EASTERN HOOLOCK GIBBON
Geographic Distribution and Habitat
Eastern hoolock gibbons live in the forests between the Chindwin and Irrawady rivers that flow through the northern half of Myanmar.
Eastern hoolock gibbon groups keep extensive home ranges. As such, these apes make use of a number of forest habitats, including scrub, semi-deciduous and broadleaf forests. Primary evergreen forest, however, is where these fruit aficionados thrive best.
Earlier research speculated that a population of gibbons living in northeast India might have been eastern hoolocks based on their coat color. A recent genetic analysis, however, determined these to be western hoolock gibbons, already found in India. A similar situation in Yunnan, China, eventually led to the discovery of a brand new species of hoolock gibbon in 2017 dubbed, the Skywalker hoolock gibbon.
These findings have important implications for hoolock gibbon conservation. In brief, knowing the range and habitats of these three species allows for proper conservation planning. When conservationists understand where each species lives, they gain a better understanding of what they eat, as well as how they interact with their particular environments. In turn, this helps to determine the best and most crucial steps to take for their conservation in the wild. Furthermore, this information helps those working in gibbon rehabilitation centers know which individuals in their care are compatible as potential enclosure mates, as well as where to release them back into the wild if and when that time comes.
The boundary between eastern and western hoolock gibbons’ geographic distributions is currently unclear. Some scientists hypothesize the existence of an intermediate zone where these two species—estimated to have diverged 1.49 million years ago—mate with one another, producing hybrids. This has yet to be concluded, however.
Size, Weight, and Lifespan
Male and female eastern hoolock gibbons are similar in size. These apes are small, measuring an average of 21.5 inches (54.5 cm), head and torso. Their heads make up only a fifth of that measurement, at 4.3 inches (11.1 cm). Their petite size is one of the factors that afford them the ability to navigate the canopy with stunning grace, speed, and agility.
The weight of eastern hoolock gibbons has not been scientifically measured at this time. Their western hoolock cousins weigh between 13 and 19 pounds (6–9 kg).
Based on what we know about other species, eastern hoolock gibbons can likely live up to 35 years in the wild.
Walking on two legs.
Active during daylight hours.
The stalk that joins a leaf to a stem.
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With their compact bodies and long arms, gibbons are often mistaken for monkeys. But they are not. Gibbons are apes. While several physiological traits give them away, the most visible indicator is their lack of tails.
Compared to other apes like bonobos, chimpanzees, gorillas, and orangutans, gibbons are significantly smaller. Thus the former four are known as “great” apes, while the latter are “small” apes. In the past, the term “lesser ape” was used, but this misnomer has gone out of use since the comparative form of the word implies to some people that gibbons are not as intelligent or as significant as their great ape cousins.
Each of the 20 gibbon species currently known to science shares a general shape and appearance: compact torsos, long arms, hook-shaped hands, long legs, and grasping feet make them some of nature’s most astonishing tree-dwelling acrobats. A gibbon at rest on a branch may appear unassuming—perhaps even clownish to some in the way they lounge and stretch, or scrunch themselves into a tight ball. But a gibbon navigating the canopy—leaping, bounding, brachiating—moves with such energy, precision, dexterity, and grace as to seem an entirely different creature altogether.
The more than 20 species of gibbons share many characteristics and features but each species has its own unique traits that distinguish it from all the rest. Like all gibbons, eastern hoolocks display obvious sexual dichromatism: males sport a black fur coat while those of females are a brighter silvery gray color with darker patches on their chest and underarms. The buffy-colored fur that follows the contours around a female’s eyes and mouth gives her the appearance that she’s wearing a mask. Males have no such facemask. The fur on their faces is black except for two pronounced white patches on the brows above their eyes.
To the naked eye, eastern hoolock gibbons are virtually indistinguishable from their western hoolock cousins. One tiny visible difference between males, however, can be found in the eyebrow markings. The distinctive white eyebrows of both species are closer together on western hoolock males than those of the eastern hoolock males.
Eastern hoolock gibbons eat a variety of different foods including seeds, young and mature leaves, petioles, flower buds, and insects. But their most precious staple is fruit. Figs are at the top of their list.
