SOUTHERN YELLOW-CHEEKED CRESTED GIBBON
Geographic Distribution and Habitat
The southern yellow-cheeked crested gibbon, also known as the red-cheeked gibbon, lives in the tropical rainforests of southern Vietnam and southeastern Cambodia. Tall evergreen and semi-evergreen forests offer these arboreal primates ideal living conditions, but they are occasionally found in mixed bamboo and woodland forests as well.
Southern yellow-cheeked crested gibbons were once thought to live further north, in central Vietnam and Cambodia, as well as Laos. However, scientists discovered in 2010 that this northern population is, in fact, a new crested gibbon species, the northern yellow-cheeked crested gibbon. With this discovery came the realization that the southern yellow-cheeked gibbons’ range is smaller than previously believed.
Once upon a time, all crested gibbons were believed to be the same species, Nomascus concolor. Gradually, more detailed research found this initial classification was not accurate. New methods and technological advances for gathering and analyzing genetic, morphological, and auditory data have helped us gradually reveal the six distinct crested gibbon species known today, one of which is the southern yellow-cheeked crested gibbon. Today, Nomascus concolor only refers to the black crested gibbon found in northern Vietnam, northwestern Laos, and southwestern China.
Before 2010, there was thought to be only one species of yellow-cheeked gibbon, Nomascus gabriellae. After the 2010 discovery of Nomascus annamensis, the geographic distribution of this species was split in two. This novel yellow-cheeked variety in the northern part of the former distribution was given a shiny new scientific name, Nomascus annamensis, while the species in the southern region kept their original name. The species’ common names have also been given qualifiers that match where they live in relation to each other, one to the north and one to the south.
Size, Weight, and Lifespan
When it comes to size, southern yellow-cheeked crested gibbons show hardly any sexual dimorphism. Males and females may weigh anywhere from 11 to 26 pounds (5–12 kg). Males measure roughly 18 to 20 inches (45–50 cm) in length (head and body) and females between 18 and 19 inches (46–48 kg).
While their lifespan has not been studied, some species of gibbons are known to live upwards of 30 years in the wild.
Gibbons look similar to monkeys, but they are actually apes. Just like other apes such as bonobos, chimps, gorillas, and orangutans, they lack tails. However, compared to these “great apes,” gibbons are remarkably small, a necessary trait since they spend their entire lives in the canopy. The only other apes who spend a significant amount of time in trees are orangutans.
Southern yellow-cheeked gibbons, like other crested gibbon species, exhibit striking variations in appearance based on sex and age. Infants are all born a buffy yellow color. They remain this color until, at about six months, their coats turn black. Males will retain their black color for the rest of their lives, but females transition back to a yellow pelage as they reach adulthood. As adults, males sport this species’ signature yellow cheek patches, which in some individuals may also show up as orange or even reddish, and their chest may develop a subtle rusty-red hue. Adult females may or may not keep a small patch of black fur on the crest of their head, and they may also have slightly dark chest hair.
Crested gibbons are notoriously difficult to identify in the wild, which is precisely the reason why researchers have historically had trouble classifying them. Because females of each species often look so similar, the only reliable distinguishing feature is usually the males’ cheek patches.
Southern yellow-cheeked gibbons have a strong preference for fruit, especially figs. Research shows 39% of their feeding time is devoted to eating figs alone compared to the 43% they spend eating other fruits.
Figs and fruit are not available year-round, however, so their diets vary seasonally. Then, they turn to other sources of nutrition. What’s on the menu in these dryer months? Mature and young leaves, other parts of plants and trees, parts of flowers, insects, bird eggs, vines, and various epiphytes.
Behavior and Lifestyle
Gibbons are arboreal (tree-dwelling), living high in the canopy. Here, they have everything they need to live happy and healthy lives while staying out of the way of predators. They rarely, if ever, venture to the lower levels of the forest.
Southern yellow-cheeked gibbons are diurnal and notoriously early risers. Each morning, beginning at around sunrise, male and female pairs sing to each other. The lilting and haunting arias 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.
