Hylobates klossii

Geographic Distribution and Habitat

Kloss’s gibbon, also known as the Mentawai gibbon, the dwarf siamang or the bilou, is native to the four Mentawai Islands of Siberut, Sipora, North and South Pagai. These islands are in Southeast Asia, off the west coast of Sumatra, Indonesia. Annual rainfall on these islands can reach 157 inches (4 m). 

These are special islands, whose separation from Sumatra since the last Ice Age have allowed for the distinct evolution of plants and animals found nowhere else in the world. The Kloss’s gibbon is one of six unique primate species of these islands, where the locals call this gibbon species the “bilou.” The upper canopy of tropical evergreen and swampy monsoon forests are where the bilou are at home.

Kloss's gibbon range, IUCN 2020

Size, Weight, and Lifespan

Females are slightly larger than males, with adult females weighing an average of 13 lbs (6 kg) and adult males weighing an average of 12 lbs (5.6 kg). The body length of an adult is between 17 and 25 in (44–63 cm). They live up to 25 years in the wild.


Like all gibbons, Kloss’s gibbons have no tails and very long arms. Their arms are longer than their legs, and they use their arms as their primary mode of transportationswinging from tree to tree (called brachiation). Their hair is all black. They have black, deep-set eyes and their mouth has a slight downturn, giving them a serious and thoughtful expression.

Photo credit: Klaus Rudloff/Creative Commons

The Mentawai island forests are filled with fruit like ratawan, lanzones, and durian. With such fruit, perhaps it is no surprise that Kloss’s gibbons are primarily frugivores. They also eat leaves and flowers. 

But these gibbons are not full herbivores. They supplement their plant-based diet with some eggs, insects, and small vertebrates.

Behavior and Lifestyle

Kloss’s gibbons are diurnal and are active up to 10 hours a day. They start their day singing, first the males and then the females.

They are territorial—meaning they have a defined space they consider to be the home of their group, which they defend. They spend much of their days moving about in that space—up to 5,000 ft (about 1,500 m) a day.

They are arboreal (tree-dwelling) and prefer to travel by swinging from tree to tree. These trees can be up to 33 ft (10 m) apart! Witnesses report that it looks like they are “flying.” But even though they prefer swinging, they can move short distances on the ground. When they do travel on the ground, they move bipedally—which means they stand upright on their two rear legs while walking, running, and hopping.

When the day is done and it is time to sleep, they may return to the same tree for rest, even though they don’t build nests (a good sleeping tree is a good sleeping tree!) And they don’t lie down to sleep, but they sleep sitting up. They have buttock pads that make this way of sleeping comfortably.

Before the break of dawn, the songs of the bilou let the forest know it is time for a new day.

Fun Facts

What’s in a name? The Kloss’s gibbon is named for English ethnologist and zoologist, Cecil Boden Kloss (1877–1949), who also has several reptiles and a mole that bear his name.

The gibbon family name, Hylobatidae, is Greek for “forest walker.”

Daily Life and Group Dynamics 

The family unit of a Kloss’s gibbon is also the social unit. Groups consist of a monogamous pair of parents and their children, between 2 and 6 members.

Groups have their own territory and will defend it against other family groups of gibbons.

The father and adolescent and sub-adult sons protect their home ranges, which vary from 50 to 86 acres (20 to 35 hectares) of which they consider 25 acres (10 hectares) theirs to defend against other groups of their own and other species.

Children will leave their family group when they reach late adolescence (when they are 6 or 7 years old) and form their own new group with a partner.

They use strategy and self-sacrifice to protect their group. When encountering humans, they have been seen to use “decoy behavior,” where one puts their life at risk by trying to attract human attention to it, while the others in their group escape.


Male Kloss’s gibbons begin their songs in a chorus before daybreak. Their songs are complex and unique, but have in common a series of rising and falling notes in phrases, accentuated by trills. The males may “solo”  from 10 minutes to a full concert length of 2 hours! Post-dawn, the females begin to sing in a female chorus. The phrases of the female’s songs are longer, with an accelerating trill to a long crescendo, and a concluding phrase that in music is called a “coda.”

