Nomascus leucogenys
northern white-cheeked gibbon
(Also: white-cheeked gibbon)

Nomascus leucogenys is better known as the white-cheeked gibbon. This species is found only in Southeast Asia. They primarily populate Laos, Vietnam, and Southern China. In Vietnam, N. leucogenys is found to the southwest of the Song Ma and Song Bo Rivers. A close relative, Nomascus concolor, is found northeast of the Song Ma River and northeast of the Song Bo River. The geographical separation is crucial to distinguishing these two gibbon species because N. leucogenys and N. concolor are extremely similar in appearance. ()

Nomascus leucogenys live in the canopy of subtropical rainforests. White-cheeked gibbons hardly ever descend to the forest floor. ()

Nomascus leucogenys are not sexually dimorphic in size. Both males and females grow to weigh an average of 5.7 kg. Likewise, both sexes reach similar lengths, from 45 to 63 cm long. White-cheeked gibbons are, however, dimorphic in fur color. All infants are born with cream-colored fur. At two years of age, the infants' fur changes from cream to black, and they develop white patches on their cheeks. At sexual maturity, males stay black with white cheeks. Females turn back to the original cream color and they lose the majority of their white cheek color. Like all species of gibbons, white-cheeked gibbons do not have tails. They have exceptionally long forelimbs and hindlimbs. Their bodies are built for an arboreal lifestyle. They have an opposable hallux and an opposable pollex. This makes grasping food and holding branches easy. Furthermore, their hands are hook shaped, facilitating brachiation. The body of N. leucogenys is small and they have a remarkably upright posture. Their molar teeth are bunodont and their canines are large and showy. The dental formula is 2/2, 1/1, 2/2, 3/3. Nomascus leucogenys was considered a subspecies of crested gibbon, N. concolor, until 1989. The main difference between the two species is the “mohawk” tuft at the top of the head of N. leucogenys. ()

The average lifespan of Nomascus leucogenys in the wild is twenty-eight years. ()

Nomascus leucogenys individuals are arboreal and live mainly in the canopy of forests. They travel through the trees by brachiation. While in the trees, white-cheeked gibbons spend a great deal of time eating. White-cheeked gibbons live in small families comprising a male adult and a female adult that mate monogamously. They usually have three to four offspring within the group. There is a hierarchy among the family. The female is dominant followed by her female offspring, male offspring, and the adult male is last. Nomascus leucogenys individuals spend much of their free time playing and grooming. White-cheeked gibbons also use vocalizations throughout the day to signal territory, and they are used in mating rituals. ()

Home Range

Nomascus leucogenys live in an area that spans about seventy-five to one hundred acres. They defend their territory. ()

Nomascus leucogenys individuals signal territory by using vocalizations. They also use vocalizations in mating behaviors. In order to signal aggression, N. leucogenys resort to the common threat of opening their mouth wide to show their teeth. White-cheeked gibbons spend much of their time grooming and playing. Grooming and playing allow individual gibbons to form bonds. It is also likely that chemical cues, such as pheromones, are used to communicate reproductive state. ()

Nomascus leucogenys are primarily frugivorous. They especially enjoy eating the pulp of fruits. They are important seed dispersers for some plants. In general, N. leucogenys eat and forage with their family. Unlike other primates that spend half of the day foraging and the other half of the day slumbering, white-cheeked gibbons search for food throughout the day. Early in the morning, they forage high in the canopy. When the sun begins to heat the canopy, they retreat to lower trees in the understory. They are frugivores, but along with fruit, white-cheeked gibbons also eat leaves, flowers, and insects. The type of food that they eat depends on precipitation. When there is a great deal of precipitation, fruit is plentiful and they do not have to travel far to find food. Conversely, they travel great distances in search of food when there is little rainfall. ()

The main threat to Nomascus leucogenys is forest clearing, and therefore, their main predator is humans. It has also been documented that in North Vietnam, some people have hunted N. leucogenys for their meat. Nothing is known about other specific predators of Nomascus leucogenys, but eagles of the family Accipitridae, owls of the family Strigidae, and Panthera pardus are known to prey on N. concolor. Nomascus leucogenys, like N. concolor, live in the canopy and that makes them easy prey for large birds and arboreal carnivores. Gibbons are very agile and remain vigilant in their high, inaccessible habitats, all of which help them avoid predation. ()

Nomascus leucogenys are known to be excellent seed dispersers because they eat fruit. They drop seeds when they eat and when they excrete. ()

There is no known evidence that Nomascus leucogenys effect humans in a negative manner. This is also true for members of the related species Nomascus gabriellae. ()

Miles and Caldecott (2005) reported that Nomascus concolor are kept as pets in Vietnam when infants are plucked from their mothers. As the infant enters adulthood they become a problem and are often abandoned by their owners. They are also kept at zoos around the world. It is unclear whether these authors studied Nomascus concolor or Nomascus leucogenys. ()

Nothing is known about the conservation status of Nomascus leucogenys, but members of the related species Nomascus concolor are endangered due to deforestation, logging, hunting and military activities. ()

A great deal of molecular research has been done recently on Hylobates phylogeny and greater ape phylogeny. There are four discrete divisions of lesser apes and these four subgenera include Bunopithecus, Hylobates, Symphalangus, and Nomascus. These are monophyletic groups. Before there was thorough DNA evidence, all white crested gibbons were considered members of a single species Hylobates concolor. Chromosomal evidence that is available today allows scientists to distinguish the white cheeked gibbon as a member of the subgenus Nomascus, which are characterized by having a diploid number of 52 chromosomes. Nomascus is found from southern China to southern Vietnam, and is found on Hainan Island. Fur coloration, anatomical data, and vocal data are also used to identify different species of gibbons. Within the subgenus Nomascus, there are four separate species that include N. concolor, N. gabriellae, N. sp. cf. nasutus, and N. leucogenys. White-cheeked gibbons have therefore been re-named Nomascus leucogenys. There are also subspecies within each species. For example, recent DNA evidence has distinguished Nomascus leucogenys leucogenys as the northern white-cheeked gibbon and Nomascus leucogenys siki as the southern white-cheeked gibbon. Due to the recent taxonomic advancements using DNA, it is not clear what species is being discussed in much of the older literature. ()