Many gibbon species practice both early-to-wake and early-to-bed lifestyles, keeping active only for as long as the sun is up. When the sun is up, they are quite busy. Though they take breaks after meals, when they are not feeding, they are likely traveling to find food or socialize. Adults stay alert, keeping watch for threats while little ones play.
Gibbons are territorial. 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. Grooming acts as an important method of reinforcing their strong familial bonds.
Gibbons use a form of locomotion known as brachiating. 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 goal. 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 km/h)! While brachiation is their go-to, gibbons improvise their movements as they go, leaping great distances and falling from great heights in order to reach the next tree. They even frequently walk in a bipedal fashion along a branch, moving swiftly and holding their arms out for balance. Due to this technique, gibbons actually 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 by the upper arms and torso and even minimize 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 gibbons 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. Since it happens so high off the ground, gibbon play is considered some of the riskiest in the world. One slip-up during a game of chase could easily mean the end of a young gibbon’s life. As a result, gibbon parents (both Mom and Dad) pay careful attention to their young while still giving them the space to practice and try out new moves.
Gibbons are learners. They aren’t born knowing how to survive in their habitat but develop the necessary knowledge and skills as they age, like being able to identify which foods are edible (and tasty) and which animals are predators. To learn this information, little ones rely on their parents to show them, a phenomenon some researchers argue qualifies as cultural transmission.
Southern yellow-cheeked gibbons living in evergreen forests, where nutritious food is more readily accessed, tend to keep smaller home ranges than those living in bamboo or degraded forests, where food is often more spread out.
Because they are often mistaken for monkeys and receive less attention than their better-known great ape relatives, gibbons are sometimes referred to as “the forgotten apes.”
Gibbon songs use their own syntax and grammar. They also sing in regional dialects, which is why researchers are able to use auditory data to identify different species.
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 southern yellow-cheeked gibbons wakes just before dawn when their canopy home catches the sun but the forest below remains dark. They spent the night in separate trees, and Mom and Dad exchange songs to find each other. Their lilting calls help the pair re-establish their special bond and also act as a warning for strange gibbons to stay away.
The family members spend their morning bonding and breakfasting high in the canopy. Throughout the day they rest and travel to find more food. By nightfall, the family chooses one or two trees in which to sleep.
Southern yellow-cheeked gibbon families are known to change up their routines depending on seasonal conditions, eating more fruit in the wet season than in the dry for instance. When temperatures and rainfall are low, the gibbons spend less of their day traveling and more of it eating, which helps them save and store energy. In general, what’s on a family’s agenda seems to change according to a number of variables including climate, the time of year, what foods are available, what the weather’s doing, and probably also just how they happen to be feeling that day.
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 to keep away.
Mating pairs of southern yellow-cheeked gibbons sing each morning when the sun first hits the top of the trees. The male and female take turns in a haphazard call-and-response pattern that gradually 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. He may also make loud booming calls that seem to shake the whole jungle around him.
Gibbons are not born instinctively knowing how to sing. It takes years of practice to perfect these highly specialized calls. Young gibbons begin by trying to mimic their parents, making a lot of mistakes at first. After a few years, they start to get the hang of it and learn each section of the duet in the order it’s sung. This process may look different in species, however. In some, it’s been observed that young males originally only practice their mother’s song. Some researchers presume this prevents them from accidentally triggering their father’s territorial defenses. Southern yellow-cheeked gibbons, only begin to make the call of a male as subadults, making them with greater frequency and for longer durations the closer they get to adulthood, when they finally say good-bye to their natal group.
Like so many other social primates, southern yellow-cheeked gibbons use a wide variety of facial gestures and body language to communicate. Family members also sing together and groom each other as a way of reinforcing bonds.
The full complexity and meaning of gibbon vocalizations and other modes of communication are paramount for their conservation and will only become clearer with more research.
