Geographic Distribution and Habitat
The yellow-cheeked gibbon (Nomascus gabriellae) is known by many names, including the gold-, red-, or buff-cheeked gibbon. They live in the tropical jungles of southern Lao People’s Democratic Republic, southern Vietnam, and southeastern parts of the Kingdom of Cambodia. As this species is not yet thoroughly researched, the full extent of its range may be larger than currently expected.
While other gibbon species may be comfortable living in mountainous regions at higher elevations, yellow-cheeked gibbons have a distinct preference for wet lowland and evergreen forests. High in the crown region of the canopy, beyond the reach of predators, they have access to everything they need to survive. Within such complacency, gibbons hardly ever venture any closer to the ground. It is rare—and potentially disconcerting—for them to ever visit the forest floor.
Yellow-cheeked gibbons are somewhat adaptable and occasionally make do living in bamboo and deciduous forests as well.
Size, Weight, and Lifespan
In weight and size, yellow-cheeked gibbons show little difference between sexes. Males and females range from 24 to 31 in (60–80 cm) in length and can weigh between 15 and 24 lb (7–11 kg).
Currently, the lifespan of yellow-cheeked gibbons in the wild is unknown, but some captive individuals have lived up to 50 years.
Small-statured and uniquely shaped for life in the trees, many people mistake gibbons for a type of monkey. Gibbons are apes, however, sharing their side of the primate family tree with orangutans, chimpanzees, bonobos, gorillas, and humans. Gibbons belong to the “lesser apes” branch of the family trees, while orangutans, chimpanzees, bonobos, gorillas, and humans are “great apes.” Like all apes, gibbons do not have tails—a feature that prominently distinguishes them from monkeys.
Except for orangutans, gibbons are the only apes who spend a significant amount of their time in trees. Their compact torsos and long limbs are uniquely adapted for this lifestyle. They have hook-shaped hands with four long fingers that are exactly what they need to grasp and swing under branches with ease. While gibbons do have opposable thumbs, theirs are significantly shortened in comparison to other apes. In this form, they remain useful for grooming but do not obstruct movement during brachiation. Also called arm swinging, brachiation is a form of arboreal locomotion in which primates swing from tree limb to tree limb using only their arms. The advantage of this subtle adaptation is clear in a gibbon’s improvisational acrobatics, swinging with amazing agility through the crown canopy at seemingly impossible speeds, dangerously high above the ground.
Male and female yellow-cheeked gibbons are relatively similar in size but are strikingly different in coloration. As infants, both sexes sport a golden-yellowish fur coat. As they mature, their coats turn black. A male retains his black coat through his adulthood, developing the light-buff cheek patches for which the species is named shortly after puberty. These extend from his eyes to just below his mouths.
As she reaches maturity, a female sheds her youthful black coat and returns to a light pelage. Slightly browner than the one she sported as an infant, she retains a cap of black fur on the top of her head as a token of her maturity. Around their edges, her ears also keep some black fur. A rim of white fur comes to frame her face. Her hair takes on a slight gray tint around her chest and a reddish and brownish hue around her genitals.
Fruit is the main staple of the yellow-cheeked gibbon’s diet. They are not entirely frugivorous, however. Especially in the dry season, when ripened fruit is scarce, they may turn to eating shoots, leaves, flowers, and occasionally insects or bird eggs to supplement her dietary needs.
More remains to be studied concerning the yellow-cheeked gibbon’s diet.
Behavior and Lifestyle
Gibbons are almost exclusively arboreal (live in trees) and very rarely leave the comfort and safety of the crown canopy. Here, with easy access to any in-season fruits and beyond the reach of predators, they are able to live a life that is virtually free of stressors.
To get around in the trees, a gibbon primarily uses a type of locomotion known as brachiation. Grasping a branch with his hook-shaped hand, he swings himself underneath it. Then, by harnessing the momentum from his swing, he propels himself forward to grasp another branch with his other hand. He continues this process indefinitely, in a chain of dynamic and energetic movement that outperforms even the most daring and practiced of human trapeze artists.
While other species of apes occasionally rely on brachiation, gibbons are the true virtuosos of this method for movement, taking it to a level of efficiency that any orangutan, chimp, bonobo, or human could never even dream of replicating. At peak speed, a gibbon is able to launch himself up to 33 feet (10 m) between branches! Still, he manages this with utmost elegance and grace, his movements flowing seamlessly into the next, coming to rest on a branch again in a calm and controlled manner: perfectly composed.
Yellow-cheeked gibbons are diurnal and notoriously early-risers. Each morning, beginning at around sunrise, male and females sing to each other. The lilting and haunting arias that they exchange can be heard for over half a mile (up to a kilometer). These are not only useful for locating each other after a night sleeping apart, they also help the pair establish their territory, letting any nearby gibbons know where they are and not to come too close.