While males and females don’t generally duet, mating pairs can be an exception, and may duet a few hours after dawn.

Each of these gibbons has a one-of-a-kind distinctive voice and way of phrasing their song that allows others to know them at great distances. Their choruses, with their break-out solos, may be the way in which individual groups let each other know just how powerfully they will defend their territory against intruders.

The stirring music of the Kloss’s gibbon is much admired by their human relatives. Researchers note that the songs of Kloss’s gibbons are considered the most complex and beautiful of any gibbons. Researchers also posit that the differences between the songs of Kloss’s gibbons and other gibbons may be due to adaptations to avoid the most dangerous ape: humans. Humans, for the last 2,000 years, have been their most successful predator on the Mentawai islands. Some researchers theorize this is why singing does not occur throughout the day, or in male-female duets. Perhaps same-sex choruses reduce individual group risk. Males sing longer than females do, but while it is still relatively dark. Females sing for less time, just at the break of day. Males are present during the female chorus, but do not duet, perhaps to keep watch and protect their mate?

In addition to vocalizations, Kloss’s gibbons also communicate through smell, facial and body gestures, play, and grooming.

Reproduction and Family

Family is everything to Kloss’s gibbon. Mating for life, a bonded pair will become the nucleus of their own group. They mate throughout the year. Babies gestate for 7 to 8 months. One baby is born at a time. Intervals between births is 2 to 3 years. A baby may nurse for as long as two years, and both parents participate fully in their care. 

Social structure within the family is communicated through play, grooming, and vocal communication. 

Both male and female adolescents reach sexual maturity at 6 to 7 years, at which point they will become ready to find their own life mate. 

Parents have been known to assist with their adolescent children finding their own space and helping to protect it. It’s good for family to be close by!

Photo credit: © markindo/INaturalist/Creative Commons
​Ecological Role

​Kloss’s gibbons, like all the native primates of this region, are the essential seed dispensers for replenishing the trees. The number of bilou in the morning chorus, or the relative quiet, reveal the health of the Mentawai forests.

Conservation Status and Threats

The Kloss’s gibbon is classified as Endangered by the International Union for Conservation of Nature (IUCN, 2015), appearing on the IUCN Red List of Threatened Species. 

Commercial logging, palm oil production, deforestation due to other agribusiness, and hunting are all huge threats to the continued survival of this species. In addition, babies are stolen from the wild (meaning the mother and father must be killed to take their baby) for humans to keep as pets.

The most significant threat to Kloss’s gibbon–be it habitat loss due to agribusiness, hunting or the pet trade–varies from island to island, but on all islands they are endangered with their populations decreasing.

Recent census estimates reported to the IUCN that the entire Kloss’s gibbon population has declined over 50% since the 1980s.

A short distribution survey of Kloss’s gibbons conducted in 2017 reported “massive habitat changes” in South Siburt with the disappearances of forests, and in Sipora and South Pagai, none were seen or heard.

Conservation Efforts

Kloss’s gibbons are listed in Appendix 1 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. 

They are protected by Indonesian law, and as was reported in 2006, more than half of the remaining Kloss’s gibbons live in the Siberut National Park, a government protected area. The protections in this area, however, lack enforcement.

There is a substantial list of specific recommended actions to help Kloss’s gibbon, from creating greater protected spaces to increasing education. 

It is important to also note that the threats to the unique native animals of the Mentawai islands are not in isolation. There is also the existential threat to practices of the native people, who underwent forced conversions in the 1950s and the traditional animist religion of the Mentawai people was deemed illegal. In the traditional Mentawai religion, people have a sacred relationship to the bilou. After an easing of the law in the 1990s, there are now about 2,000 people that still practice their ancestral ways and live in the forest.

Each of us, no matter where we are in the world, can make a difference in the choices we make that can directly or indirectly affect saving species.

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Written by Laura Lee Bahr, November 2021