Nomascus nasutus

Geographic Distribution and Habitat

The cao-vit gibbon (Nomascus nasutus) is an ape species found primarily in the trees of a small forest on the border of northern Vietnam and southeast China.

Cao-vit gibbons—sometimes known as eastern black-crested gibbons—have had a rough go of survival. Decades ago, the ape species covered an expansive forest east of the Red River throughout Vietnam and China. But population numbers began to decrease, and by the 1960s cao-vit gibbons (pronounced cow-veet) were considered to be extinct.  

In 2002, to the surprise and delight of Vietnamese scientists, a small population of cao-vit gibbons was rediscovered along the northern Vietnam border. Years later, more were found in China. Today, cao-vit gibbons have made a comeback, but they are still the world’s second-rarest primate with a small population of just 135.


Cao-vit gibbons were once thought to be either conspecific or a subspecies of the Hainan black crested gibbon (Nomascus hainanus). But differences in molecular data, vocal communication, and fur coloration separate the cao-vits from other crested gibbons. 

Cao-Vit gibbon range, IUCN 2008

Size, Weight, and Lifespan

Gibbons are classified as lesser or “small” apes because of their small size compared to great apes like gorillas and chimpanzees. They are slender primates with long limbs, including the longest arms of any primate.

Like all apes, cao-vit gibbons do not have tails. Apes in the genus Nomascus average 12 pounds (5.5 kgs) in weight with head and body length ranging from 15.7 to 25.5 inches (40–60 cm). Cao-vit gibbons have no sexual size dimorphism, meaning males and females are the same size.

Because of the cao-vit gibbon’s small population, information about their lifespan is limited. However, gibbons are known to live longer than many primates. For example, the Borneo gibbon (Hylobates muelleri) has been reported to live up to 60 years.


Cao-vit gibbons are sexually dimorphic in appearance. Females’ coats are primarily buff-colored, but with distinctive markings. Their black faces are surrounded by a broad light buff-colored ring. A large black patch starts narrowly at the top of the head near the face and broadens and it downward to the shoulder blades and narrows again to a point further down the back. They have dark fronts, to varying degrees, unlike other female crested gibbons who have lighter bellies. Males are black or dark brown with a small occipital crest (the crest at the top of the head).

Gibbons are small apes. Since they are apes, cao-vit gibbons lack a tail. Their long limbs are all they need to swing gracefully, and with tremendous athleticism, through the forest canopy. They spend about 12% of their time walking upright (bipedally), but their flat, flexible feet were designed for climbing trees and clinging to branches. Our early human ancestors had similar feet and hands to present-day gibbons.

Photo credit: ©Zhao Cao. Seeking permissions.

Cao-vit gibbons are primarily frugivorous, but in the lean season (the period between harvests) they eat lots of leaves and flowers. Most of their time is spent feeding from branches and twigs throughout the middle canopy of the forest. They’re also known to enjoy insects and bird’s eggs when available.

Behavior and Lifestyle

With their slender bodies and long limbs, cao-vit gibbons are built for arboreal (tree-dwelling) locomotion. Their preferred way to travel is brachiation, a method by which they use their long arms to swing from tree limb to tree limb.

Adult females are cautious swingers and prefer bridging over brachiation when their young are in tow. Adult males take riskier routes that involve more leaping through the trees. Gibbons are very fast, swinging through the treetops at a speed of up to 35 mph (56 kph).

Fun Facts

This species got its name from the sound of one of their vocalizations, “cow-veet.”

The cao-vit gibbon was considered extinct in the 1960s, but was rediscovered in 2002. 

There are 135 cao-vit gibbons left in the wild, making them the second-rarest primate in the world (the Hainan gibbon is the first).

Daily Life and Group Dynamics

Cao-vit gibbons are diurnal. They spend their days swinging through the forest canopy and their nights sleeping in the trees. 

Gibbons typically pair for life and maintain small family groups of about 6 individuals. However, polygynous behavior has been observed in cao-vit groups, with one male mating with no more than two females.

Most gibbons are very territorial and they defend their habitats with daily vocalizations. The home range for a group is quite large for gibbons: 321 acres (130 ha). Social interactions among groups of gibbons are light, but occasionally they will groom each other.


Vocalization is a significant part of being a gibbon; cao-vit gibbons were even named after their distinct “cow-veet” call. Songs are emitted as single-frequency bands that sound similar to the calls of rainforest birds. 

Cao-vit gibbons sing to defend their territory, attract mates, and communicate with other cao-vits. They are most vocal at dawn, after emerging from their sleeping trees. Males and females are known to perform morning duets.  