High in the crown canopy where predators cannot reach them, yellow-cheeked gibbons spend the day feeding and locating food, little ones find time to play, and parents relax or groom each other or their offspring.
Gibbon play is considered some of the riskiest in the natural world. Though living high in the canopy, safe from predators, provides the perfect stress-free environment for play behavior to thrive, it also means a single slip-up during a playful chase or wrestle can have fatal results for a yet-competent infant. Naturally, as a result, gibbon parents (both the male and the female) pay close attention to their young.
Unless they are trying to a find a mate, gibbons tend only to socialize with their immediate family members. All others are generally considered outsiders who are chased away whenever they enter another family’s territory. Grooming acts as an important method of reinforcing their strong familial bonds.
Developing throughout their lives, coloration plays an important role in signaling yellow-cheeked gibbons’ sex and maturity.
Yellow-cheeked gibbons were adversely affected by the use of napalm bombs, defoliants, and other herbicides by the United States during the war it waged in Vietnam: agents that had disastrous and decades-long effects for the environment.
Researchers have recognized a basic syntax and grammar in gibbon songs, making them an unique model for studying the evolution and development of language.
Gibbons live in small nuclear families consisting of a mother and father and up to four of their offspring. Mom and Dad both 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.
A family of yellow-cheeked gibbons wakes just before dawn, when their canopy home catches the sun but the forest below remains dark. If they spent the night in separate trees, Mom and Dad exchange songs to find each other. Their lilting calls also act as a warning for any strange gibbons in the area not to come too close.
Family members spend their morning re-establishing their bonds, feeding, or traveling to find food. They eat periodically throughout the day, from morning until the late afternoon, with plenty of leisure time for resting and playing between the group’s mid-morning and mid-afternoon meals. By nightfall, the family chooses one or two trees in which to sleep.
The gibbon family’s routine is affected by the changing of the seasons. As different fruits ripen during the rainy season, the family generally spends more of their day traveling between fruiting trees and less of their time feeding. During the dry season, when fruit stops ripening, the family foregoes travel and spends more time chowing down on the less-nutritious (but abundant and readily available) leaves with which they must effectively supplement their diet.
Like many other social primates, yellow-cheeked gibbons use facial gestures and body language to communicate. Family members also groom each other as a way of establishing and reinforcing their bonds.
Gibbons are well known for their loud, far-reaching, and acoustically complex vocalizations, often 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 family members reconnect after a night sleeping in separate trees, as well as warning unfamiliar gibbons who happen to be in the area to keep away. Some studies have shown that they may also be used as alarm calls. However, much research remains to be done to find out the full nature of these vocalizations.
Mating pairs of yellow-cheeked gibbons sing each morning when the sun first hits the top of the trees where they have been asleep. The male and female take turns in a haphazard call-and-response pattern that ultimately overwhelms the competing songs of insects and birds in the still-dark jungle below.
The repertoire of calls these apes produce is distinctive to their sex. The female’s consists of a series of high-pitched and haunting notes that successively increase in pitch, volume, and speed—an auditory experience that resembles an approaching ambulance siren. At climax, her call suddenly shatters into a staccato chirp of notes that fall in pitch and bend in timbre. The male’s call is also a series of notes—each as high-pitched and haunting as the female’s—that lilt upwards in tone until they flatten into a squeak.
Infants and adolescent gibbons develop their singing skills with help from their mother. Hers is the song that both her male and female offspring practice. Researchers reason that this is a method for still-developing males to communicate their immature status to the father. As they get closer to maturation, the male parent becomes increasingly aggressive towards their male offspring as he readies to leave to start his own family. Therefore, by practicing his mother’s call only, an immature male gains important singing skills but also avoids upsetting his father by inadvertently triggering his territorial defenses.
The full complexity and social function of gibbons vocalizations and other modes of communication will only become clearer with more research.
Mature yellow-cheeked gibbons establish monogamous male-female relationships that form the foundation for gibbons’ chief social unit: the nuclear family. Though instances of gibbon infidelity and separation have been observed in the wild, these relationships are generally exclusive and last long-term.
Over the course of their partnership, the male and female may produce a new offspring every 2–3 years. The raising of their young is a process that both parents take very seriously. The large amounts of time they must invest toward the healthy development of their child means that caring for a newborn is only possible one at a time.
After seven months in the womb, the female births a single infant. Helpless beyond clinging to his mother’s fur, she nurses her newborn for up to two years. Only after he is weaned is the female once more able to become pregnant. It is not uncommon for families to grow as large as five members at one time.
Once weaned, it takes another 6–8 years before their offspring matures enough to strike out on his own. During this time—through play, and by observing and mimicking his parent’s behaviors—he practices and hones all the important skills necessary to make it as a wild gibbon. With this knowledge, the newly matured gibbon leaves his family to find a female partner, and establish a family of his own.
