SKYWALKER HOOLOCK GIBBON

Hoolock tianxing

Geographic Distribution and Habitat

Skywalker hoolock gibbons (Hoolock tianxing), also known by their less galactic (and definitely less Hollywood blockbuster-inspired) name of Gaoligong hoolock gibbons, are lesser apes native to the countries of China and Myanmar. In China, these primates are found in western Yunnan Province in the Nujiang River Valley (known to locals as the Angry River Valley), where home is the Gaoligongshan Nature Reserve within the Gaoligong mountains at the southeastern boundary of the Himalayas. Considered a biodiversity hot spot, these mountains are home to the world’s highest elevation and highest latitude tropical rainforests and host varying climates across diverse geography. The Gaoligongshan Nature Reserve inches west into northeastern Myanmar (formerly known as Burma), where the Chindwin River (a tributary of the Irrawaddy River) separates the China and Myanmar populations. Home to the Myanmar Skywalker hoolock gibbons is between the Irrawaddy-Nmai Hka River and the Salween River (also known as the Thanlwin River), the easternmost range of all hoolock gibbon species.

the angry river

China’s Nujiang River derives its name from the valley’s earliest inhabitants—the Nu minority, in whose language, the word “nu” means “dark.” It gets its colloquial name of “Angry River” for its loud sound of rushing water, which can be heard from miles away during the summer rainy season. During winter months, the Angry River becomes serene. The river is also known by the name Salween as it flows from the Tibetan plateau south. Emptying into the Andaman Sea, in coastal Myanmar, the river takes the name Thanlwin.

Forest dwellers, Skywalker hoolock gibbons inhabit primary evergreen, scrub, mountainous broadleaf, semideciduous hill, and montane forests. Most of the population lives in unprotected areas where they make their home in small forest patches defined by rough terrain. Those who live in the mountains of the Gaoligongshan Nature Reserve reside at altitudes up to 8,858 ft (2,700 m), where the annual mean temperature is about 55 °F (13.0°C). The “cold season” (November through March) brings temperatures below 41°F (5.0°C); however, temperatures can plummet to 28.4°F (2.0°C)—making the region the coldest known gibbon habitat. Snowfall is not uncommon, and the peaks are often snowcapped. The Myanmar population (less studied by researchers), resides at altitudes between 250 to 5,000 ft (76.2 to 1,524 m).

TAXONOMIC NOTES

Initially considered a population of the eastern hoolock gibbon (H. leuconedys), today the Skywalker hoolock gibbon, discovered by a team of researchers in 2017, enjoys distinct species status. Differing genetic characteristics, coat color patterns, and tooth morphology between the two primates led researchers to this scientific conclusion. The Skywalker is a monotypic taxon; that is, it has no subspecies. Researchers now believe that the Skywalker hoolock and eastern hoolock gibbons diverged from one another about 490,000 years ago.

Skywalkers belong to the family known as Hylobatidae. Prior to receiving the genus Hoolock, the Skywalker hoolock gibbon and its two “sibling species”—the eastern hoolock gibbon and the western hoolock gibbon (H. hoolock)—had been recognized by scientists as belonging to the genus Hylobates and to the subgenus Bunopithecus. To date, the only known representative of this extinct subgenus are the skeletal remains of a gibbon or gibbon-like ape (Bunopithecus sericus), from the Middle Pleistocene epoch (about 1 million years ago), discovered in Sichuan (Szechuan), China.

Size, Weight, and Lifespan

Because of their smaller size, gibbons are classified as lesser apes. However, of the 20 gibbon species, hoolock gibbons are the second largest, after their big “cousins” the siamangs, who belong to the genus Hylobates and tip the scales at nearly 31 pounds (14 kg).

Adult male hoolock gibbons weigh about 15 pounds (6.8 kg); females are more petite, weighing 13 pounds (5.9 kg) when full grown. Average height is 32 inches (81 cm). Like all apes, hoolock gibbons have no tail.

In the wild, hoolock gibbons have a lifespan between 20 and 25 years. Captive hoolock gibbons have been reported as living longer than 40 years. Of course, this longer lifespan must be examined for the primates’ quality of life: enrichment, mental stimulation, ability to engage in natural behaviors, etc., balanced against their loss of freedom.