While there are similarities in their songs, there are many variations due to each individual’s unique voice. Gibbons even have regional accents, just like humans!

Reproduction and Family

Cao-vit gibbons live in small groups of one male, one or two females, and their young. Females typically have a single infant every four or five years, with births occurring between October and February.

Females travel through the forest canopy with their young clinging to their belly. Scientists often have difficulty determining a newborn’s gender because they are constantly attached to their mother for their first few months of life. Young will stay in the same family group for up to 10 years, or until their same-sex parent kicks them out.

Photo credit: ©Zhao Chao. Seeking permissions.
Ecological Role

Gibbons play an important ecological role in seed dispersal, which helps the forest stay healthy and sustainable. Seed dispersal also benefits the local communities that use the forest’s resources.

Conservation Status and Threats

The cao-vit gibbon is listed as Critically Endangered by the International Union for Conservation of Nature (IUCN, 2015), appearing on the IUCN Red List of Threatened Species.

There are approximately 135 cao-vit gibbons in the wild, making them the second-rarest primate in the world. The small population is a threat to the survival of this species—but considering the cao-vit gibbon was thought to be extinct until 2002, there is hope that conservation efforts can help increase the population.

Like many other primates, cao-vit gibbons live in a restricted range that is susceptible to habitat loss and degradation. Their remaining home ranges are threatened by logging, wood harvesting, and cultivation for livestock grazing. Hunting and trapping are also extremely detrimental to the very small population and the species’ long-term viability. Other concerns such as inbreeding, disease outbreaks, and climate change threaten the cao-vit gibbon’s survival.


On January 3, 2024, a scientific article, Vocal fingerprinting reveals a substantially smaller global population of the Critically Endangered cao vit gibbon (Nomascus nasutus) than previously thought, asserted these conclusions about cao vit gibbon populations:

“The cao vit gibbon (Nomascus nasutus) is one of the rarest primates on Earth and now only survives in a single forest patch of less than 5000 ha on the Vietnam–China border. Accurate monitoring of the last remaining population is critical to inform ongoing conservation interventions and track conservation success over time. However, traditional methods for monitoring gibbons, involving triangulation of groups from their songs, are inherently subjective and likely subject to considerable measurement errors. To overcome this, we aimed to use ‘vocal fingerprinting’ to distinguish the different singing males in the population. During the 2021 population survey, we complemented the traditional observations made by survey teams with a concurrent passive acoustic monitoring array. Counts of gibbon group sizes were also assisted with a UAV-mounted thermal camera. After identifying eight family groups in the acoustic data and incorporating long-term data, we estimate that the population was comprised of 74 individuals in 11 family groups, which is 38% smaller than previously thought. We have no evidence that the population has declined—indeed it appears to be growing, with new groups having formed in recent years—and the difference is instead due to double-counting of groups in previous surveys employing the triangulation method. Indeed, using spatially explicit capture-recapture modelling, we uncovered substantial measurement error in the bearings and distances from field teams. We also applied semi- and fully-automatic approaches to clustering the male calls into groups, finding no evidence that we had missed any males with the manual approach. Given the very small size of the population, conservation actions are now even more urgent, in particular habitat restoration to allow the population to expand. Our new population estimate now serves as a more robust basis for informing management actions and tracking conservation success over time.”

The authors of the publication: Oliver R. Wearn, Hoang Trinh-Dinh, Chang-Yong Ma, Quyet Khac Le, Phuong Nguyen, Tuan Van Hoang, Chuyen Van Luong, Tru Van Hua, Quan Van Hoang, Peng-Fei Fan & Tho Duc Nguyen 

Conservation Efforts

The cao-vit gibbon is listed in the Convention on International Trade in Endangered Species (CITES) Appendix I, and in annex IB of the Decree No. 32/2006 ND-CP in Vietnam.

Since rediscovering the cao-vit gibbon, Fauna & Flora International (FFI) has made great efforts in protecting the species. FFI works with local partners to create community patrol groups, reduce threats, and secure official protection for their habitats. Their habitats are also protected by the development of protected areas through the Cao Vit Gibbon Conservation Areas in Vietnam and Bangliang Nature Reserve in China. In China, all species of gibbons are protected as National First Class Protected Animals.

Despite drastic threats to the species, cao-vit gibbons have made progress since their rediscovery in 2002. Population estimates have increased from approximately 45 individuals in 2015 to 135 in 2022. To ensure that future generations continue these conservation efforts, a “junior ranger” program was created for local secondary schools to educate students about cao-vit gibbons and forest diversity.


Written by Maria DiCesare, August 2022