Nomascus siki

Geographic Distribution and Habitat

Southern white-cheeked gibbons are endemic to Southeast Asia, east of the Mekong River, in central Laos and Vietnam. The wet tropical climate of this area nurtures the perfect habitat for these arboreal apes. Primary lowland forests and tall primary broadleaf forests provide the type of dizzying canopies where they can thrive. Karst forests, with their tall limestone formations jutting out of the landscape, also provide the security and support their unique lifestyle requires.

Historically, this species had a large distribution. As it turns out, however, a 2011 study found that many of these were a newly identified species dubbed the northern yellow-cheeked crested gibbon (Nomascus annamensis), thus significantly shrinking the southern white-cheeked gibbon’s range. Researchers are still in the process of settling the true geographic distribution of southern white-cheeked gibbons to the north, where they may overlap with populations of northern white-cheeked gibbons.

Additionally, this species was once found between 100 and 6,200 feet (30–1,900 meters) above sea level. But today, they are typically only found at higher elevations in Laos. This is due to a recent, massive conversion of lowland forests in Vietnam. In general, this species is more common in Laos.


Crested gibbons undergo a series of changes to their appearance throughout their lives. While these changes follow a pattern, each species exhibits differences in how this process unfolds as they mature. Sometimes these differences are obvious, other times they are frustratingly subtle. Suffice it to say, this phenomenon has created a great cause for debate in classifying crested gibbons for decades and has resulted in somewhat of a taxonomic mess that researchers are still working to clean up.

Initially, most scientists believed there was one general taxon, dubbed Nomascus concolor, and a small variety of subspecies. However, thanks to modern advances, further research into crested gibbons has revealed that they form a quite biologically diverse subset of the gibbon family. New methods and technological advances for gathering and analyzing genetic, morphological, and auditory data have helped us gradually reveal that a number of these subspecies do, in fact, meet the criteria to be described as distinct species.

At various times throughout this debate, the southern white-cheeked gibbon has been thought to be a subspecies of the northern white-cheeked gibbon, the northern yellow-cheeked crested gibbon, or possibly a hybrid of the two. While genetic and acoustic data have already resolved that the southern white-cheeked gibbon is a species in its own right, there is still some confusion about how far and wide their kind is distributed.

Southern white-cheeked gibbon range, IUCN 2008

Size, Weight, and Lifespan

When it comes to size, southern white-cheeked gibbons show hardly any sexual dimorphism. Males and females may be anywhere from 18 to 24 inches (47–64 cm) tall and normally weigh between 15 and 20 pounds (7–9 kg) on average.

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 other apes, gibbons are remarkably small, which is necessary 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 white-cheeked gibbons, like many other gibbon species, exhibit striking variations in appearance based on sex and age. Based on data collected before northern and southern white-cheeked gibbons were found to be separate species, infants are born beige and then change depending on their sex. Males turn black sometime after their first birthday but before they turn two. During this time, they also develop their signature white cheek patches. Females transition to a light orange-yellow color and gain a dark streak on the top of their heads, along with white hair surrounding their faces.

Distinguishing southern white-cheeked gibbons from other crested gibbon species with the naked and untrained eye can be quite challenging. This difficulty has historically posed problems for researchers trying to identify and classify them. Female southern white-cheeked gibbons in particular make this task even harder, as they appear essentially the same as females of their two closest neighbors, northern white-cheeked gibbons and northern yellow-cheeked crested gibbons.

Males, on the other hand, are a bit easier to differentiate. Southern white-cheeked gibbons have smaller and less bushy cheek patches compared to their northern white-cheeked neighbors. Additionally, their white fur tends to extend and surround their mouths, and the white color of their patches makes them immediately distinguishable from northern yellow-cheeked crested gibbons.

Young male southern white-cheeked gibbon in Laos.

Southern white-cheeked gibbons eat a variety of fruits, leaves, and invertebrates. Their diets vary depending on the seasons. For instance, they eat more fruit during the wet season than in the dry season when a lack of fruit forces them to rely more on young leaves.

Everything that makes up a healthy gibbon diet can be found in the middle and upper levels of the canopy. As such, they rarely need to venture to lower levels of the forest. Gibbons typically only go to the forest floor when something is wrong.

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.

Many gibbon species practice both early-to-wake and early-to-bed lifestyles, keeping active only for as long as the sun is up. While 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’ primary form of locomotion is 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, 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. 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 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 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-up 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 while still giving them the space to practice and try out new moves.

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.

Fun Facts

Southern white-cheeked gibbons were likely 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 on the environment.

Researchers have recognized a basic syntax and grammar in gibbon songs, making them a unique model for studying the evolution and development of language. They also exhibit regional dialects.