Fruit is only abundant for about half the year, however. In the dry season, gibbons rely more on other sources of nutrition.
Eastern hoolock gibbons keep expansive home ranges and travel frequently to reach fresh areas in which to forage—hopefully where the fruit has just become ripe. They transport themselves through the canopy using a style of locomotion called brachiation. Hanging from a branch by one arm, they swing their body forward to grasp the next, continuing in this alternating pattern until they reach their destination.
While all apes brachiate, gibbons are the true virtuosos of this method. Not only do they navigate the canopy with style and grace, they can also reach speeds of up to 35 mph (56 kmh)! 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.
Gibbons are the virtuosos of brachiation because they have evolved a special set of physiological adaptations not found in other ape species. Firstly, their wrists are composed of ball and socket joints. These not only allow for specialized, biaxial, movement but significantly reduce the amount of energy required in the upper arm and torso and even minimizes stress in their shoulders. Additionally, the joints in their shoulders can rotate a full 360 degrees, greatly increasing their range of motion and overall dexterity. Long arms help them to bridge the distances between branches with ease while their hook-shaped hands ensure a firm and consistent grasp. Gibbons’ thumbs are shorter than other apes, keeping them useful for grooming without getting in the way during brachiation. Of course, all these traits wouldn’t be nearly as useful had gibbons not also evolved such well-developed cerebellums. This section of the brain helps to coordinate movements and regulate muscle activity, giving them exceptional balance, poise, and spatial perception.
A young gibbon is not a natural acrobat, however. Young gibbons must develop their bodies and skills for several years before they are ready to take on the canopy completely on their own. Playtime is the perfect chance to practice. Gibbon play is considered some of the riskiest in the world since it happens so high off the ground. One slip during a game of chase could easily spell the end of a young gibbon’s life. As a result, gibbon parents (both Mom and Dad) pay careful attention to their young.
Unless they are trying to find a mate, gibbons tend to socialize with their immediate family members only. All others are considered outsiders and are chased away whenever they enter another family’s territory. Grooming acts as an important method of reinforcing their strong familial bonds.
Scientists have recently confirmed that eastern hoolock gibbons are not found in China, nor in India, as some once speculated.
Eastern hoolock gibbons inhabit mountainous regions with elevations up to 8,860 feet (2,700 meters).
Gibbons are sometimes called “long-armed apes.”
So perfectly adapted are they to living in trees that finding gibbons on the ground is actually cause for great concern. Not only is this atypical behavior for them, but exposure to certain bacteria at this level of the forest can actually cause them to become fatally ill.
Singing patterns of gibbons has helped researchers to establish migration patterns of gibbons throughout their evolutionary history.
Gibbons live in small nuclear families consisting of a mother and father and up to two of their offspring—though they may fly solo for part of their lives prior to starting a family of their own or if their mate dies.
A family of eastern hoolock gibbons wakes early, just as the branches of their canopy home catch the rising sun. They immediately look for breakfast, traveling together until they find something palatable to chow down on. Their bellies full, Mom and Dad then engage in one of gibbons’ most distinctive behaviors: they sing! Their call-and-response style song fills the surrounding jungle.
Mated pairs perform these duets as a way of bonding but also to warn other gibbons in the area to keep their distance. Now the family can relax a bit knowing no stranger gibbon is likely to come by unannounced. Still, while Mom rests, Dad keeps an eye out for anything amiss, now and then letting down his guard to play with the kids.
Both Mom and Dad 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 has already started to wind down. They find a comfortable tree to sleep in where they huddle together for the night. While the bedtime of eastern hoolock gibbons has not been recorded, their western hoolock cousins are known to settle in quite early—1.5 to 4 hours before sunset!
There is still a great deal to discover about the daily lives and group dynamics of eastern hoolock gibbons, which will be crucial to their successful conservation in the wild.
Like other social primates, eastern hoolock gibbons use facial gestures and body language to communicate. Family members also groom each other as a way of establishing and reinforcing their bonds.
