Hylobates funereus

Geographic Distribution and Habitat

The northern gray gibbon, also known as the eastern gray gibbon or the North Bornean gray gibbon, is endemic to Borneo—the third largest island in the world and shared by three countries: Malaysia, Indonesia, and Brunei. This species lives in select areas of Sabah and Sarawak in Malaysia, as well as in Kalimantan in Indonesia. Their current distribution is fragmented and restricted to certain pockets of suitable habitat within these regions.

As arboreal primates, northern gray gibbons spend the majority of their time in the forest canopy, rarely descending to the ground. They prefer mature primary and secondary forests, including Borneo’s tropical evergreen dipterocarp forests, which provide them with safety and abundant fruit. They can also be found in the Sundaland heath forests (also known as Kerangas forest), which is a unique habitat type in Borneo.

Although they are most commonly associated with lowland and montane forests, it’s interesting to note that northern gray gibbons have been observed living at elevations of up to 5,700 feet (1,700 meters) above sea level. This suggests some adaptability in their habitat preferences.


The northern gray gibbon was previously considered a subspecies of Müller’s gibbon (Hylobates muelleri) until 2013 when it was reclassified as a distinct species based on genetic and morphological studies.

Size, Weight, and Lifespan

Gibbon species exhibit varying degrees of sexual dimorphism, where one sex is typically larger, usually the male.

Northern gray gibbons show less pronounced sexual dimorphism compared to other gibbon species, but conflicting sources make data on their size and weight unclear. Based on all available data, a reasonable estimate for size for both males and females might fall between 1.4 feet (42.6 cm) and 2.0 feet (60.9 cm). But future research may confirm different statistics.

Data for Müller’s gibbons presents similar issues, making it difficult to use as a reference. However, one source from 1998 found that the average male Müller’s gibbon weighed 12.2 pounds (5570 g) and the average female weighed 11.5 pounds (5250 g). Northern gray gibbons are likely of similar weight.

The average lifespan of most gibbon species in the wild is considered to be around 30 years.


Gibbons are often mistaken for monkeys due to their compact bodies and long arms, but they are actually apes. Unlike other apes—such as bonobos, chimpanzees, gorillas, and orangutans—gibbons are smaller and are often referred to as “small” apes, while the former four are known as “great” apes. The term “lesser ape” has fallen out of use as it implies that gibbons are less intelligent or significant than their great ape counterparts.

There are currently 20 known gibbon species, all sharing a similar general shape and appearance, with compact torsos, long arms, hook-shaped hands, long legs, and grasping feet. They are remarkable tree-dwelling acrobats, displaying astonishing energy, precision, dexterity, and grace while navigating the canopy through leaping, bounding, and brachiating.

Northern gray gibbons are named for their overall gray or gray-brown fur, dominant on the back of their heads and backs. The outer parts of their limbs, elbows, and fingertips may be the same color or slightly paler, while the inner parts of their limbs and chest are black or dark brown. They have a prominent white mark across their brow, wider at the center, extending from the top of their heads down to their faces. Exact patterns may vary by individual.

In regions where their ranges overlap with Müller’s gibbons in the southern areas, northern gray gibbons tend to have black toes and fingers, though the reason for this is not clear at present.

Photo credit: Burungborneo/Creative Commons

Northern gray gibbons have a varied diet that is mostly vegetarian. While they prefer fruits, they also consume leaves, flowers, seeds, and nectar. Insects serve as a source of protein when other food sources are scarce.

Many gibbon species’ diets change with the seasons. During the wetter seasons when fruits are ripe and abundant, they may travel long distances to obtain them. In dryer periods when fruits are scarce, they rely on less preferred leaves for sustenance. Although these leaves provide fewer nutrients and are less calorie-dense, and may be harder on their digestive systems, their abundance in the rainforest ensures they are readily available and easily obtained.

Behavior and Lifestyle

Gibbons are known for their expansive home ranges and frequent travel to find fresh foraging areas, especially where fruit has just ripened. They use a style of locomotion called brachiation to travel through the canopy. Brachiation involves hanging from a branch by one arm and swinging the body forward to grasp the next branch, continuing in an alternating pattern until they reach their destination.

Gibbons are considered the virtuosos of brachiation among apes. They navigate the canopy with style and grace, reaching impressive speeds of up to 35 miles per hour. They also improvise their movements, leaping great distances and even walking bipedally along branches to reach the next tree. This unique form of locomotion means that gibbons walk upright more than any other wild ape.