Daily Life and Group Dynamics

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 southern white-cheeked gibbons wakes early, just as the branches of their canopy home catch the rising sun. Dad is the first to rise. Having slept in a separate tree, the first order of business is to find each other. To do this, gibbons do something that few other primates do: sing! Dad’s song consists of several high-frequency notes issued in one long chain, occasionally interspersed with a loud “boom” call. This not only alerts the rest of his family to where he is but tells any gibbons in the vicinity to stay back; this is his territory.

Mom’s song is a series of notes that increase in pitch; the intervals between them are made shorter and shorter until she finishes with a loud “twitter” call. This begins the pair’s daily duet, which plays out as a call-and-response for up to 30 minutes.

Reunited, the family looks for breakfast, traveling together until they find something palatable to chow down on. Once their bellies are full, they take a break. Mom rests while 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 dusk, the family has already started to wind down. Mom finds a comfortable tree for her and the kids to sleep in where they huddle together for the night, and Dad finds his own tree nearby.

There is still a great deal to discover about the daily lives and group dynamics of southern white-cheeked gibbons, which will be crucial to their successful conservation in the wild.


Like other social primates, southern white-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.

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 southern white-cheeked gibbons sing before rejoining as a family unit and finding breakfast. The male and female take turns in a call-and-response pattern that fills the jungle around them with lilting, squeaky, and sometimes booming 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 fact, acoustic data plays a role in helping researchers determine when types of gibbons are separate species.

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 toward 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 southern white-cheeked gibbons’ communication methods are not well studied but can only become clearer with more research.

Reproduction and Family

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. There is also 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 always 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 white-cheeked gibbon is pregnant for around 200 days. Her newborn is tiny and relies on her completely. Being beige in color, he blends into his mother’s light-colored fur almost completely. 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 on his own. Between three and six years, he leaves his parents to start a family 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.

Close-up of a young southern white-cheeked crested gibbon in Khammouane province, Laos
Ecological Role

All gibbons play critical roles as seed dispersers. When southern white-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.

Conservation Status and Threats

The southern white-cheeked gibbon is classified as Critically 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 white-cheeked gibbons face a host of threats in the wild, habitat loss being the most worrisome, especially in Vietnam. According to Global Forest Watch, Vietnam lost at least 15% of its natural forests in the last fifteen years thanks to human activity and, if trends continue, may lose up to 45% more in the next 30 years. In Laos, as well, the lush primary forests these gibbons call home are, more and more, being converted to farmland, to build human settlements, and to facilitate mining operations.

While the felling of their forests is a huge threat to gibbons, even minor destruction has immediate and adverse effects on their well-being. Because they are strictly arboreal, they 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. In such cases, 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 them sick and even kill them.

As the gaps in their forests widen, gibbon populations become hopelessly 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 likely doomed. 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 white-cheeked gibbons are regularly hunted and killed in Vietnam and Laos for their meat or to be used in traditional medicines. Humans also capture young gibbons to keep or sell them as pets. 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, rehabilitation is a long and time-consuming process, with no guarantee they will be released back into the wild.

The difficulty researchers still have when it comes to classifying crested gibbons and knowing the range and distribution of a species causes problems for those 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 also cause issues for conservationists. Species like the southern white-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 white-cheeked gibbons and their habitat, the follow-through enforcing those protections is not always consistent. This undermines not only the protections but all efforts to conserve gibbon populations in the wild.

Conservation Efforts

The southern white-cheeked 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. Additionally, it is legally protected in both Vietnam and Laos and is found in at least 10 protected areas across both countries.

In Vietnam, where habitat loss and fragmentation represent their biggest threats, southern white-cheeked gibbon conservation efforts remain few. While efforts to protect and rehabilitate Vietnam’s gibbons do exist, there is currently no project protecting or monitoring this particular species. Phong Nha-Ke Bang National Park and Bac Huong Hoa Nature Reserve do undertake regular, dedicated patrols to protect their gibbons from poachers.

Hunting represents a bigger issue for the southern white-cheeked gibbons in Laos than habitat loss, but little is done to curb this trend other than strict enforcement of gun laws. Thanks to certain political and economic interests, forest conversion is currently on the rise in this country, and if efforts are not made to reverse these trends, the country’s gibbons may soon face a similar situation as those in Vietnam do today.

The IUCN notes the importance of conducting new surveys to determine the distribution of southern white-cheek gibbons once and for all. Additionally, ecological and behavioral studies will help conservationists know how to best help this species in the long run.

Association Anoulak (anoulak means “conservation” in the Lao language) is currently investigating the taxonomic status and distribution of gibbons within Nakai-Nam Theun National Protected Area, where they believe the resident gibbon population may be both northern and southern white-cheeked gibbons, one of the two, or a genetic hybrid. As this protected area is one of the largest remaining contiguous forests on the Indochinese peninsula, their results are likely to have important implications for gibbon conservation in this region.


Written by Zachary Lussier, July 2023