly westerly, hoolock gibbons are found only in China, Myanmar, northeastern India, and Bangladesh (Mukherjee 1984; Yang et al. 1987; Ma et al. 1988; Mukherjee et al. 1992). Hoolock gibbons are the only ape present in India and within China, hoolock gibbons are found only in the western Yunnan province (Yang et al. 1987; Ma et al. 1988; Mukherjee et al. 1992; Das et al. 2006). In addition, they are only found west of the Salween River in Myanmar (Groves 1972). In eastern India, the hoolock is restricted to the states of Assam, Arunachal, Pradesh, Nagaland, Manipur, Mizoram, Tripura, and Meghalaya. Thus, in general they are found in India south and east of the Brahmaputra, Lohit, and Dibang Rivers (Groves 1972; Choudhury 1987; 2006). Within Bangladesh, hoolock gibbons are only found in the southeast and northeast (Feeroz et al. 1995).

The distributions of the two species are demarcated by the Chindwin and Irrawaddy Rivers in Myanmar, with H. leuconedys occurring to the east and H. hoolock to the west of the watercourses (Groves 1967; Ma et al. 1988). The distribution of H. leuconedys continues east as far as the Salween River in Myanmar (Groves 1972). However, there are recent reports of hoolock gibbons close in appearance to H. leuconedys reported in the Lohit district of Arunachal Pradesh, India, the most easterly district in India which may require revision of the current geographical division between species (Das et al. 2006).

It is important to remember however, that the distribution is not continuous and in many cases is quite fragmented (Choudhury 1991). There are estimated to be roughly 200 western hoolock gibbons left in the wild in Bangladesh (Biswas et al. 2003; Molur et al. 2005; Walker et al. 2007) and there is no published information on the status of western hoolock gibbons in Myanmar (Kakati 2006; Walker et al. 2007).


Hoolock gibbons are exclusively forest-dwelling and depend on a contiguous canopy. They are found in broad-leaved, moist deciduous forest; mixed evergreen/deciduous forest with tall deciduous trees and an evergreen understory; sub-tropical broadleaf forests, from mountainous, hill, valley, slope and lowland forests to wet evergreen and semi-evergreen forests often consisting of interspersed trees and bamboos as well as various conditions of forest, including primary, secondary, and regenerating (Khan & Ahsan 1986; Mukherjee 1986; Choudhury 1990; 1991; Islam & Feeroz 1992; Mukherjee et al. 1991-1992; 1992; Lan 1994; Choudhury 1996; 2000; Ahsan 2001; Sati & Alfred 2002; Das et al. 2003; Choudhury 2006; Kakati 2006; Srivastava 2006). The best habitats have continual canopy of tall trees with thick undergrowth (Mukherjee et al. 1991-1992; Bai et al. 2007). Typical tree species found in hoolock habitats include Mangifera, Bixa, Dipterocarpus, Zyzygium, Protium, Entada, Lagerstoemia, Shorea, Albizzia, Artocarpus, and Ficus (Mukherjee 1986; Muzaffar et al. 2007). Hoolock gibbons are found at altitudes under 50 m and up to and above 2500 m (Choudhury 2006).

In general, hoolock gibbon habitats are tropical with wet, hot summers with monsoon rains and dry cool winters. Annual rainfall is usually between 130 and 400 cm (51.2 and 157.5 in) (Choudhury 1991). For example, in Assam, India, the annual temperature usually ranges between 7°C (44.6°F) and 32°C (89.6°F) (Choudhury 2000).


Hoolock gibbons are primarily omnnivorous, consuming, in some habitats, over 100 species of plants although sometimes the number of species is far lower (Alfred 1992; Islam & Feeroz 1992; Ahsan 2001). The species and number of food species can vary greatly between study site (based partially on availability and density) and they also consume some prey items, predominantly invertebrates but also birds' eggs (Mukherjee 1986; Alfred 1992; Ahsan 2001). In the diet, figs dominate and fruits are very important however lianas, flowers, shoots, petioles, exudates, nectar, and other plant foods, including lichens, are also consumed (Islam & Feeroz 1992; Ahsan 2001; Bujarbarua & Das 2001). In one study, when expressed as a percentage, fruits comprised 65% of the diet (60% of this is figs), buds, young shoots, and leaves 25%, and flowers and insect 10% (Alfred 1992). In most studies, proportions were roughly similar, with fruit, especially figs, predominating, followed by other plant parts and to a lesser extent, insects (Tilson 1979; Gittins & Tilson 1984; Ahsan 1992; Islam & Feeroz 1992; Feeroz et al. 1994; Ahsan 2001; Hazarika & Gupta 2005). However, sometimes leaves are nearly as important as fruit in the diet (Hazarika & Gupta 2005).