Most of all, however, gibbons are well known for their loud, far-reaching, and acoustically complex vocalizations typically referred to by researchers as “songs.” While the characteristics of such songs vary from species to species, their functions seem to be mostly universal: helping mated pairs to bond as well as warning outsider gibbons to keep away.
Mated pairs of eastern hoolock gibbons sing after eating their morning meal. The male and female take turns in a haphazard call-and-response pattern that fills the jungle around them with lilting and squeaky notes.
The repertoire of calls gibbons produce is distinctive to their species and sex. Researchers have even found evidence that gibbons exhibit regional dialects. In some species young gibbons, both male and female, develop their singing skills by copying their mother only. Researchers reason that this is a method for still-developing males to communicate their immature status to the father. As male offspring get closer to reaching sexual maturity, the male parent becomes increasingly aggressive towards them. By practicing his mother’s call only, an immature male practices important singing skills but also avoids upsetting his father by inadvertently triggering his territorial defenses.
The full complexity and function of eastern hoolock gibbons’ communication methods are not well studied but can only become clearer with more research.
Like all gibbons, eastern hoolock gibbons establish monogamous male-female relationships that form the foundation for gibbons’ chief social unit: the nuclear family. While these relationships generally last throughout their lives, they may occasionally end in separation. Additionally, there is evidence that pairs may not mate exclusively with one another even if they do stay together. So, while gibbons can be considered socially monogamous, it is no longer assumed that they are also sexually monogamous. However complicated their relationships may be, the family unit structures a young gibbon’s life as each parent invests their time and energy in their young’s survival and success.
Research into the mating rituals of eastern hoolock gibbons has not been conducted at this time, but their western hoolock cousins have been observed to mate in the mornings, before 10:30 am. The male initiates an intimate grooming session with his mate that ends in copulation.
A female eastern hoolock female becomes pregnant during the dry season. Her pregnancy can last from 180 to 225 days. She gives birth in the wet season when fruits are abundant, ensuring she has plenty of readily available calories to give her the energy required of motherhood.
Her newborn is tiny and relies on her completely. For the first several months, he clings to his mother’s fur for warmth and protection and nurses for up to two years. Over the next two years, he gradually begins to learn the knowledge and practice the skills he will need as an adult. At around six years of age, he is considered a subadult, functioning more or less on his own but still not ready to depart to start his own family. In the meantime, his mother has given birth to his baby sister or brother. He watches how his mother and father care for her or him, committing it all to memory for when he becomes a parent.
At eight years old, he has reached sexual maturity. At this stage in their development, both male and female gibbons leave their families to start their own. Before finding a partner, however, they may spend a short time on their own.
All gibbons play critical roles as seed dispersers. When eastern hoolock gibbons eat their beloved fruits the seeds pass through their digestive systems intact. They then drop to the ground in their feces miles away from where they were eaten. Through this process, the forest is constantly being regenerated, and the biodiversity of forests is preserved.
The eastern hoolock gibbon is classified as Vulnerable by the International Union for Conservation of Nature (IUCN, 2017), appearing on the IUCN Red List of Threatened Species.
All species of gibbons are threatened by deforestation caused by human activities such as logging for timber, land development for building, and conversion to farmland. By making fruit and other food sources harder to come by, habitat loss severely damages gibbons’ ability to thrive in the wild.
As their forests are cut down and degraded, the patches that remain become more and more separated. For a completely arboreal species, like gibbons, even the fragmentation caused by a one-lane road can upset navigation through the canopy. If they can’t find a way to span a gap in the trees, gibbons may choose to descend to the forest floor in order to cross, risking both contact with harmful ground bacteria that can make them sick.
As the gaps in their forests widen, gibbon populations become confined to these fragments, and family groups are pushed closer together. Competition for space and resources increases, ensuring more territorial disputes become physical. Under these conditions, some gibbons are bound to perish.
Those gibbons left are also doomed, however. As fragmentation limits gibbon-to-gibbon contact, gene flow slows to a halt, forming genetic bottlenecks. Lack of genetic diversity causes huge problems for primates. In only a few generations, once healthy gibbon populations may become vulnerable to diseases and parasites, or develop birth defects that are passed on to the next generations indefinitely. All of these changes decrease the chances that new generations live long enough to procreate at all.