Gibbons have evolved physiological adaptations that make them well-suited for brachiation. Their wrists have ball and socket joints that allow for specialized, biaxial movement and reduce energy expenditure in the upper arm and torso, while also minimizing stress in their shoulders. Their shoulder joints can rotate a full 360 degrees, increasing their range of motion and dexterity. Long arms and hook-shaped hands help them bridge distances between branches with ease and maintain a firm grip. Short thumbs keep their hands useful for grooming without hindering brachiation. Additionally, gibbons have well-developed cerebellums, a section of the brain that helps coordinate movements and regulate muscle activity, giving them exceptional balance, poise, and spatial perception.

Young gibbons do not start as natural acrobats and require several years of development before they can fully navigate the canopy on their own. Playtime is an important opportunity for them to practice their skills, although it comes with risks as it often happens high off the ground. Gibbon parents, both males and females, pay close attention to their young to ensure their safety.

Gibbons are social animals but tend to socialize mainly with their immediate family members, as they are not gregarious (that is, they do not socialize within other gibbon groups). They consider outsiders as intruders and chase them away from their territory. Grooming is an important activity that reinforces strong familial bonds among gibbons.

Gibbons are diurnal and known for being early risers. They begin their morning routines, including singing to each other, shortly after sunrise. Their melodic and haunting duets can be heard over half a mile (up to a kilometer) away. These duets help gibbons locate each other after spending the night sleeping apart and also serve as a warning to nearby gibbons not to come too close.

Daily Life and Group Dynamics

Gibbons live in small nuclear families, typically consisting of a mother, a father, and up to two offspring. However, they may also live alone for a portion of their lives before starting their own family or if their mate dies.

A family of northern gray gibbons starts their day early, just as the sun rises and illuminates the branches of their canopy home. During the night, the mother and her offspring slept together, while the father slept in a different tree. To reunite, they communicate through singing. The mated pair takes turns crooning their parts of a duet, which serves as a way of bonding and also as a warning to other gibbons in the area to keep their distance.

Once the family is together again, they search for breakfast, traveling as a unit until they find food to eat. Afterward, the family takes some time to relax. While the mother rests, the father keeps a watchful eye for any potential danger, occasionally taking breaks to play with the young offspring.

Both the mother and father take on the responsibilities of caring for their young, and family members develop close bonds with one another as they feed, play, groom, and sleep together throughout the day. By early evening, the family begins to wind down. They find suitable trees to sleep in, with the mother and offspring huddling together for the night, and the father finding his own sleeping spot. While the specific bedtime of northern gray gibbons has not been recorded, some gibbon species are known to settle down quite early, ranging from 1.5 to 4 hours before sunset.


Like other social primates, northern gray gibbons communicate using facial gestures and body language to establish and reinforce their bonds. They are also well known for their loud and complex vocalizations, often referred to as “songs,” which serve various functions such as bonding between mated pairs and warning other gibbons to stay away.

Mated pairs of northern gray gibbons engage in a call-and-response pattern of singing in the morning to locate each other. The male and female take turns producing lilting and squeaky notes that fill the jungle around them.

The vocal repertoire of gibbons is distinctive to their species and sex, and researchers have even found evidence of regional dialects among them. In some species, young gibbons develop their singing skills by copying only their mother, which is thought to be a way for immature males to communicate their status to their father. However, this does not appear to be the case for northern gray gibbons.

Northern gray gibbons also make alarm calls to warn their families about nearby or approaching predators. Loud hoots are used as signals for others to react, and parents may follow up with a higher pitch and faster duet. Young gibbons typically join in the hooting but not in the duet.

Despite their fascinating communication methods, the full complexity and function of northern gray gibbons’ vocalizations and gestures are not well studied and require further research to better understand.

Reproduction and Family

Like all gibbons, northern gray gibbons establish monogamous male-female relationships that form the foundation for their social structure, the nuclear family. While these relationships generally last for their lifetime, occasional separation may occur, and there is evidence that pairs may not mate exclusively with each other, indicating that gibbons may be socially monogamous but not necessarily sexually monogamous. Despite the complexities of their relationships, the family unit is crucial in a young gibbon’s life, with both parents investing time and energy in the survival and success of their offspring, regardless of biological relatedness. Observations of male gibbons taking on a fatherly role towards young gibbons whose biological fathers have passed away have also been documented in the wild.

Research on the mating rituals of northern gray gibbons is currently limited, but they appear to mate year-round. The gestation period for females is approximately 210 days, and once born, the newborn is tiny and heavily reliant on the mother for warmth and protection. Nursing continues for up to two years before weaning, and over the next two years, the young gibbon gradually learns the knowledge and skills needed for adulthood.

Between six and eight years of age, the young gibbon enters the subadult phase, functioning more independently but not yet ready to leave and start its own family. Meanwhile, the mother gives birth to another baby sibling, and the young gibbon observes and learns from its parents’ care. By around eight years old, the gibbon reaches sexual maturity, and both male and female gibbons typically leave their families to establish their own territories.