While variable between study sites and study groups, in Bangladesh, hoolock gibbons spend their days feeding (30.8%), foraging (25.8%), resting (27.5%), traveling (7.4%), with the rest of the day spent in other activities, including calling, territorial behavior, and play (Ahsan 2001). Although variable, in general between study sites, feeding, moving and resting activities predominate followed by other activities (Alfred 1992; Feeroz & Islam 1992; Islam & Feeroz 1992; Gupta 1994; Ahsan 2001; Hazarika & Gupta 2005). Social activities including play and grooming take up less than 10% of the day (Gupta et al. 2005).

Diurnal hoolock gibbons are usually active, starting at dawn or in the early morning, for 8-10 hours each day (Mukhergee 1986; Feeroz & Islam 1992; Gupta et al. 2005). Hoolock gibbons start activity early in the morning and they awake earlier in the summer than in the winter (Gittins & Tilson 1984; Alfred 1992; Ahsan 2001). As they leave their sleeping sites, hoolock gibbons usually defecate and urinate (Ahsan 2001). Feeding peaks during the morning hours while movement peaks right before hoolock gibbons move to their sleeping trees at the end of the day with a second smaller peak in the morning (Islam & Feeroz 1992; Ahsan 2001). Resting activity peaks in the early afternoon (Ahsan 2001). Calling occurs only before noon, and then usually in the earlier morning (Islam & Feeroz 1992; Ahsan 2001). The end of the activity period is usually several hours before sunset possibly to avoid feeding overlap with sympatric monkeys (Ahsan 2001; Gupta et al. 2005). In the summer when the days are longer, more resting and social behavior is seen (Gittins & Tilson 1984).

The home range of hoolock gibbons varies with study site, and can range between 0.15 and 0.30 km² (0.06 and 0.1 mi²) up to 3-4 km² (1.2-1.5 mi²), with some intermediate sizes as well as some overlap with neighboring groups (Tilson 1979; Gittins & Tilson 1984; Alfred 1992; Islam & Feeroz 1992; Choudhury 1996; Ahsan 2001; Gupta et al. 2005). Average day ranges can vary between 600 m (1968.5 ft) and around 1350 m (4429.1 ft) but single day ranges can be less than 300 m (984.3 ft) and more than 3000 m (9842.5 ft) with the highest day ranges occurring during the summer (Mukherjee 1986; Islam & Feeroz 1992; Ahsan 2001; Gupta et al. 2005).

In captivity and in the wild, hoolock gibbons generally sleep with the knees tucked up into the chest and the arms around them in a hunched position (McCann 1933; Jindal & Sharma 1982). In the wild, sleeping trees are usually taller than those nearby and are normally located on slopes or hilltops. Usually, hoolock gibbon groups do not all sleep together in the same tree but instead in several nearby trees (Ahsan 2001).

Several species of bird (magpies, drongos, and laughing thrushes), sometimes chase hoolock gibbons when they are in food trees, with drongos being more aggressive, continuing confrontations until hoolock gibbons leave (Islam & Feeroz 1992). Hoolock gibbons compete for food with the Malayan giant squirrel and do not allow them to remain nearby. Primate species with which hoolock gibbons may be sympatric include Phayre's leaf monkeys (Trachypithecus phayrei), capped langurs (T. pileatus), slow loris (Nycticebus coucang), rhesus macaques (Macaca mulatta), Assamese macaques (M. assamensis), pigtail macaques (M. nemestrina) and stump-tailed macaques (M. arctoides) (Mukherjee et al. 1992). When in contact with other sympatric primates, langurs, lorises and macaques are usually tolerated by hoolock gibbons although agonism can occur (Mukherjee 1986; Islam & Feeroz 1992). Pigtailed macaques are often avoided because of the large group sizes of that species (Feeroz & Islam 1992). The general ability of hoolock gibbons to coexist with sympatric primates as well as other species with which it has food overlap is partially attributable to differences in daily activity patterns (Islam & Feeroz 1992).

Potential predators of hoolock gibbons include eagles, vultures, leopards, and pythons (Gupta et al. 2005).

Content last modified: August 13, 2008

Written by Kurt Gron. Reviewed by Alan Mootnick.

Cite this page as:
Gron KJ. 2008 August 13. Primate Factsheets: Hoolock gibbon (Hoolock) Taxonomy, Morphology, & Ecology . <>. Accessed 2014 April 21.