BORNEAN GIBBON

Hylobates muelleri

Geographic Distribution and Habitat
The Bornean gibbon, also known as the grey gibbon or Müller’s gibbon, is endemic to the island of Borneo, which is split between the nations of Indonesia, Malaysia, and Brunei. They occur throughout southeast Kalimantan, the Indonesian portion of the island, compromising 73% of the island’s area. The southwest region of the island is home to the closely related Bornean white-bearded gibbon.

They live in the lush tropical evergreen and semi-evergreen rainforests for which the island is famous. The Bornean gibbon shares its genus, Hylobates, with six other species, and other gibbon species are split into three more genera. Molecular data suggests that these four genera are as distinct from one another as a human is to a chimpanzee. Hylobates is the most widely distributed of the four gibbon genera, with species ranging throughout the whole of southeast Asia.

Bornean gibbon range, IUCN 2020

Size, Weight, and Lifespan
​The total body length of a Bornean gibbon is 17 to 25 inches (44–64 cm), and they weigh between 9 and 18 lbs (4–8 kg). They live up to 25 years in the wild and one individual lived to be about 60 years old in captivity.

Appearance
Bornean gibbons have a light brown coat that transitions to black on the chest, face, and insides of the arms, with a white frame around their face. They have buttock pads and pronounced canine teeth, and the base of their thumb begins at their wrist instead of their palm, allowing for an extended range of movement, although it limits their ability to grasp small items.

There is no pronounced sexual dimorphism in Bornean gibbons. As apes, Bornean gibbons do not have tails. One of their most striking features is their extremely long arms, particularly the extended ulna and radius bones of their forearms. This feature, along with a plethora of other small skeletal tweaks, adapt the species to their method of locomotion: brachiation.

Photo credit: Greg Hume/Creative Commons

​Diet
Bornean gibbons are largely frugivorous, being particularly fond of figs. A smaller portion of their diet consists of leaves.

Behavior and Lifestyle
Gibbons have the distinction of being the fastest non-flying arboreal mammals in the world, reaching speeds of up to 35 mph (56 km/h). Bornean gibbons’ main method of locomotion is brachiation. Their long arms and wide grip allow for extremely graceful and fast movement through the trees. They can cover 49 feet (15 m) in a single swing. Gibbons are so adapted to brachiation that when speeding through the trees, they tuck their legs close to their body so they stay out of the way. On the rare occasions that these tree-dwellers descend to the ground, they hold their hands above their heads for balance, and to keep their hands from dragging on the ground, as they walk bipedally.

What Does It Mean?

Arboreal:
Physically adapted to living primarily or exclusively in trees.

Bipedally: 
A biped is an animal that walk on two legs. Bipedally refers to animals moving on two legs.

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.

Deforestation:
The permanent cutting, clearing, and removal of trees to convert forest land for other use, such as pasture, cropland, or plantations.

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

Frugivore:
An animal who feeds on fruits.

Monogamous:
Having only one sexual partner. 

Sexual dimorphism:
Distinct difference in size or appearance between the sexes of an animal in addition to difference between the reproductive organs themselves.

Taxonomy: 
A branch of science that encompasses the description, identification, nomenclature, and classification of organisms.​

Visit the Glossary for more definitions

Daily Life and Group Dynamics 
Bornean gibbons usually live in groups of about three to four, generally a mated pair and their offspring. It is not unusual to see solitary individuals who have left their family groups and not yet established their own territory. Gibbons are active for about eight to ten hours per day. They are diurnal, rising shortly before dawn and going to sleep for the night before sunset. They spend most of their day foraging for food in the forest canopy.

Bornean gibbons spend relatively little time engaging in social interaction—only about 5% of their day is devoted to grooming and social play, relatively low for a primate species. This may simply be due to the small group sizes typical of the species. Behaviors of males and females are similar, though in one study, males were found to groom females and play with young more often than adult females. It is unclear if this is true of the species as a whole.

Bornean gibbons are fiercely territorial. Their home range is 84 acres (34 ha) on average, and about 75% of that area is actively defended. The apes regularly call at and chase away intruders and vocalize in the morning to defend their territory. Physical violence is rarely needed to defend territory.

Fun Facts

Although the songs of each Bornean gibbon may all sound the same to a human, they are actually highly individualized. Computer analysis can identify individual females based on their songs with a 95.7% accuracy!​

Communication
​Gibbon singing has been studied more extensively than other species characteristics, because it is an important trait in determining their taxonomy. In groups, Bornean gibbons are very vocal, with males singing loudly every morning before sunrise, and females joining them between sunrise and about 10 AM. Duets are dominated by the female, and her song, called her “great call,” involves a series of loud whooping followed by a rapid series of notes, reminiscent of the sound of a ping pong ball being dropped on a tabletop. Their daily duets last about 15 minutes on average, and these songs are very iconic of Bornean rainforests. Unpaired males sing more loudly than paired males, likely to help them attract a mate. Unpaired females rarely sing at all. Other forms of communication include grooming, social play, gestures, facial expressions, and body postures to communicate.