Gibbons have one of the longest development cycles among primates, surpassed only by orangutans and humans. Due to the significant time and effort invested by parents in raising their offspring, gibbons usually raise only one young at a time to avoid becoming overworked.

Photo credit: ©lappuggla/ iNaturalist/Creative Commons
Ecological Role

Gibbons, including the northern gray gibbons, play vital roles as seed dispersers in ecosystems. When they consume fruits, the seeds remain intact as they pass through their digestive systems. Subsequently, when gibbons defecate, these seeds are dropped on the ground, often miles away from where they were originally consumed. This process contributes to the constant regeneration of forests and helps to preserve the biodiversity of these ecosystems. In regions like Borneo, which is renowned for its exceptional biological diversity, this seed dispersal process is of utmost importance for maintaining the health of fragile ecosystems.

Conservation Status and Threats

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

All gibbon species are threatened by deforestation caused by human activities such as logging, land development, and conversion of their habitats to farmland. Habitat loss makes it harder for gibbons to find food and severely damages their ability to thrive in the wild.

As their forests are cut down and degraded, the remaining patches become more fragmented. For arboreal species like gibbons, even a one-lane road can disrupt their navigation through the canopy. Gibbons may choose to descend to the forest floor to cross a gap, risking contact with harmful ground bacteria or getting hit by traffic.

Gaps in their forests widen over time, and gibbon populations become confined to fragments, pushing family groups closer together and increasing competition for space and resources. This leads to more territorial disputes and physical conflicts, resulting in some gibbons perishing.

Even the surviving gibbons are not safe. Fragmentation limits gibbon-to-gibbon contact, slowing gene flow and forming genetic bottlenecks. Lack of genetic diversity makes gibbons vulnerable to diseases, parasites, and birth defects that can be passed on to future generations, decreasing their chances of survival and reproduction.

Gibbons are also hunted and killed by humans for meat and traditional medicines. Young gibbons are captured for the pet trade, resulting in the killing of parents who try to protect their young. Keeping gibbons as pets is detrimental to their health as they require ample space and social interactions with their own species, which is impossible to replicate in captivity. They also require a specific diet that can only be found in their natural habitat.

Most gibbons rescued from the pet trade are found in malnourished and unhealthy states, often on the verge of death. Captivity leads to unhealthy behaviors such as self-harm and repetitive movements, which are difficult to unlearn. Rehabilitating gibbons is a challenging and time-consuming process with no guarantee of successful return to the wild.

In Borneo, one of the most biologically diverse places in the world, these threats are particularly pervasive. Between 1973 and 2017, Borneo lost an estimated average of 1% of its original forest cover each year due to logging, farming, mining, dam building, and other human activities. This trend is expected to continue in the future. Borneo’s ecosystems are irreplaceable, and once they are gone, so are the countless endemic species that rely on them, including northern gray gibbons.

Climate change exacerbates the destruction of Borneo’s habitats. Borneo’s fragile ecosystems make it vulnerable to changes in temperature and rainfall, resulting in more frequent and intense El Niño events, causing storms and heatwaves. Forest fires, once a natural part of Borneo’s cycles, are now a concern due to the loss of precious remaining forests. In a place like Borneo, these threats compound each other, making it difficult to reverse the downward spiral.

Conservation Efforts

The northern gray 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. Malaysia, Brunei, and Indonesia are all signatories.

Northern gray gibbons are currently found in several protected areas, including Gunung Mulu National Park, Lambir Hills National Park, Kutai National Park, Kayan Mentarang National Park, Crocker Range National Park, Kinabalu National Park, and Ulu Temburong National Park, primarily in Malaysia. However, there are no protected areas for northern gray gibbons in Brunei.

In addition to these protected areas, northern gray gibbons are also found in Malaysia’s Danum Valley Conservation Area, which is home to the Danum Field Center. Established in 1986 to support scientific research in one of the world’s oldest rainforests, the Danum Field Center is a well-equipped and highly respected field site and is considered one of the best in Southeast Asia.

In January 2023, the Gibbon Conservation Society, based in Malaysia, founded a project in Sabah focused on rehabilitating northern gray gibbons, along with their close relatives, Abbot’s gray gibbons. This is the first gibbon rehabilitation center in the state and currently provides care for five northern gray gibbons, most of whom were victims of the illegal pet trade. While assessments are ongoing, the project managers are hopeful that all five gibbons will eventually be able to return to the wild.

As the world’s first IUCN-accredited gibbon rehabbers, the Gibbon Conservation Society is dedicated to gibbon conservation in general. In addition to rehabilitation efforts, they also work extensively to educate local and global communities about the importance of gibbon conservation, the threats faced by gibbons in the wild, the cruelty of keeping gibbons as pets, and what steps they can take to help.


Written by Zachary Lussier, April 2023