WHAT'S IN A NAME?

A lot…if you are eponymously named after a supergalactic Jedi master. Excited scientists, along with legions of fans of the wildly popular Star Wars movies, could barely contain their glee after the 2017 discovery of the Skywalker Hoolock gibbon. Nor could American actor Mark Hamill, whose alter ego and hero figure Luke Skywalker combats evil by harnessing a mysterious and metaphysical power known as “The Force”—a long time ago in a galaxy far, far away. Hamill enthusiastically “tweeted” the gibbon’s discovery on his Twitter (social media) account.

Hold on, Star Wars fans. This gibbon’s celestial nomenclature is rooted not in Hollywood—but in early Chinese theology, including Confucianism, the Chinese belief system concerned with personal ethics and morality. A chief precept of this religion is the concept of heaven as a metaphysical force that controls the world. In an ancient Chinese book of divination called the I Ching, or “Book of Changes” (which strongly influenced the teachings of venerated philosopher Confucius, “father of Confucianism”), the spiritual energy force of scholars is compared to the planetary movement of heavenly bodies— such as stars. Indeed, the species’s scientific name Hoolock tianxing loosely translates to “heaven’s movement” or “skywalker”—an homage to its Chinese phrasing where the word/character “tian” (天) translates to “heaven” or “sky,” and the word/character “xing” (行) translates to “star.” The name gives reverence to gibbons who, in early Chinese culture, were regarded as mystical scholar-officials, or junzi, and also acknowledges this gibbon’s ability to walk high up in the forest canopy, through the sky.

Appearance

Some say that these lean, long-limbed, furred primates with expressive faces bear resemblance to Ewoks, those furry fictional warrior-creatures of Star Wars movies. Okay, so those who say this are ardent Star Wars fans with an affinity for film hero Luke Skywalker. Never mind all that.

All hoolock gibbons are characterized by thick, shaggy coats and long limbs (arms are longer than legs, almost ridiculously). To distinguish Skywalker hoolock gibbons from their eastern and western siblings, Mother Nature has tweaked certain morphological traits for subtle differences in appearance. Specifically, she turned her attention to their eyebrows, beards, and genital tufts.

The species is sometimes referred to as white-browed gibbons due to their conspicuous facial markings. While white eyebrows are a distinct feature of the genus, these ornamental strips of hair are slightly different in each of the three species. The white eyebrows of Skywalker hoolock gibbons are thinly painted streaks that curve downward and are separated by a larger gap than seen in eastern hoolock gibbons, whom Skywalkers were initially thought to be; western hoolocks have been pranked with a unibrow. And unlike eastern hoolock gibbons, Skywalkers have no white hairs springing from beneath their eye sockets. In female Skywalker hoolock gibbons, Mother Nature never finished painting the furry white rings outlining their dark faces, as she completed for both eastern and western hoolock female gibbons. Both sexes, across the three species, have a flat, black hairless muzzle with thin lips. Dark eyes assess their forested environment while their ears, obscured by furry shafts, listen to the sounds of nature while alert to danger. Beards of male Skywalkers are either black or brown, while eastern hoolocks sport white or buff-colored beards; the beards of western hoolocks are black.

The genitals of adult male Skywalkers are garnished with black or brown tufts of fur; in western hoolocks, these tufts are either black or faintly grizzled. Silvery white tufts decorate the testicles of the eastern hoolock gibbon species.

Sexual dichromatism is pronounced in hoolock gibbons, which means that the coloring of their fur coat (pelage) differs between males and females. The underside of adult male Skywalker hoolock gibbons is dark brown, while the fur covering their back is described as a brownish overlay, more obvious under bright light (think how bright sunshine makes your hairstylist-applied highlights pop!). Adult female Skywalker hoolocks have yellowish white fur coats that become reddish blonde as they age. Females may have lighter-colored fur covering their hands and feet; some researchers suggest this coloration is an allopatric differentiation, or geographic speciation.

