Western Hoolock Gibbon, Hoolock hoolock
WESTERN HOOLOCK GIBBON
Hoolock hoolock
Geographic Distribution and Habitat
Western hoolock gibbons (Hoolock hoolock), also called white-browed gibbons, reside in the countries of Bangladesh; northeastern India—within the states of Arunachal Pradesh, Assam, Manipur, Meghalaya, Mizoram, Nagaland, and Tripura—where their distribution is restricted to points south of the Brahmaputra and east of the Dibang (Dingba Qu) Rivers; and northwestern Myanmar, west of the Chindwin River. The Chindwin River separates western hoolock gibbons from their cousins, the eastern hoolock gibbons, Hoolock leuconedys—though the two species share a hybrid zone at the river’s northern headwaters.
Some wildlife researchers speculate that the species might also have a presence in extreme southeastern Tibet. But those western hoolock gibbons who had once made their homes on the plains of Arunachal Pradesh are gone, displaced when their habitat was razed and transformed into agricultural tracts of land.
Tropical evergreen rainforests, tropical evergreen and semi-evergreen forests, tropical mixed deciduous forests, and subtropical broadleaf hill forests provide the species with its habitat. Western hoolock gibbons prefer dense forest vegetation with contiguous canopy, where they reside at altitudes up to 8,202 feet (2,500 meters), often in mountainous terrain. They sometimes occupy thickets of bamboo (known as “brakes”) where they have been found at ground level. The species also occurs in plantations of hollock, a large evergreen tree also known as the East Indian almond tree (Terminalia myriocarpa), and in plantations of ajar, a deciduous tree with bright pink to light purple flowers, also known as Queen’s Crape Myrtle or Queen’s flower (Largerstroemia flosreginae). Both hollock and ajar are cultivated for medicinal properties: the bark of the hollock tree is used as a diuretic, cardiac stimulant, and an antioxidant; leaves of the ajar tree are used to treat diabetes and kidney problems, whereas its bark is commercially harvested.
Gibbons are known as “lesser” or small apes and belong to the family Hylobatidae, which includes four genera: dwarf gibbons (Hylobates), crested gibbons (Nomascus), siamangs (Symphalangus), and hoolock gibbons (Hoolock).
The term “lesser ape” has fallen out of favor with some wildlife biologists, who suggest that it may lead to the incorrect connotation that gibbons are less intelligent than their great ape cousins (gorillas, chimpanzees, orangutans, and bonobos). Not so, these wildlife biologists assert. The preferred term is “small apes.”
Before being recognized as a unique species in 2005, scientists had considered the western hoolock gibbon to be conspecific (belonging to the same species) with the eastern hoolock gibbon. That is, they thought these two gibbons were one and the same. But nope. When they reclassified the western hoolock gibbon, they also moved it from its historic genus, Bunopithecus (associated with an extinct species of gibbon-like primate) to Hoolock. Today, the genus Hoolock is shared by western, eastern, and skywalker (Hoolock tianxing) hoolock gibbons.
The western hoolock gibbon is the parent species of two subspecies. Hoolock Hoolock Hoolock, the nominate species, shares geographic range with its parent. The Mishmi Hills Hoolock gibbon (Hoolock hoolock mishmiensis), identified as a subspecies in 2013, occurs between the Lohit and Dibang (Dingba Qu) Rivers in Assam and Arunachal Pradesh.a
Size, Weight, and Lifespan
Hoolocks are the second-largest gibbon species after siamangs (Symphalangus syndactylus); siamangs live in the forests of Indonesia, Malaysia, and Thailand.
Adult western hoolock gibbons stand 32 inches (81 centimeters) tall. Average weight for adult males is 15.2 pounds (6.9 kilograms); average weight for adult females is 13.2 pounds (6 kilograms), an example of sexual dimorphism.
Lifespan in the wild is at least 25 years, provided these lesser apes are given the opportunity to thrive in their natural environment.
