Trachypithecus poliocephalus

Geographic Distribution and Habitat

The Cat Ba langur (Trachypithecus poliocephalus), also called the Cat Ba hooded black leaf monkey, golden-headed langur, and Tonkin hooded black langur, is endemic to Cat Ba Island off the northeastern coast of Vietnam.  A species is “endemic” when it is native to a particular area and does not live naturally anywhere else in the world. Cat Ba langurs are critically endangered, and are considered by some to be the second-most endangered primate species in the world. The entire species is confined to a 8.5 square mi (22 square km) area—for reference, that’s only slightly larger than the footprint of the Atlanta International Airport. Based on data from two individuals, they seem to have an elevation preference of sea level to 430 feet (130 m).

Cat Ba Island is the largest of the Cat Ba archipelago’s 366 islands, which together form both a UNESCO Man and Biosphere Reserve and Cat Ba National Park. The area is known for its astounding geology, dominated by karst. Karst is a landscape that is formed by limestone deposits in the ground. As the limestone is worn away over time, it leaves behind interesting geologic structures, such as ridges, cliffs, towers, and caves. The Cat Ba archipelago is quickly becoming a major tourism destination, which is proving to have major consequences for the fragile langur population.


Cat Ba langurs were formerly split into two subspecies: Trachypithecus poliocephalus poliocephalus and T. p. leucocephalus. However, further study resulted in the latter, known by its common name white-headed langur, being recognized as a full species. T. poliocephalus, the Cat Ba langur, is now considered a “monotypic species,” or one with no subspecies.

Cat Ba langur range, IUCN 2020

Size, Weight, and Lifespan

Females weigh an average of 15–18 pounds (6.8–8 kg), and the slightly larger males weigh an average of 18–21 pounds (8–9.5 kg). Their head and body length is 21 inches (53 cm) on average, with their tail a bit longer than their body, likely around 28 inches (70 cm) on average. They live to an age of about 20 years.


Cat Ba langurs are covered in long, thick black hair over their bodies. They have a silver chevron running from the tops of their thighs to their lower back, and their black faces are framed in pale tan and golden orange. The hair on their crown grows upwards, forming a cap on the tops of their heads. They have impressively long arms and tails. Baby Cat Ba langurs are a striking golden orange color from head to tail—they are easy to spot in contrast to their parents’ dark bodies. As they grow, their bodies gradually turn black as they develop their adult coloration, a process which is finished by the time they are about a year and a half old. In fact, scientists use the changing coat colors of young Cat Ba langurs to help age them. A purely golden baby is likely only a few weeks old, as their coat and skin colors start changing around their second month of life. This method of aging young is only so helpful; however, as the rate at which their colors change can vary and is believed to be genetic. Adult males and females look mostly the same, with adult males a bit larger, and females possessing a white pubic patch of hair on their groins that the males lack.


Cat Ba langurs are folivorous, meaning that their diet is mostly composed of leaves. About 10–25% of their diet is composed of supplementary foods like fruit and flowers. They have been reported to eat from nearly 100 different plant species in their habitat. In the dry season, they tend to eat more leaves, likely to make up for the lack of other supplemental food during this time of the year.

Behavior and Lifestyle

Cat Ba langurs are semi-arboreal, meaning they spend some of their time in trees and some of it on the ground. They are also diurnal, meaning that they are awake during the day. They usually sleep on ledges or in caves, and come back to the same sleep site night after night. When these areas are disturbed, such as by noisy humans, it can be very disruptive to the langurs because they are so loyal to their chosen sleep sites. Cat Ba langurs spend most of their day resting (about 57% of it, based on one study), about 18% of it foraging for food, 13% socializing, and 10% moving about. In the dry season, they spend more time foraging, while in the wet season, they socialize more. Socialization primarily consists of grooming and, for the younger langurs, playing with each other. The langurs are usually found on the slopes and cliffs that are so characteristic of their unique habitat.

Fun Facts

It is believed that about 12,000 years ago, Ca Ba langurs became confined to Cat Ba Island due to the rising sea level caused by the melting of glaciers when the last ice age ended.

Cat Ba langurs are known to use caves to protect themselves from temperature extremes.

Daily Life and Group Dynamics

The average Cat Ba langur group size is about six individuals, and the entire species is split into 10–11 groups, as of 2019. Because all Cat Ba langurs face disturbance from humans at this point, it is unknown what their natural group size is. Though most groups are composed of one male with multiple females, there are also some single-sex groups and solitary males that travel solo. It is estimated that the average home range size is 50–125 acres (20–50 ha), although this may be altered from the langurs’ natural range sizes due to human disturbance of their habitat. Occasionally, a new male attempts to take over a group, and this can sometimes lead to violence, even infanticide—the killing of infants. As humans have disturbed the natural population significantly, there is now a surplus of young males compared to females, and take-overs and infanticide occur more often than they would naturally. Adult males are the most solitary, and actively seem to avoid young langurs. When disputes happen within the group, they most often occur between adult females.


Research into the specific communication methods among Cat Ba langurs is unfortunately lacking, but they likely communicate through vocalizations. They also use visual communication, such as body posturing and facial expressions, to communicate with one another. The sense of touch is also likely important in group bonding, as members engage in “allogrooming,” or the grooming of one another.