Diet

Skywalker hoolock gibbons are omnivorous, meaning that they eat both plant and animal foods, though only a fraction of their diet comes from animal prey. Nearly half their diet consists of various fruits, consumed from 36 plants species. Figs are their absolute favorite; these tiny, teardrop-shaped fruits are packed with nutrition. When fruits are unavailable, Skywalkers feed on leaves and buds. Invertebrates (including insects and spiders) along with the occasional bird chick or egg, followed by stems and flowers complete their diet.

Reportedly, females eat more than males, particularly during spring and autumn. No reference is found as to whether the girls’ bigger appetite might be linked to breeding season. However, springtime breeding begins around April, so we might conjecture a connection.

Behavior and Lifestyle

Like all gibbons, Skywalker hoolock gibbons are arboreal. Befitting their name, they reside in the canopy and advance through their forested habitat swiftly and with great agility, swinging from tree to tree with their ridiculously long arms outstretched (a form of locomotion known as “brachiation”) and grasping branches with their strong, hook-shaped hands. Adapted ball-and-socket wrist joints and adapted shoulder joints aid this momentum, helping to relieve fatigue in their arms. The gibbons’ powerful legs help propel them forward. Flexible feet help them “stick their landing.”

While brachiation is their primary form of locomotion, these hoolock gibbons also leap, climb, and jump as they navigate their habitat. They are capable of walking upright (bipedally), though do so awkwardly and rarely—for less than 1-minute intervals—whether along branches or on the seldom occasion they are forced to the ground, when the distance between trees is too far for them to swing or leap. Their long arms, held above their head, assist with balance in these alternative modes of locomotion. Quadrupedal locomotion (using all four limbs), is even more rare with gibbons, though researchers have reported this behavior.

Skywalker hoolock gibbons are diurnal animals, meaning they are active during daylight hours. After rising at dawn and taking care of bathroom business (peeing or pooing), they spend most of the day foraging. Their impressive brachiation allows them great success. Not only are they able to travel up to 35 miles per hour (56.3 kph) while bridging gaps 50 feet (15.2 m) wide, they are able to swing out and grab fruits growing at the end of branches—a feat that most of their competition is unable to do. When eating, they either sit on their tushes or hang suspended from a support. What they cannot do is swim; gibbons avoid crossing waterways because if they fall in, they are likely to drown. In fact, rivers often serve as species boundaries.

When not foraging and eating, Skywalkers spend their days grooming one another, playing, resting, and singing (gibbons’ lives revolve around song). They are less active during winter months and retire to their “sleeping trees” earlier in the day.

On cold winter mornings, they may sun themselves. Lying on their tummies upon the highest branches of trees, closest to the sky, they expose their backs to the sun’s rays, warming their bodies.

About 2 to 3 hours before sunset, Skywalker hoolock gibbons tuck themselves in for the night—before any nocturnal predators have a chance to find them. Predators include cloud leopards, tigers (a potential threat to the Myanmar population), large birds of prey (such as eagles and vultures), and huge snakes (such as pythons). Unlike great apes (and many other primates), gibbons do not create nests. Rather, they ensconce themselves within the tallest trees, upon thin branches less than 4 inches (10 cm) near the crown of the tree, to sleep overnight. They sleep sitting up, with arms wrapped around their knees that are tucked into their chest. At least Mother Nature has provided these gibbons with “seat cushions.” Thickened skin on their buttocks, known as ischial callosities, enables them to comfortably sit on branches.

Skywalker gibbons deliberately and strategically choose a different sleeping tree nearly every night. Proximity to food sources and predator avoidance are two strong considerations. Typically, family members sleep in neighboring trees—except during the winter. Winter months in Yunnan province can be very cold, so Skywalker hoolock gibbons use thermoregulation (body heat) to keep warm—yep, they sleep together in a huddle, body against body; they also tend to linger longer in their sleeping trees, because who wants to get out of bed when the weather is frigid? They are also likely to sleep at lower elevations, where temperatures are milder and food sources are more readily available.

Fun Facts

Legends from China’s Jin dynasty (1115-1234) claim that noble scholars would be transformed into gibbons, while people of lower standing would devolve into dust or insects.

