SOUTHERN WHITE-CHEEKED GIBBON

Nomascus siki

Geographic Distribution and Habitat
Southern white-cheeked gibbons are primarily found in southern Laos and north-central Vietnam, east of the Mekong River. There is an overlap between the ranges of southern white-cheeked gibbons and northern white-cheeked gibbons.

They gibbons prefer lowland broadleaf evergreen and karst forests and live at elevations around 98.4–328 feet (30–100 m). They live in a subtropical climate with an average temperature of 75 degrees Fahrenheit (23.9 Celsius). During the rainy season from May to October, the average annual rainfall lies between 4.5 and 6.2 feet (1.36–1.9 m). Summers are typically hot and dry with westerly winds.

Southern white-cheeked gibbon range, IUCN 2008

Size, Weight, and Lifespan
One of the reasons why gibbons are known as “lesser apes” is due to their reduced body size compared to great apes. White-cheeked gibbons exhibit little to no sexual dimorphism in size, with both males and females measuring between 19 and 25 inches (45.7–63.5 cm) long. Males weigh around 12.3 pounds (5.6 kg) and females typically weigh about 12.8 pounds (5.8 kg). 

White-cheeked gibbons can live up to 28 years in the wild.

Appearance
Southern white-cheeked gibbons are sexually dichromatic, that is, the pilage of each gender is quite different, clearly identifying female from male, although they are roughly the same size. Females have golden and reddish fur with black faces and black or dark brown hair on the top of their heads. They exhibit white fur around their faces, similar to a halo. Males have coarse black fur and black skin on their faces with white fur on their cheeks. They also have pronounced crests of hair on the crown of their heads. 

Infants are born with a whitish buff coat that turns black after the first two years of life. When they reach sexual maturity, males remain black while females change to a whitish golden colored coat.

Gibbons are very acrobatic primates and are perfectly adapted for an arboreal lifestyle. They have long arms and fingers that allow them to travel through the forest at up to 10 feet (3 m) per swing. Their shoulder joints, specifically, allow for a greater range of motion when swinging.

What Does It Mean?

Brachiation:
Also called arm swinging, it’s a form of arboreal locomotion in which primates swing from tree limb to tree limb using only their arms.

Karst forest:
A distinctive type of landscape consisting of limestone. Karst landscapes also feature caves, underground streams, and sinkholes on the surface. Where erosion has worn away the land above the ground, steep rocky cliffs are visible. 

Monogamy:
A mating system in which a male and female mate exclusively with each other.

Sexual dichromatism:
When male and female animals of the same species display different colorations.

Visit the Glossary for more definitions

Young male southern white-cheeked gibbon in Laos.

Diet
White-cheeked gibbons are frugivorous and spend most of their time searching the canopy for fruit. Leaves are another important source of food for them, as well as flowers, leaf buds, and insects.

There are seasonal differences in fruit intake due to availability or scarcity of food items. Fruit is widely available during the rainy season and white-cheeked gibbons tend to travel less during this time. While foraging for food, gibbons often use a specialized form of locomotion called brachiation. Some scientists suggest that brachiation is an adaptation for their frugivorous lifestyle.

In the dry season they travel greater distances and spend more time foraging in order to find resources. Their dependence on leaves increases since fruit is typically scarce in the dry season.

Behavior and Lifestyle
Southern white-cheeked gibbons are arboreal and mainly live in the canopy of the forest. They travel by brachiating through the trees, or swinging themselves through the trees by grasping the branches overhead. Gibbons are uniquely anatomically adapted for brachiation in the length of their forelimbs, their hook-like fingers, and the mobility of their shoulder joints. While in the trees, they spend most of the time eating.

Gibbons are diurnal primates and spend most of the morning and afternoon foraging, while resting midday. They tend to travel between .50 and .95 miles (.80–1.53 km) each day within their home range. At night, they sleep high in the branches huddled together.

Fun Facts

Gibbons have the longest arm length relative to body size of any primate.

Adult female gibbons are dominant in their family social structure.

White-cheeked gibbons are small members of the ape family. Like all apes, they do not have tails. 

Daily Life and Group Dynamics 
Gibbons are primarily monogamous, and form long-lasting pair-bonds, which is a characteristic rarely found in other ape species with the exception of humans. Groups consist of the adult male and female that mate monogamously, and 3 to 4 offspring. There is a hierarchy within the group and the adult female is dominant followed by her female offspring, then male offspring; the adult male is last.