Appearance
Hoolocks are distinguished by extremely long arms, comparatively long legs, and wild hair that manifests itself in a plush, dense coat that covers their sturdy body. Like all gibbons, western hoolocks display obvious sexual dichromatism, that is, different coat colorations.
The pelage of males is charcoal-black and encroaches on its dark, hairless face; a pointy beard accents the chin. Above deep-brown eyes sit a pair of furry white eyebrows, giving dramatic contrast to the darkness and suggestive of an expression of surprise. A dark, grizzled tuft of fur accents the male’s genitals. The pelage of females is buff colored, ranging from light beige and brown to gray and yellow. Mother Nature has powdered a white ring of fur outlining the eyes and muzzle.
Like all gibbons, western hoolocks have hook-shaped hands with opposable thumbs; like all apes, they lack a tail.
Western hoolocks look nearly identical to their close cousins, eastern hoolock gibbons. No wonder the two species were once regarded as one and the same. But if you look closely at the males of each species, you’ll see that the white furred eyebrows of western hoolocks are set closer together—not quite a unibrow, nevertheless a small distinction that allows a keen observer to tell the two species apart.
Diet
The species is mostly frugivorous, meaning these hoolocks eat lots of fruits. They have a penchant for figs, but jackfruit and other ripe, pulpy fruits are also enjoyed. More than 76 plant species are included in their diet. Whenever fruits are scarce, particularly at the end of the dry season, their diet becomes more folivorous—meaning they eat more leaves. In addition to leaves, western hoolock gibbons’ plant-based menu includes flowers, buds, lichen, plant shoots, and bark. They also consume a smattering of non-plant-based foods: invertebrates, bird eggs, silkworms, ants, and other insects. Insects become a dietary staple during monsoon season.
The species has evolved to rely on its biodiverse ecosystem, so scientists are gravely concerned with the increasing loss and transformation of natural habitat through crop monocultures (cultivation of a single crop) and homogeneous forests (lacking biodiversity).
Behavior and Lifestyle
Western hoolock gibbons are both diurnal—meaning they are active during daylight hours—and arboreal—meaning they spend most of the time in the trees. These forest swingers get about by swinging from tree branch to tree branch, suspended by their long arms and using their hands to grip, high up in the forest canopy. Known as “brachiation,” this mode of locomotion allows the hoolocks to move quickly as they travel. They are also able to jump 32 feet (10 meters) or more. The occasional cartwheel certifies western hoolock gibbons’ credentials as aerial gymnasts. While brachiation is their go-to, gibbons improvise their movements, leaping great distances and falling from great heights in order to reach the next tree. They even frequently walk bipedally along a branch, holding their arms out for balance. This technique actually means that gibbons walk upright more than any other wild ape. When hoolock gibbons walk upright, whether across tree limbs or, on those rare occasions when they must descend to the ground (occurring when trees are too far apart to swing or leap, or to access an isolated food source), they extend their large arms to the side of overhead, like a tightrope walker holding a pole, to steady their balance (and to prevent their hands from dragging on the ground).
Like other gibbon species, hoolocks appear to have favored travel routes in the forest canopy. And like other gibbon species, western hoolocks avoid water when on the ground. Scientists suggest that a fear of drowning fuels their avoidance behavior. Though other accounts suggest that these apes are capable swimmers, if need be.
While eating, western hoolocks either sit upright on a tree limb or hang by one arm, securely gripping the branch above for support with one hand and feeding themselves with the other. They are known to sunbathe on high branches, particularly on winter mornings, allowing the warm rays of the sun to warm their backs. Overnight, they sleep in tall, emergent trees (those that tower above the main canopy layer) in a hunched position with their arms wrapped around their knees tucked into their chest, huddled together for warmth and safety. A family group may sleep together in one tree or side by side in two different trees.
Predators of western hoolock gibbons include large birds of prey (infant hoolocks are most vulnerable), pythons, and leopards. Domestic dogs are a potential threat to those hoolocks residing near human settlements.