The remains of an extinct genus, Junzi imperialis, approximately 2,200 to 2,300 years old—perhaps a gibbon—were found inside a tomb, excavated in 2004, believed to be that of Lady Xia, grandmother of China’s first emperor (259-210 BC). During Lady Xia’s time, gibbons were kept as pets by people of high social status. Junzi imperialis is believed to have gone extinct after the last ice age and because of anthropogenic activity, a portent, perhaps, to the current dire situation faced by Skywalker gibbons (and virtually all wildlife species in our world). According to scientific analysis, Junzi imperialis, or Lady Xia’s gibbon, is distinct from Bunopithecus sericus, the extinct gibbon species from 1 million years ago.

Daily Life and Group Dynamics

Gibbons are highly social animals with a lifestyle similar to that of great apes. A family group is comprised of a set of parents and their young offspring, typically one or two children. Youngsters leave their natal (birth) group upon reaching adulthood so they can create their own families. Sometimes, a parent has to give a child a nudge—no moving into the basement until you’re 30, like seen in human primates! On occasion, a lone adult establishes a solitary territory; this situation occurs when he or she is unable to find a mate.

Each family group of Skywalker hoolock gibbons occupies a home range from 0.06-0.1 sq mile (0.15-0.30 sq km) to 1.2-1.5 sq mile (3-4 sq km). An average day’s travel varies between 0.37 mile (0.6 km) and 0.8 mile (1.3 km). But Skywalkers may traverse a distance of just 0.19 mile (0.3 km)—or as much as 1.9 mile (3 km). They vary their foraging patterns within their home range on a seasonal basis. When fruits are out of season and Skywalkers must rely on leaves for their primary nutrition, they do not venture far from home.

About 205 species of wild animals live in the Gaoligongshan National Nature Reserve, so Skywalker gibbons have lots of company. Sympatric species include Assamese macaques (Macaca assamensis), Phayre’s leaf monkey (Trachypithecus phayrei), the Bengal slow loris (Nycticebus bengalensis), Chinese pangolin (Manis pentadactyla), large Indian civet (Viverra zibetha), red pandas (Ailurus fulgens), and leopards (Panthera pardus). More than 500 bird species live here, too, including annoying drongos (Dicruridae), who dive-bomb Skywalkers and harass them until the primates abscond a feeding tree.

Myanmar Skywalker gibbons share their habitat with a few additional interesting species. These animals include a large ungulate known as a takin (Budorcas taxicolor), also called cattle chamois or gnu goat; musk deer (Moschus), though their populations are declining; and two species of tigers: the Bengal (Panthera tigris tigris) and the Indochinese (Panthera tigris corbetti). These wild cats are heavily hunted, however, and their numbers are dwindling. Elephants and rhinoceros also once roamed this region—but they are gone. Over 120 bird species live here.

Communication

Vocalizations:
Gibbons are known for their loud, prolonged songs that ring out into the jungle, reverberating for miles. Their songs are specific to their species and include specific characteristics that are inherited, not learned. In hoolock gibbons, males and females emit the same vocalizations. The songs of Skywalker hoolock gibbons are distinguished by high-pitched, rhythmic notes, a distinction that helped researchers identify these primates as a singular species.

Like all gibbons, Skywalkers sing in the hope of attracting a mate, to identify individuals within and outside of a family group, and to intimidate interlopers from entering their home range. Songs are usually performed from their sleeping trees, 1 to 2.5 hours after dawn, and last 20 to 30 minutes.

Lone, adult Skywalkers of reproductive age sing their hearts out to one another in the hopes of finding a soulmate.

While out foraging, gibbons use loud long-distance song bouts to warn family members in neighboring areas of a potential predator threat. Those gibbons in the neighboring groups respond by joining the singing, matching the specific predator song—demonstrating, researchers say, that the primates understand the difference between song calls.

To mark their family’s territory, and dissuade outsiders, a paired adult male and female sing a loud duet; an individual never sings solo—unless s/he is separated from one another by human disturbances. The gibbons go silent when gunshots ring out. Researchers documented one group of Skywalkers who stopped singing and calling to one another for 68 days—after hearing gunfire within their home range.

Songs may be accompanied by threatening body postures and facial expressions. If this musical and physical display fails to convince an intruder to leave, Skywalkers gives chase.

Tactile:
As with most primate species, mutual grooming helps Skywalker hoolock gibbons to establish, and reinforce, social bonds with one another. Their opposable thumbs aid this effort. Playtime between youngsters helps them to understand social boundaries.