White-cheeked gibbons spend most of their free time playing and grooming. They vocalize throughout the day in mating rituals and to claim territory.

Communication
Gibbons are known for their singing and prolonged vocal calls. Pair bonded males and females sing duets with each other throughout the day. The development of the duet begins during the juvenile period when young gibbons practice the vocal songs they will one day perform. Young gibbons imitate parts of the females’ and males’ song.

Gibbon songs are sex specific and individually recognizable. It’s suggested that the purpose of gibbon song bouts are for the strengthening of pair bonds and intergroup communication, which includes territory defense or defense of mates from neighboring groups.

Males have large throat sacs that expand to produce a “booming” call followed by a series of other sounds, especially a call known as the “long” call. Females are known for their “great” calls and “twitter” calls. Male and female pairs take turns going back and forth with their specific calls. While making these calls, body displays are also performed. Males may swing between branches and shake them vigorously. These calls can last anywhere from 5 to 17 minutes.

Lesser known communication types in gibbons are olfactory communication, facial expressions, and gestures.

Reproduction and Family
Gibbon family groups usually include one to four offspring with the breeding pair. Pairs typically produce one offspring every 2–4 years. When the infant is born, he or she clings to the mother for about 2 years. After the 2-year period, the infant is weaned. Parental care in white-cheeked gibbons is not restricted to just the mother. Both males and females share the responsibilities. During the period of parental care, infants learn to groom, distinguish between food sources, and learn basic social interactions such as social dominance, playing, and communication.

At about three years of age, offspring reach physical maturity. Offspring of both sexes are thought to be independent and disperse from their natal group at sexual maturity (around 6–7 years).

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

Ecological Role
As fruit eaters, gibbons are important seed dispersers in their rainforest ecosystem.

Southern white-cheeked gibbons are classified as Critically Endangered by the International Union for Conservation of Nature (IUCN, 2015), appearing on the IUCN’s Red List of Threatened Species, and their numbers continue decline. It is estimated that only 600 mature individuals remain.

Declining populations are due to the significant and ongoing threats of hunting, live capture and forest loss. The two countries in which this species occurs, Viet Nam and Lao PDR, have already lost significant tropical forest cover this century, and will likely lose substantially more should current deforestation rates continue over the next few decades. 

In Vietnam specifically, gibbon populations are heavily fragmented due to logging and expansion of agricultural.

The high density of humans in their range continues to threaten the southern white-cheeked gibbon and their habitat. Hunting is also a large threat in both Vietnam and Lao PDR, where body parts from this species are used in traditional medicine and food. They are also victims of the illegal pet trade.

Conservation Efforts
Southern white-cheeked gibbons are listed on the Convention on International Trade in Endangered Species (CITES) Appendix I. They occur in a mixture of protected areas and national parks throughout their range and are legally protected in Vietnam; however, there is inadequate enforcement against poaching and forest encroachment, which still leaves them at risk. More conservation measures are needed, such as the prevention of poaching and wildlife trade, minimizing habitat disturbances and anthropogenic impacts. Additional research and field surveys throughout the southern white-cheeked gibbon’s range is needed to help better define the distribution of not only the southern white-cheeked gibbon populations, but other relative taxon like the yellow-cheeked gibbon (N. gabriellae) and northern white-cheeked gibbon (N. leucogenys).

In Vietnam, Phong Nha-Ke Bang National Park and Pu Mat National Park hold two of the largest populations of gibbons. In Lao PDR, one of the largest population is in Nakai Nam Theun National Biodiversity Conservation Area.

References:

  • ​http://pin.primate.wisc.edu/factsheets/entry/white-cheeked_gibbon/behav
  • https://www.iucnredlist.org/species/39896/1027236
  • https://animaldiversity.org/accounts/Nomascus_leucogenys/– 
  • Dooley H and Judge D. 2007. Vocal responses of Captive Gibbon Groups to a Mate Change in Pair of White-Cheeked Gibbons (Nomascus leucogenys). Folia Primatol. 78:228-239. 
  • Rafacz ML, Margulis SW, Santymire RM. 2013. Hormonal and Behavioral Patterns of Reproduction in  Female hylobatids. Animal Reproductive Science. 137:103-112. 

Written by Tara Covert, May 2020. Conservation status revised Jul 17, 2020.