As western hoolock gibbons brachiate through the forest, travel speeds can reach up to 34 mph (55 km/h)—making them the fastest of nonflying mammals!
A family group consists of a mated pair and their young offspring. Average group size is three individuals (mom, dad, and child), but a group can be as large as six when the family unit includes four offspring. Young hoolocks leave their natal (birth) group upon reaching adulthood (when they attain sexual maturity) in search of another unrelated group to join or to begin their own family group. Solitary individuals, usually subadults who have been forced to leave home, sometimes occur. These loners establish their own solitary territories.
The species is fiercely territorial and defends its home range boundaries with ardent vocalizations (loud morning songs), intimidating postures, and by chasing away intruders when necessary. Home range for most populations ranges from 19.8 to 156 acres (8 to 63 hectares). Low fruit availability is associated with a larger home range, as the hoolocks must search farther for food sources.
Western hoolock gibbons awaken at dawn. Seasonal fluctuations affect their daily activity budget. During winter months, they linger in their sleeping nest (just like we like to linger beneath our bed covers on cold and dark winter mornings). They also spend more time feeding and less time traveling (like we are prone to eat more and exercise less during the winter season). During summer months, they get an earlier start on the day’s activities and also make more time for socializing afforded by the longer hours of daylight.
After leaving their nest each morning, the hoolocks first take care of bathroom business. They then devote the next 8 to 10 hours of their day to feeding, foraging, napping, playing, and grooming. Feeding peaks during the morning hours, while early afternoon is for napping. Playing occurs at whim, and grooming is a relaxing social activity.
When foraging for fruits, it’s not uncommon for the hoolocks to be harassed by several species of birds, including laughing thrushes, magpies, and drongos, who fly aside the apes and generally annoy them until the hoolocks depart. The hoolocks have more luck in intimidating the Malayan giant squirrel (Ratufa bicolor), another food source competitor.
Sympatric primate species include Phayre’s leaf monkeys (Trachypithecus phayrei), capped langurs (T. pileatus), slow loris (Nycticebus coucang), rhesus macaques (Macaca mulatta), Assamese macaques (M. assamensis), stump-tailed macaques (M. arctoides), and pigtail macaques (M. nemestrina).
Generally, western hoolock gibbons’ interactions with sympatric primates are peaceful and without incident. Researchers posit that differences in daily activity patterns mitigate any would-be confrontations over food sources. The only primates they tend to avoid are pigtail macaques, due to that species’ large group sizes.
Gibbons are known for their loud, early-morning songs. Western hoolock gibbons have their own morning symphonies, specific to their species, with characteristics that are inherited, not learned, and are performed from their sleeping trees just after dawn, lasting 15 to 18 minutes. Rich, melodic, and harmonious, these vocalizations are a blend of alternating high and low notes, typically sung as a male-female duet. Unlike some gibbon species, male and female hoolock vocalizations are alike. Though a small throat sac on the female may amplify her notes. Singing in unison, the mated couple begins their song with an introductory sequence, gradually quickening their notes to become more intense with each successive call, finishing with a great-call sequence that reverberates throughout the forest. Apart from the musical enchantment, these loud morning songs allow a mated couple to announce and defend their territory.
Olfactory (scent) communication is not well documented. But wildlife biologists posit that secretions from glands in the sternum (breastbone) and inguinal area (groin) deliberately left on certain substrates may convey certain messages.
The most commonly observed tactile communication is allogrooming. Western hoolock gibbons love to groom one another, a pleasant pastime that also helps to establish social bonds. Their opposable thumbs help them perform this activity.
Western hoolock gibbons adhere to a monogamous social structure with the mated male and female forming the core of the family group. Reproductive maturity occurs between 8 to 9 years of age, when the hoolocks are considered fully mature. (Reproductive age for captive western hoolock gibbons is reported as 6 to 8 years of age.)
