WESTERN HOOLOCK GIBBON

Hoolock hoolock

Geographic Distribution and Habitat
Western hoolock gibbons, also known as white-browed gibbons, are the only apes that live in the Indian sub-continent. They thrive in the dense forests that extend from east of the Brahmaputra River in northeast India, through Bangladesh, and into western Myanmar. There may be some populations living in extreme southeastern Tibet.

The landscape in the region is a mosaic of dense tropical evergreen rainforests, subtropical broadleaf hill or mountain forests, and subtropical monsoon evergreen broadleaf forests. Although these gibbons can be found in flood plains, they commonly reside at altitudes ranging between 260 to 4,900 feet (80–1500 m) and only a few individuals are known to live at altitudes above 8,000 feet (2,500 m) in Manipur (northeastern India).

In 2005, the western and the eastern hoolock gibbons were classified as two distinct species.

In 2013, a new western hoolock gibbon population was found and determined to be a western hoolock gibbon subspecies. The Mishmi Hills hoolock gibbon, Hoolock hoolock mishmiensis, resides between the Lohit and Dibang Rivers in Assam and Arunachal Predesesh.

Western hoolock gibbon range, IUCN 2018

Size, Weight, and Lifespan
There is not much difference in size between males and females; they weigh between 13 and 19 pounds (6–9 kg) and stand 23–35 inches (60–90 cm) tall. They can live up to 35 years in the wild and up to 60 years in captivity.

Appearance
Male hoolock gibbons have a black pelage, whereas females are a creamy whitish color with dark hair on the chest and neck. Their coats appear thick and soft. Their faces are striking because their markings almost look painted on. The white eyebrows form a straight line above the eyes of the males and the females’ black faces look as if framed with white fur. Their eyes, nostrils, and lips appear delicate and well-defined.

Their torsos and arms are elongated; their legs are short. Hands and feet are thin and long with an opposable thumb and big toe. The finger flexors are short, which helps the gibbons grasp onto branches—as if they had hooks. This is great adaptation to their mode of locomotion, called brachiation, as their hook-like fingers allow them to hang without expending much energy while staying stillas is the case when they forage, for instance.

It is very common for males and females to become thinner as they age and loose strength in their legs; old females are also easily recognizable because the hair on their necks, shoulders, and chests is not as thick as in younger individuals.

What Does It Mean?

Endemic:
Native or restricted to a certain area or country.

Brachiation:
Also called arm swinging, is a form of arboreal locomotion in which primates swing from tree limb to tree limb using only their arms.

Desiccated:
Dried-out.

Pelage: 
The fur, hair, or wool of a mammal.

Visit the Glossary for more definitions

Photo: Vijay Cavale/Creative Commons

Diet
Western hoolock gibbons are frugivorous and folivorous. When fruit is abundant, their diet is mostly composed of ripe fruit; when it isn’t, they eat leaves, flowers, and shoots. In Hallongapar Gibbon Wildlife Sanctuary in Assam, India, they have been observed eating a lot more non-plant food items than in any othermostly during the pre-monsoon (March–May) and monsoon (June–September) seasons, when insects are readily available. It is, therefore, possible that in small isolated forest patches, other western gibbons also consume more non-plant food items to survive than observed. Non-plant items include caterpillars, tasar silkworms, black ant eggs and larvae, and other insects rolled in dry leaves, as well as spiders.

To feed on black ant eggs and larvae, the western hoolock gibbons destroy the nests with their hands, which they shake often to avoid bites.

Western hoolock gibbons like to feed from trees in the fig family, the shala tree (also known as sal tree), and other trees that are native to the regions where gibbons are found. Indeed, fig and persimmons are important staples in their diet. However, they do not forage at all in exotic trees imported for plantations. In Bangladesh, for instance, the gibbons never feed on the hard or desiccated fruit of the ironwood and teak trees.