Yellow-cheeked gibbons act as important seed-dispersers, especially for the Pacific walnut. The full extent of their ecological role is in need of more and deeper research, however.
High in the crown canopy, gibbons are not often targets for any predators.
The yellow-cheeked gibbon is classified as Endangered by the International Union for Conservation of Nature (IUCN, 2008), appearing on the IUCN Red List of Threatened Species.
Although living high in the canopy keeps gibbons relatively safe from predation, it also means they are extremely vulnerable to forest degradation. Unfortunately, the lowland evergreen forests where yellow-cheeked gibbons thrive have a long history of being cut down to make space for farmland and to facilitate other human projects, including gold mining.
Additionally, it is highly probable that yellow-cheeked gibbons have been the innocent victims of human warfare. During the 1960s and ‘70s, the United States military destroyed nearly 7.5 million acres (3 million hectares) of natural forest in Vietnam as a result of the war it waged there. Although the species’ full range at that time is impossible to know, many of these affected areas would likely have been habitats for yellow-cheeked gibbon families.
The chemical and biological weapons used during the warfare, like napalm, did much more than cause immediate suffering for the flora and fauna caught in their wake. Heavy use of herbicides and defoliants, like the notorious “Agent Orange,” had long-lasting effects on these ecosystems. Without foliage, the soil where these chemicals were sprayed often dried out and successively eroded, slowing the forests’ natural recovery. In places where the forests could not recover fast enough, more aggressive plant species, like bamboo, eventually came to dominate. Although we can never know the full details of how these events affected yellow-cheeked gibbons directly, the evidence does suggest that they would have gravely affected the species over the course of several decades, if not longer.
The U.S. military also used these weapons in areas of the Lao People’s Democratic Republic and the Kingdom of Cambodia, where yellow-cheeked gibbons are also known to make their homes. Thus, the U.S. war effort in Vietnam is likely to have had ill effects for yellow-cheeked gibbon populations throughout their native Southeast Asia.
Today, yellow-cheeked gibbons are regularly hunted and killed for meat or to be used in traditional medicines. Humans also capture young gibbons to keep or sell them as pets. Considering how much time and care gibbon parents put into developing their young, it is extremely difficult for an orphaned or kidnapped gibbon to recover from being separated from his or her parents. Even when an orphaned gibbon survives her capture and confinement, the infant’s rehabilitation is a long and time-consuming process, with no guarantee for proper re-release into the wild.
As a result of all these variables, yellow-cheeked gibbon numbers are currently on a downward trend.Status
In order to reverse these negative trends, the IUCN recommends more regulation and more consistent enforcement of gibbon hunting and trafficking. Curbing current logging and agricultural trends that disturb native habitats are also important steps toward conserving yellow-cheeked gibbon populations in the wild. In general, the yellow-cheeked gibbon is a severely understudied species; more research (especially genetic analysis) is necessary to better understand this ape’s full nature so that humans can better protect it.
Fortunately, conservation efforts for yellow-cheeked gibbons are on-going and still evolving. Populations already exist in a number of protected natural areas in both Vietnam and the Kingdom of Cambodia. In southern Vietnam, yellow-crested gibbons are known to live in Cát Tiên and Bù Gia Mập national parks. In the Kingdom of Cambodia, yellow-crested gibbons live with protections in Virachey National Park, the Phnom Prich Wildlife Sanctuary and the Seima Biodiversity Conservation Area.
The Doa Tien Primate Center (DTPC) was established at Cát Tiên National Park in 2008. The center rescues and rehabilitates primate species local to the area, including the pygmy slow loris, the black-shanked douc, the silvered langur and the yellow-cheeked gibbon. Since its founding, the center has become a champion for yellow-cheeked gibbon conservation in particular.
Located on the eastern side of the Dong Nai River, opposite Cát Tiên, Doa Tien houses and feeds orphaned or other adversely affected yellow-cheeked gibbons—always with the hope and intention of re-releasing them into the wild. The majority of their gibbon charges are orphans whose abilities to care for themselves in the wild have been inhibited by the loss of their parents.
Proper rehabilitation for a yellow-cheeked gibbon is complex and often takes years. A recent 2019 release that Dao Tien facilitated for two adult gibbons was the culmination of a 10-year process!
When a yellow-cheeked gibbon in their charge is deemed fit for release, she is fitted with a tracking collar. Such a mechanism is designed to be as unobtrusive as possible; its use is necessary to monitor the gibbon in the days and months following her release. If workers at DTPC perceive that her welfare is in any sort of danger, they take action to intervene lest their years of hard work in rehabilitating her should end in vain.
DTPC also works to educate local populations about gibbon conservation threats so that, one day, the need to care for and rehabilitate orphaned gibbons may become less of a trend.
Written by Zachary Lussier, January 2020