Olfactory:
Hoolocks are fitted with glands in their breastbone (sternum) and the groin (inguinal region), which researchers believe may play a role in olfactory (scent) communication, through the corresponding secretions.

Reproduction and Family

Rare among primates, gibbons are monogamous. Skywalker hoolock gibbons are no exception: Adult males and females form long-term stable bonds with one another.

Sexual maturity (ability to reproduce) for males and females occurs at about 8 or 9 years of age (captive gibbons attain sexual maturity 2 or 3 years earlier). China’s Skywalker population breeds only once every 3 to 5 years, contrasted to Myanmar’s Skywalker hoolock gibbons who breed every 2 to 3 years. The species’s low fertility rate keeps impacts the overall low population number.

After a gestation period of about 7 months, a female gives birth to a single infant. Births occur between November and March. While infant eastern hoolock gibbons are born with the same color coat as their mothers, an adaptation and safeguard that allows them to blend with mom, Skywalker infants are cloaked in a creamy white fur coat that gradually changes color over the next several months, to either brownish-black or yellowish-white (depending whether infant is male or female, respectively).

For the first few months of their lives, infants clings tightly to their mothers’ waists. Mothers nurse their babies for 18 to 24 months, at which time young Skywalkers are considered weaned. While mothers are the primary caregivers, dads may lend some assistance. They at least act as protectors for their offspring. Age of independence also 8 or 9 years of age (same as sexual maturity).

Researchers have documented the species’s four-step mating ritual. (1) As the dominant partner in this copulatory event, the female Skywalker raises and presents her buttocks to the male—twerking him. (2) Next, the male saunters over to the female and accepts her less-than-subtle invitation. (3) The willing male then mounts the female and sexual intercourse commences. (4) After their amorous interlude, the two take a rest.

Ecological Role

Skywalkers eat lots of fruit, which is a lot of roughage, which—after digesting and passing through the gibbons’ intestinal tract, results in lots of poop! Their poop is good, because it is loaded with the seeds of the fruits they eat. Deposited throughout their habitat, these seeds help to regenerate new forest growth.

Conservation Status and Threats

The Skywalker hoolock gibbon is classified as Endangered by the International Union for Conservation of Nature (IUCN, July 2019), appearing on the IUCN Red List of Threatened Species. These gibbons also received the ignoble distinction of being included in the IUCN’s 2018 “Primates in Peril” listing as among the world’s 25 most endangered primates—that’s only one year after they were “discovered.”

Habitat loss has led to severe fragmentation, placing Skywalker hoolock gibbons at grave risk of extinction. China’s remaining Skywalkers live in small clusters, separated by impassable, human-constructed barriers. In fact, the scientists who discovered these gibbons needed to ascend 8,202 ft (2,500 m) within Gaoligongshan Nature Reserve to find suitable gibbon habitat—the forest below had been entirely logged. Although China has since banned commercial logging, illegal logging persists.

Destruction of forestland for the cultivation of tsaoko—also known as black cardamon, has further decimated pristine gibbon habitat. This “cash crop,” a primary source of income for local farmers, is responsible for a 50-percent decrease in Yunnan’s gibbon populations. As the tallest trees are taken down, the gibbons’ food choices and sleeping sites disappear, forcing the animals to expend more energy as they forage greater distances—leaving them vulnerable to hunting and to predators. Individuals living in isolated fragments cannot easily find mates to procreate. Their solo songs are of unrequited love.

Hunting poses an additional threat to the species’ survival. Humans kill these primates for their flesh (“bushmeat”), desired for its culinary yewei or “wild flavor.” The gibbons are also killed (poached) for medicinal use; some indigenous people of the Gaoligong region believe that the brain of hoolock gibbons can be used to cure childhood epilepsy and headaches.

The Myanmar population of Skywalker hoolock gibbons is less studied, in part due to political unrest in the region, but they are similarly imperiled. Slash-and-burn agriculture, gold mining, logging, and hunting are prevailing anthropogenic activities in the region. Habitat loss has increased significantly in recent years. Scientists warn that unless anthropogenic activities cease, more than half the Myanmar population will be wiped out within the next two decades.