Behavior and Lifestyle
The groups’ home range varies from 19 to 155 acres (8–63 ha) in most areas. In Tripura and Arunchal Pradesh (northeastern India), however, some groups have a very large home range of 490–980 acres (200–400 ha). Unfortunately, due to deforestation, everywhere they live, more and more groups find themselves isolated in deteriorated forest patches, unable to cross over to a different part of the land and with fewer opportunities for food and mating.

Daily Life and Group Dynamics 
Hoolock gibbons live in small monogamous family groups of up to six individuals. They descend in the early morning hours from their sleeping sites, which are usually located in the tallest trees at high elevations, to the valley below where they like to forage. They forage in trees, hanging by one arm or sitting on a branch.

After feeding, they perform their songs for up to 20 minutes.

The gibbons travel an average of half a mile (about 1 km) per day, swinging from branch to branch very swiftly at speeds that can reach 12 mph (20 km/h), and earning them the nickname “acrobats of the canopy.” Although strictly arboreal, swinging is not their only mode of locomotion. They can walk upright on two legs with arms extended out like tightrope walkers. In fact, the only primates that walk on two legs more regularly than gibbons are humans.  

Fun Facts

The closest relatives to the gibbons are the great apes and humans.

American naturalist and zoologist Richard Harlan and Dr. Burrough, one of his contemporaries, first described the “hollack” or “hoolock” gibbon in a publication in 1834.

Charles McCann, who was the first naturalist to study hoolock gibbon behavior, reported seeing a male carrying an infant, but to date this is the only account of a male carrying an infant. At the time, McCann shot the male to bring him back home as a museum specimen.

Because so little was known about gibbons for so long, zoos held them in large groups instead of family groups until the 1960s.

Communication
Gibbons communicate with other groups through song. It is their way of attracting mates and of announcing to neighboring groups that the territory they occupy is off-limits. These songs can be heard several miles away.

The duets that an adult male and female perform probably announce to the world that a pair is together. These are composed of coordinated gender-specific sequences that are repeated once or several times. They always start with the female “great call” to which the male replies.

The females’ great calls consist of a series of notes produced at increasing speed. Males and females don’t share the same repertoire of notes. So, while females repeat the same great call once every few minutes, males utter short phrases, which are never identical to one another. These start with simple notes that increase in complexity as the song progresses.

Scientists see no evidence that the gibbons “learn” their songs or calls from other gibbons. Interestingly, in captivity, it is not unusual for these primates to engage in synchronous call bouts with neighboring pairs of a different species.

Reproduction and Family
Gibbons are monogamous and form strong family bonds. Mated pairs stay together for several years. Families are usually composed of one adult male and one adult female and their offspring. Both males and females become sexually active at adulthood (around 8 years of age), at which time they leave their family to start their own.

Females can reproduce every three years in the wild and are pregnant for 210 days, about 7 months. They give birth to a single offspring. Infants are carried by the mother.

Juveniles are weaned at about two years of age, when their size is half that of an adult and a thick white ring around their face starts appearing. They also start developing their singing voice at that age. They travel independently but stay in close proximity to their mothers and play a lot with their siblings.

Adolescents (4–6 years old) start traveling longer distances away from their mothers and males occasionally perform a solo morning song while females start singing with their moms.

Sub-adults (6–8 years old) are three quarters the size of adults and develop a thick layer of white fur around their faces. Their voice is strong but is a slightly higher pitch than the adults’ voice. At that stage in their lives, adult males regularly perform morning solos.

Upon reaching adulthood, males frequently perform solos, singing in the morning, and females perform duets with adult males.

Ecological Role
They are important seed dispersers of large and small fruit trees.

Conservation Status and Threats
The International Union for Conservation of Nature lists the western hoolock gibbon as Endangered, appearing on the IUCN’s Red List of Threatened Species (IUCN, 2017).