Mating occurs between March and May, with births occurring from November through February. A female gives birth after a 7-month pregnancy, usually to a single infant but occasionally to twins. For their first few months of life, infants cling tightly to their mother’s waist, for safety, comfort—and warmth. Infants are born with a hairless underside. Their overcoat is cream or buff-colored. By six months of age, the pelage of males begins to darken in color before eventually turning black. Only the eyebrows remain white. The pelage of females remains buff-colored into and throughout adulthood.
Mothers nurse their offspring for two years, at which time young hoolocks are considered weaned. The species’ interbirth interval is between two and three years.
These small apes greatly contribute to habitat regeneration. Western hoolock gibbons are important seed dispersers, thanks to their largely frugivorous diet. All those undigested seeds from the many varieties of fruits they eat are excreted in their feces, which act as fertilizer, as they travel throughout their tropical environment, encouraging new plant growth.
The western hoolock gibbon is classified as Endangered by the International Union for Conservation of Nature (IUCN, September 2017), appearing on the IUCN Red List of Threatened Species. The overall population has suffered an alarming decline, from over 100,000 individuals a few decades ago to less than 5,000 individuals today, confirming researchers’ grim prediction of a total population reduction of at least 50 percent over three generations (2001-2015, 2016-2030, and 2031-2045).
In Bangladesh, researchers consider the species to be Critically Endangered. A 2021 study published in the conservation/environmental science news platform Mongabey reveals a population drop of about 84 percent in the past four decades, with an estimated population (at that time) of only 469 individuals.
Habitat loss poses a significant threat to the species’ survival as forests continue to be decimated, particularly in Bangladesh, northeastern India, and Myanmar. Coal mining, oil mining, and transformation of pristine forest into tea plantations, and the harvesting of bamboo for use in papermills, along with the associated pollution of “slash-and-burn” agriculture, are but a few of the anthropogenic activities depleting the gibbons’ habitat. The resulting fragmented populations of small groups put the species at risk of becoming locally extinct. In Myanmar, political unrest adds a further conservation detriment.
Additional threats throughout the species’ range include rampant poaching and international wildlife trafficking. Indigenous groups hunt the gibbons for their flesh, known as “bushmeat”; traffickers kidnap the gibbons and transport them across country borders so these lesser apes can be used in traditional medicine; kidnapped infants are funneled through the illegal wildlife pet trade.
Conservation status for Hoolock hoolock hoolock, the nominate subspecies, is currently under review by the IUCN. It had been classified as Vulnerable, but the more dire status of Endangered is being considered.
The Mishmi Hills Hoolock gibbon is classified as Endangered by the IUCN, facing the same threats as its parent.
The western hoolock gibbon is listed in Appendix I of the Convention on International Trade in Endangered Species (CITES), an international agreement between governments whose goal is to ensure that international trade in specimens of wild animals and plants does not threaten their survival. The species is also included in Schedule 1 of the Indian Wildlife (Protection) Act of 1972 and in Bangladesh’s Wildlife Conservation and Security Act 2012 that prohibits the sale, purchase, import or export of wildlife.
Protected areas include Namdapha National Park in the Changlang District of Arunachal Pradesh, India. With the largest contiguous stretch of protected habitat, the park is considered a stronghold for this species. Other protected areas are found throughout the species’ range. Unfortunately, wildlife protection laws are often ignored, and poaching regularly occurs.
Wildlife biologists have called for an urgent conservation initiative that includes the hiring of additional forest officials to enforce protection laws, and the hiring of border guards and intelligence agents to strengthen surveillance and crack down on poaching and wildlife trafficking.
Education is a key conservation imperative. Promoting the western hoolock gibbon as a keynote species and cultivating an appreciation for this lesser ape, particularly with indigenous people, will help to ensure its place in our world. To point, based on their observations in Arunachal Pradesh, researchers have posited that the species might already be revered by certain groups—a stark contrast to those who view the gibbons as commodities.
Conservation organizations working to protect the species’ habitat include the World Land Trust and the Gibbon Conservation Alliance.
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Written by Kathleen Downey, April 2025