Gibbons are regularly hunted and killed by humans for meat or to be used in traditional medicines. Humans also capture young gibbons to keep or sell as pets. Poachers are only interested in small and helpless infants. Since no gibbon parent is going to give up their young without a fight, they always have to kill the parents. Thus, many gibbons die in order to capture just one infant.
Becoming a pet is a harrowing experience for any primate. Gibbons especially need copious amounts of space and opportunities to freely associate with members of their own species in family-style arrangements that are impossible to replicate in captivity. Additionally, they require a diet that only their natural habitat can provide, and foods found in pet stores or supermarkets just won’t do.
If they don’t die of starvation or disease, most gibbons rescued from the pet trade are found in an extremely unhealthy state when confiscated. Thin, malnourished, and often ill, many are on the verge of death. Having been confined to a small cage for so long and deprived of the intellectual challenges they would naturally face in the wild, gibbons kept as pets develop unhealthy habits such as biting and scratching themselves or rocking back and forth repetitively. Once learned, these kinds of behaviors are hard for gibbons to unlearn. Rehabilitating gibbons is a challenging and time-consuming process that takes years with no guarantee that any individual will ever be prepared to return to the wild.
Eastern hoolock gibbons experience their own host of threats unique to their situation in Myanmar. A long history of social and political unrest in the regions where they live has exacerbated all of the issues stated above and made the few laws and protections that exist harder to enforce. Rampant corruption in politics over the last several decades has allowed commercial logging companies to operate unchecked and with impunity. The recent influx of some 50,000 settlers in Hukaung Valley Tiger Reserve has greatly increased hunting in the reserve, threatening its entire ecosystem.
A series of recent gold rushes in northeastern Myanmar has created even more problems for the gibbons residing there. Mining not only increases and accelerates deforestation and forest fragmentation but damages natural water systems by diverting rivers, destroying riverbanks, and causing fluctuating water levels.
Due to the unlawful nature of their activities, gold miners in Myanmar frequently use mercury in their extraction of gold. When they heat the mercury during this process, it escapes as vapor into the atmosphere; and when they do not properly dispose of waste material, it seeps into the soil and water where it immediately enters the food chain. Mercury is well-known for its negative effects on biology. Ingesting or inhaling this heavy metal can wreak havoc on a creature’s nervous, digestive, and immune systems, and does irreparable damage to the lungs and kidneys. While it has mostly gone out of style among large-scale gold mining companies, the types of small-scale gold miners operating in Myanmar are not affected by the moral backlash of using mercury to extract gold, and so they do it without worrying about potential repercussions to their business or the environment.
The eastern 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.
Eastern hoolock gibbons live in a small number of protected areas, including Bumhpabum, Hponkan Razi and Htamanthi Wildlife Sanctuaries, and the Hukaung Valley Tiger Reserve. In 2002, the Mahamyaing Sanctuary was founded specifically as a gibbon refuge.
Of course, protections do nothing without adequate enforcement and education of local peoples about the important roles gibbons play in their environments. Fortunately, the Wildlife Conservation Society aims to improve the gibbons’ situation in Myanmar. Focusing primarily on the regions near the Mahamyaing Sanctuary, the society puts on educational programming for local school children, teaching them about the importance of gibbon conservation and why these animals should never be hunted. It also provides limited support for increased patrols of the sanctuary and promotes ecotourism in the Hukaung Valley. Obviously more projects like this are needed in other regions, but ongoing unrest in Myanmar makes their initiation and facilitation nearly impossible at the moment.
Another project, conducted by People Resources and Conservation Foundation (PRCF) works with local Karen people native to northeastern Myanmar. With their help, the foundation has been mapping high-priority areas for gibbon conservation and writing conservation action plans to protect gibbons and their habitats. The Karen people also have long-standing and rich oral and folkloric traditions relating to their natural environment and its resources, including gibbons. PRCF has been working to document these in the native Karen language to ensure they are preserved in hopes of inspiring future generations to respect and protect these forests and the creatures living in them as their ancestors have.
Written by Zachary Lussier, September 2022