Conservation Efforts

Most primate species that have been given an IUCN classification are also listed within an appendix 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. You’ll find both the Endangered western hoolock and Vulnerable eastern hoolock here, within Appendix I. But you won’t find the Skywalker hoolock gibbon. Even more unfortunate: this primate’s body parts continue to be traded, which makes its omission confounding.

You will find the primate Bunopithecus hoolock, of the family Hylobatidae, entered into Appendix I of CITES on January 7, 1975. According to its CITES entry, this species, simply known as the hoolock gibbon, has pretty much the same range as the primate we have come to know as the Skywalker hoolock gibbon. In fact, the scientific names Bunopithecus hoolock and Hoolock tianxing appear to be used interchangeably in some earlier recorded research accounts, prior to the Skywalker’s discovery and its subsequent designation as a distinct species.

It may be that the CITES appendix has not been updated; the same holds true for China’s protected species listing. Both the Endangered western hoolock and the Vulnerable eastern hoolock are listed as a Class I protected species in China—that country’s highest conservation status. But the Skywalker is missing.

Those Skywalkers living within the Gaoligongshan Nature Reserve in China have some protections (at least in theory; conservation laws are often flouted and difficult for authorities to enforce). The reserve is under the authority of the Chinese Ministry of Forestry and also recognized by international organizations. It is a class Protected Area of World Wildlife Fund and World Biosphere Reserve and a part of World Heritage Site of UNESCO, United Nations Educational, Scientific and Cultural Organization, a specialized agency of the United Nations (UN) aimed at promoting world peace and security through international cooperation in education, arts, sciences and culture).

Unfortunately, those Skywalkers residing in Myanmar live in no protected areas.

Some conservationists in China have proposed translocating isolated groups of Skywalkers to areas that reconnect them with fragmented populations. Relocating wild species is never an optimal solution, however. Nevertheless, these conservationists argue that controversial forms of intervention are sometimes warranted to save a species from extinction. But such an undertaking as capturing and relocating Skywalker hoolock gibbons is fraught with logistical challenges, not only the region’s diverse geography but also the “red tape” surrounding government approval for such an initiative.

Other conservationists have proposed a captive breeding and reintroduction program. However, not enough Skywalkers are currently in captivity to begin such a program, and capturing these primates, while logistically prohibitive, also risks harming the animals. According to the IUCN, a number of wildlife sanctuaries are found with the Skywalker hoolock gibbons’ Myanmar range, but none harbor this species.

Scientists enamored with this mystical gibbon are experimenting with telephony to save the species from extinction. Broadcasting recorded Skywalker songs through the jungle, first by satellite telephones and then speakers and microphones at two distanced locations, the scientists have “hooked-up” lovelorn single Skywalkers by allowing them to sing to and find one another, and, optimistically, procreate. This innovative conservation plan is the brainchild of two unlikely collaborators: Beijing pop star musician Xingyu Lee and Skywalker hoolock gibbon scientist/expert/researcher/discoverer, credited with naming the Skywalker hoolock gibbon, Professor Pengfei Fan.

A conservation plan to save the Skywalkers must consider the region’s indigenous people. Cloud Mountain Conservation is a nongovernmental organization (NGO) founded in 2015 that specializes in gibbon conservation in China. The organization educates local citizens, especially schoolchildren, about the significance of Skywalkers and their habitat; supports wildlife reserves to increase anti-poaching patrols; works with local governments and forestry bureaus to protect the species; and promotes sustainable development. Currently, Cloud Mountain is helping local farmers diversify their crops, in the belief that both gibbons and indigenous people can have a more stable future.

Local lore might also play a role in helping to protect Endangered Skywalkers. While some indigenous peoples actively hunt the gibbons, citizens of the Lisu community (a Tibeto-Burman ethnic group of the mountainous regions of Myanmar, southwest China) who live around one of the three gibbon clusters in Yunnan do not. Their culture maintains strong generational taboos against the practice: The Lisu people believe that gibbons are primate gods who can forecast the weather or even death through their singing; thus, killing gibbons would bring misfortune to a hunter’s family or to a whole village.

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Written by Kathleen Downey, July 2022