Gibbons establish monogamous male-female relationships that form the foundation for their chief social unit: the nuclear family. While these relationships generally last throughout their lives, they may occasionally end in separation. There is also evidence that pairs are exclusive when it comes to mating. So, while gibbons can be considered socially monogamous, it is no longer assumed that they are sexually monogamous. However complicated their relationships may be, the family unit creates an important structure for a young gibbon’s life as each parent invests their time and energy in their young’s survival and success.
After mating, a female southern yellow-cheeked gibbon is pregnant for around seven months. Her newborn is tiny and, at first, relies on her completely. Being beige in color himself, the little guy blends almost seamlessly into his mother’s light-colored fur. He nurses for a number of months before venturing away from his mother’s warm and comforting embrace to try his first attempts at navigating his canopy home. He begins to watch what his parents do, noting what they eat as well as what forest creatures to be wary of. He also mimics their song, practicing for the day when he’ll also have a duet partner. Gradually, he gains the knowledge and skills to survive in the wild. Between seven and eight, after reaching sexual maturity, he leaves his family in hopes of starting one of his own.
Females lead similar lives and are usually between seven and eight years of age when they give birth for the first time. Because raising their young is so taxing, gibbon parents both help out, and mothers wait until their current offspring can take care of itself a little before having another child. At any given time, a gibbon family often consists of two parents, a juvenile, and an infant.
Gibbons play critical roles as seed dispersers. When southern yellow-cheeked 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 southern yellow-cheeked crested gibbon is classified as Endangered by the International Union for Conservation of Nature (IUCN, 2015), appearing on the IUCN Red List of Threatened Species, and populations are currently in decline.
As arboreal apes, southern yellow-cheeked gibbons face a host of threats in the wild, habitat loss being one of the most worrisome. According to Global Forest Watch, Vietnam lost at least 16% of its natural forests in the last 20 years due to human activity and, if trends continue, may lose up to 60% in the next 20 years. In the same amount of time, Cambodia has lost 25% and is predicted to lose up to 70% by 2045.
While the complete destruction of their forest homes threatens gibbons, even seemingly minor changes to their environment have immediate and adverse effects on their well-being. Because they are strictly arboreal, gibbons rely on continuous canopy cover to navigate. Gibbons keep large ranges and travel significant distances to find food or mate. When their habitats are fragmented, it limits their ability to move about freely. Even the building of a one-lane road can create a gap too difficult for gibbons to cross. Facing such an obstacle, a gibbon must choose to either risk making a hazardous leap or venturing to the ground where they are vulnerable to predators and, in the case of a road, oncoming traffic. Additionally, the forest floor is home to a host of bacteria not found in the canopy that can make gibbons sick and even kill them.
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 who survive are left in an unfortunate situation. 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 severely decrease the chances that new generations live long enough to ever procreate.
Southern yellow-cheeked gibbons are regularly hunted and killed in Vietnam and Cambodia for their meat or to be used in traditional medicines. Humans also capture young gibbons to keep or sell them as pets. Poachers are only interested in baby gibbons and 100% of the time have to kill the mother, and often the father as well, to get one. As pets, gibbons not only suffer mentally and physically but also miss out on learning how to be a natural gibbon in the wild. Gibbon parents put a great deal of time and energy towards helping their young develop. Therefore, it is extremely difficult for an orphaned or kidnapped gibbon to recover after being separated from his or her parents. Even when gibbons survive capture and confinement, their rehabilitation is a long and time-consuming process, with no guarantee they will be released back into the wild.
In Vietnam and Cambodia, gibbons are frequently poached from the wild and used to entertain tourists at zoos and hotels. Tourists usually just see a cute animal and are not confronted with the gibbon’s troubling past nor do they ever bear witness to its daily sufferings.
The same sort of problem is rampant across social media platforms where uninformed users are confronted with pictures and videos of gibbons kept in captivity as pets. People posting such material are careful to curate their posts so that their followers conveniently never see the daily cruelty and misery their pets endure behind the scenes. While this phenomenon seems trivial to some, studies show that media depicting primates as “cute” sends the message that they make suitable pets, priming people to accept and spread this sort of material further. Others may decide they want to get a primate of their own, fueling their trafficking and ensuring more gibbons are harmed. Unlike cats and dogs, who were naturally domesticated by humans over long periods of time, primates never make suitable pets and belong in the wild.