The hoolock gibbons’ original entire habitat covered about 100,000 square miles (160,000 sq km) and shrunk to 35,000 square miles (56,000 sq km) in 1987. The species was widespread in all the forested areas of northeastern India until the 1980s. Now it is mostly concentrated in the Namdapha National Park in the Changlang District of Arunachal. As a matter of fact, to date, the species has become extinct in eight locations in Bangladesh and ten in India; including the Chunati Wildlife sanctuary between Bangladesh and Myanmar.

There are currently 100 locations in India where the species has been recorded. In Bangladesh, scientists estimate that there are about 300 individuals spread out in 82 groups at 37 sites, 12 of which are in the northeastern part of the country.

With only 30% of its land still covered in forests, Myanmar is one of the most biologically diverse countries in Southeast Asia and probably hosts the largest remaining population of hoolock gibbons. Unfortunately, the political situation has prevented any kind of surveys and conservation efforts in most of the country for many years, with the exception of the southern Rakhine Yoma area, where a small population of these apes was recorded. The overall population was estimated at 100,000 individuals four decades ago; it is now thought to be less than 5,000 and is expected to continue declining in the coming years, mostly because of habitat loss.

Forest clearance for logging, tea, or coffee plantations and slash-and-burn agriculture are the main cause of habitat fragmentation. The natural habitat of these animals is deep forests, but because of deforestation, they are forced to the edges in close proximity to human populations, where they are hunted for meat. Often adults are killed so their babies can be captured for the illegal pet trade. Their bones and meat are also sold as a “tonic” in some traditional Asian medicines.

Conservation Efforts
The western hoolock gibbon is listed on the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) Appendix I and on schedule I of the Indian Wildlife Protection Act of 1972. It was designated as one of the top ten threatened gibbon species in a resolution resulting from the 2002 Congress of the International Primatology Society in Beijing. That same year a Population and Habitat Viability Assessment workshop was held in Bangladesh. One of the recommendations was to translocate isolated western gibbons living in degraded areas to parts of the forest where they could survive. The translocation plan is not only meant to save the lives of isolated individuals, but also to strengthen the gene pool for future generations.

Training workshops have been offered to foresters and the general public in India and Bangladesh by governmental and non-governmental organizations since 2009 to facilitate the implementation of translocation plans and to bring awareness to the plight of these primates to the locals. However, many primatologists think that translocation should be a last resort solution. Priority should be given to protecting the forest, restoring it, preventing further deterioration, and building corridors to connect the canopy of forest patches so the apes can move independently to areas where food is most abundant and they can avoid going through clearings between forest patches where they come in contact with humans.

​References:

  • ​IUCN Red List of Threatened Species​
  • Gibbon Journal Nr. 5 – 2009 – Hoolock gibbon and biodiversity survey and training in southern Rakhine Yoma, Myanmar – Thomas Geissmann, Mark Grindley, Frank Momberg, Ngwe Lwin and Saw Moses.
  • Kalia, The Lost Gibbon – documentary film by Nikhil Virdi and Nitye Sood
  • Primates – DOI 10.1007/s10329-013-0352-8 – Habitat characterization of western hoolock gibbons Hoolock hoolock by examining home range microhabitat use – Alice A. Akers, Md. Anwarul Islam, Vincent Nijman
  • Wester Hoolock Gibbon – Hoolock hoolock (Harlan 1831) – Bangladhes, India, Myanmar (2009) – Sally Walker, Sanjay Molur, Warren Y. Brockelman, Jayantha Das, Anwarul Islam, Thomas Geissmann and Fan Peng-Fei.
  • Current Science, vol. 107, no. 10, 25 November 2014 – Feeding on non-plant food items by Wester hoolock gibbon (Hoolock hoolock) – 
  • Journal of Research in Biology – ISSN: print 2231-6280 – A Review on the distribution of Western Hoolock Gibbon (Hoolock hillock) in Northeast India.
  • Duet-splitting and the evolution of gibbon songs – Thomas Geissmann
  • ​The Gibbons Conservation Alliance – Gibbons – The Singing Apes exhibition catalog – Thomas Geissmann (2014)

Written by Sylvie Abrams, July 2019