Reproduction and Family
Unlike the vast majority of mammals, and even most primates, Bornean gibbons are monogamous and mate for life. Males initiate mating more frequently than females, who may either accept copulation by bending forward, or reject by ignoring the male. Gestation lasts 207 days on average, about seven months, and usually only one offspring is born at a time. Gibbons usually produce offspring every two to three years, and nursing may last up to two years. Gibbons usually stay with their parents until they have reached sexual maturity (eight to nine years), and often help to raise their siblings. Around the age of six or seven, a young gibbon begins to detach from the family unit by maintaining a distance or sleeping separately from the group. If it is slow to strike out on its own, the parent of the same sex may give them a “hint” by displaying aggression until the offspring leaves to find a mate and establish territory.

Ecological Role
​There are no reported predators of Bornean gibbons, likely because, as extremely agile tree-dwellers, they are difficult to catch. If they are preyed upon, it is likely by avian predators and/or arboreal snakes. As frugivores, they play an important role as seed dispersers.

Conservation Status and Threats
The Bornean gibbon is classified as Endangered by the International Union for Conservation of Nature (IUCN, 2015), appearing on the IUCN’s Red List of Threatened Species. This listing is based on the species’ loss in population of over 50% during the last 45 years, or three generations.

Borneo’s forests are estimated to be 140 million years old, and it has taken a mere 40 years to destroy nearly half of the island’s natural vegetation. One cause of habitat loss is the expansion of palm oil plantations throughout the island. Palm oil is the cheapest and most important tropical vegetable oil used in the global oils and fats industry. Significant swaths of prime habitat must be deforested to make room to cultivate this high-demand crop, impacting Bornean gibbons and the many other species that depend on the habitat for survival.

Another cause of habitat loss is forest fires. While some biomes experience forest fires as part of their natural, undisturbed processes, the rainforests of Borneo are not one of them. Forest fires are increasing in frequency and severity on Borneo as humans open up and dry out the usually lush, naturally fire-resistant forests. These fires impact human and animal health, destroy critical habitat, and weaken the ability of the forest to regrow. Even in areas not ravaged by habitat loss, Bornean gibbons are under threat of hunting, sometimes for supposed medicinal qualities in their long arm bones, or collected for the illegal pet trade.

Conservation Efforts
Like the six other species of its genus (Hylobates), Bornean gibbons are listed on Appendix I of the Convention on International Trade in Endangered Species (CITES) program, a listing reserved for species in immediate threat of extinction and for whom trade is strictly controlled.

While there have been movements towards making palm oil more sustainable, a 2019 Purdue University study casts doubt that palm oil certification programs, namely the Roundtable on Sustainable Palm Oil and Palm Oil Innovation Group, actually help to reduce deforestation for palm oil plantations. If palm oil can indeed be cultivated without further harm to endangered species, more research and development is needed to achieve this.

​References:

  • ​Clink, D. J., H. Bernard, M. C Crofoot, A. J. Marshall. 2017. Investigating Individual Vocal Signatures and Small-Scale Patterns of Geographic Variation in Female Bornean Gibbon (Hylobates muelleri) Great Calls. International Journal of Primatology 38:656-71.
  • Freeman, D. 1979. The Great Apes. New York, NY: G. P. Putnam’s Sons.
  • Gatti, R. C., J. Liang, A. Velichevskaya, M. Zhou. 2019. Sustainable palm oil may not be so sustainable. Science of the Total Environment 65:248-51.
  • Haimoff, E. H. 1985. The organization of song in Müller’s Gibbon (Hylobates muelleri). International Journal of Primatology 6(2):173-92.
  • Kavanaugh, M. 1983. A Complete Guide to Monkeys, Apes, and Other Primates. New York, NY: Viking Press.
  • https://forestsnews.cifor.org/61158/zoom-in-on-borneo-ecology?fnl=en
  • https://www.iucnredlist.org/species/39888/10270564
  • https://animaldiversity.org/accounts/Hylobates_muelleri/
  • https://www.ecologyasia.com/verts/mammals/bornean-gibbon.htm
  • http://apesportal.eva.mpg.de/status/species/gibbon/hylobates
  • https://wildambience.com/wildlife-sounds/muellers-bornean-gibbon/
  • https://wwf.panda.org/knowledge_hub/where_we_work/borneo_forests/borneo_deforestation/
  • http://genomics.senescence.info/species/entry.php?species=Hylobates_muelleri
  • https://a-z-animals.com/animals/gibbon/
  • https://www.britannica.com/animal/gibbon-primate
  • https://www.nature.com/scitable/knowledge/library/primate-locomotion-105284696/

Written by K. Clare Quinlan, January 2020