The difficulty researchers still have when it comes to classifying crested gibbons and knowing the range and distribution of each species causes problems for people rehabilitating gibbons, as they need to be able to easily identify a gibbon’s species and know where an appropriate place to stage a release will be. Additionally, it affects what gibbons they can socialize with as well as what foods rehabilitators should give, or not give, to the gibbons in their care. Inaccurate classifications and distribution cause issues for other conservationists as well. Species like the southern yellow-cheeked gibbon, which were once thought to have a larger distribution than they actually do, may end up seeming like less of a priority than they really are as a consequence of inaccurate survey data.
Last but not least, there is the issue of inconsistent enforcement. While protections exist for southern yellow-cheeked gibbons and their habitat, the follow-through on enforcing them is not always consistent. This undermines not only the protections but all efforts to conserve gibbon populations in the wild.
The southern yellow-cheeked crested 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. It is also legally protected in both Vietnam and Cambodia and found in several protected areas within both countries.
Conservation projects to protect gibbons have already been established in Vietnam. The Dao Tien Primate Center (DTPC) was established at Cat Tien 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 southern yellow-cheeked crested gibbon. Since its founding, the center has become a champion for southern yellow-cheeked gibbon conservation in particular. They educate local populations and tourists about the importance of gibbon conservation through tours of their center and early morning hikes into the jungle to view gibbons in the wild.
The author of this page had the privilege of visiting the Dao Tien Primate Center in 2018. He had to wake up at 4:30 in the morning, strap on special “leech socks,” bike two miles to wait in the dark for a ferry to carry his group into Cat Tien National Park where they rendezvoused with one of the park rangers. He led them about a mile into the pitch-black jungle where they sat at the foot of a giant tree and waited, listening to the otherworldly dawn chorus of birds and insects singing all around them. Then, soon after the sun began to paint the tips of the canopy above gold, the first gibbons started dueting nearby and we took off to find them. Upon seeing the family of four (Mom, Dad, juvenile, and baby) eating high above them, the author was changed forever (and ultimately led him to write for NEPC).
Further north in Vietnam, in Cuc Phuong National Park, the Endangered Primate Rescue Center is also busy “working to ensure the future of Vietnam’s primates” – as the slogan on their website proclaims. While their project is well outside the range of southern yellow-cheeked gibbons, gibbons poached from the wild can end up anywhere. In fact, according to surveys the majority of gibbons who end up as pets are sourced from the southern regions where southern yellow-cheeked crested gibbons reside. The center works to rehabilitate gibbons and educates locals and tourists about the importance of gibbon conservation.
While these two centers do amazing work, there is need for more if we want to turn the tide for the southern yellow-cheeked crested gibbon. The IUCN recognizes the need for a number of initiatives to stop poachers including more frequent and more extensive patrolling as well as stricter gun laws. Less lenient enforcement of protections would increase confiscations of gibbons already taken from the wild, beginning their long rehabilitation journeys and hopefully leading to their release. Indeed, the longer gibbons are kept in confinement, the less likely they are to ever be able to return to the wild. The IUCN acknowledges that rehabilitation and release alone could make a huge difference in the conservation of southern yellow-cheeked gibbons.
While a few wildlife centers are looking out for gibbons in Cambodia, there seems to be less emphasis here than in Vietnam, particularly when it comes to southern yellow-cheeked crested gibbons. Part of this could be due to the fact that Cambodia seems to have healthier, more ecologically viable populations than their neighboring country to the east. Still, as an endangered species, southern yellow-cheeked crested gibbons need help wherever they can get it. Hopefully, this situation is changing.
- Bockhaus, Anna / “#PrimatesAreNOTPets: The Role of Social Media in the Primate Pet Trade and Primate Conservation” (https://scholar.colorado.edu/downloads/j3860733d)
Written by Zachary Lussier, January 2020; revised by author August 2023