Reproduction and Family

Mating among the langurs tends to occur in and around August, which is the end of the rainy season on Cat Ba Island. The mothers give birth six months later, with births peaking February through April. Little is known about other aspects of their reproductive lives, but based on similar species, all mothers in the group may help others to raise the young, a behavior called “alloparenting.” Because of the predominantly single-male multiple-female group structure, the fathers likely play a limited role in rearing the babies. The vibrant color of the babies likely make it easier for adults to keep track of them. Upon reaching maturity, male young leave to form or take over their own groups, while females stay in the group they were born into—their “natal group.”

Ecological Role

As a result of their living on a small, secluded island, Cat Ba langurs have no natural predators. In fact, their only predators are humans. They form ecological relationships with the plants they consume, and may help to disperse seeds when they eat fruits.

Conservation Status and Threats

The International Union for Conservation of Nature lists the Cat Ba langur as Critically Endangered (IUCN, 2015), appearing on the IUCN’s Red List of Threatened Species. In undisturbed habitat, it is estimated that the peak Cat Ba langur population would have been around 4,000 to 5,000 individuals. By the 1970s and ’80s, the population plummeted to just 500–600. In 2016, the total population was estimated at about 50 individuals. 

While the situation is critical, there are reasons to be hopeful. The most recent population estimates as of December 2023 are at around 80 individuals, higher than any population count since regular surveys started in 2000, reflecting the hard work of conservationists who are actively pulling the species from the brink of extinction. Births have been increasing as well, with about 30% of reproductively active females giving birth in a given year, as of 2019. There were nearly as many babies born between 2014 to 2018, a four-year span, as had been born between 2000 and 2013, the previous thirteen years. In 2023, eleven babies were born, with just one baby not surviving early infancy as of January 2024.

The main threats faced by Cat Ba langurs include poaching and habitat disturbance. Poaching is believed to be the most significant cause of the langurs’ historic population decline. They are primarily hunted for use in traditional medicine and sometimes for sport. Though poaching has declined since 2000, when strict protection measures were put in place, concerns have been raised that the growing population of langurs may result in a resurgence of poaching—a concern that is further validated by a poaching event in 2016.

Habitat disturbance has resulted from Cat Ba Island’s relatively recent popularity as a tourism destination. Because of this, there is now a growing human population on the island and more disruptive activities nearby, which can alter the langurs’ natural behaviors. For example, while the island itself is fairly well protected, noisy tour boats pass by the relatively small island regularly, and there is a growing concern that the noise could interfere with the langurs’ choice of sleep sites. There was also a case of a tourism company installing rock climbing bolts in the middle of a langur sleep site.

Both an extremely small range and an extremely small population mean that the Cat Ba langurs are constantly at a high threat risk, as a relatively minor event, like a natural disaster or disease outbreak, could spell catastrophe for the entire species. For example, the poaching incident that occurred in 2016 resulted in a population loss of about 17 individuals. That may not seem like many, until you realize that those 17 individuals represented about 25% of the population at the time. As the population grows, the risk of extinction will lessen and the species will grow more resilient.

Conservation Efforts

Cat Ba langurs are listed in Appendix II 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. They are also protected under Vietnamese law.

The Cat Ba Langur Conservation Project, administered by the Münster Zoo and Zoo Leipzig in Germany, was founded in 2000 and has played a major role in saving the species since then. The project operates two main programs. The first is a sanctuary within Cat Ba National Park and a buffer zone within Ha Long Bay, adjacent to the Cat Ba Archipelago. About 58% of the current Cat Ba langur population lives within the sanctuary. The second program is aimed at protecting those langurs that live outside the sanctuary. The conservation project works with the national park and local people to establish patrols within the langurs’ range and provide conservation education to the community. Though the highlight of their work is the Cat Ba langur, the project’s goal is the conservation of all biodiversity of the Cat Ba archipelago. Because of the inherent interconnectedness of life, when the langurs are protected, other species benefit. The program’s ultimate goal is to shift focus from a single species to a more holistic view of the island as a whole, protecting the thousands of species that inhabit it.

Further conservation actions that are needed include continuing to protect langur habitat from encroachment and disturbance, a reduction in the isolation of the subpopulations, and an increase in genetic diversity, which could be accomplished by relocating individuals between subpopulations. A 2014 population viability analysis modeled future population sizes under the scenario of significant work being done to interconnect isolated subpopulations, allowing them to act, at least genetically, more like one large population. Under this scenario, the overall population of Cat Ba langurs was dramatically increased compared to scenarios where that work was not done.

While the long-term survival of the Cat Ba langur hangs in the air, there is reason to be hopeful, thanks to the hard work of dedicated conservationists.


Hendershott, R., Behie, A. & Rawson, B. 2016. Seasonal variation in the activity and dietary budgets of Cat Ba langurs (Trachypithecus poliocephalus). Int J Primatol 37, 586–604.

Hendershott, R. 2017. Socioecology of Cat Ba langurs (Trachypithecus poliocephalus): implications for conservation (Publication No. 10758349) [Doctoral dissertation, Australian National University]. ProQuest Dissertations & Theses Global.

Hendershott, R., Rawson, B. & Behie, A. 2018. Home range size and habitat use by Cat Ba langurs (Trachypithecus poliocephalus) in a disturbed and fragmented habitat. Int J Primatol 39, 547–566.

Nadler, T. 2020. The development of pelage coloration in Cat Ba langurs (Trachypithecus poliocephalus). Vietnamese Journal of Primatology 3(2): 23-37.


Written by K. Clare Quinlan, January 2024