White-cheeked Gibbon

Genus and species: Nomascus leucogenys

Distribution and Habitat
White-cheeked gibbons are found in Laos, Vietnam, and southern China in evergreen tropical rainforests and monsoon forests.

Gibbons have a home range of about 75 to 100 acres (0.3 to 0.4 km2) and travel about one mile (1.6 km) per day through this range. They defend approximately three-quarters of their range as their group territory. Defense takes the form of calls from the center of the territory, calls from the boundaries, confrontations across the boundaries, chasing across the boundaries, and, rarely, physical contact between males. Gibbons are arboreal and spend most of their time in the canopy. They rarely stay on the ground for very long. Here at the Zoo the gibbons spend more time on the ground. You may see youngsters wrestling in the grass.

Physical Description
White-cheeked gibbons are 18 to 25 inches (47 to 64 cm) tall and weigh about 15 to 20 pounds (7 to 9 kg). Our females are slightly heavier than males, which is not typical of gibbons in the wild. They exhibit sex- and age-linked color dimorphism. All infants are a beige color. By the time they are one to one and a half years old, their coat has become black with white cheek patches. At sexual maturity (five to seven years), males remain black and females become a beige color again. The external genitalia of males and females are remarkably similar, and the sex of an individual can be hard to determine without close examination. Both sexes have long, dagger-like canines.

Social Structure
Like all gibbons, white-cheeked gibbons live in small, monogamous families composed of a mated pair and up to four offspring. They are physically independent at about three, mature at about six, and usually leave the family group at about eight, though they may spend up to ten years in their family group.

Gibbons are one of the few apes where the adult female is the dominant animal in the group. The hierarchy places her female offspring next followed by the male offspring and finally by the adult male.

Grooming is an important social activity between adults, between sub-adults, and between adults and young. Infant centered play behavior is another common social activity.

Communication
Vocalization (see gibbon communication information) is a major social investment. The basic pattern is an introductory sequence where both male and female “warm up,” followed by alternating sequences of male and female calls and of female great calls, usually with a male coda at the end. Calls are often accompanied by behavioral acrobatics.

Reproduction and Development
The menstrual cycle is 28 days, and the gestation period is seven months. White-cheeked gibbons give birth to a single offspring every two or three years. Infants cling to their mothers from birth. Newborns are often found clinging horizontally across the female's abdomen. This allows the mothers to sit with their knees up as most gibbons do. Older infants orient vertically on the abdomen. Youngsters are weaned early in their second year. Once the offspring reach full maturity they usually leave the family group and search for a territory and mate of their own.

Diet in the Wild
White-cheeked gibbons eat mostly ripe fruits, leaves, and a small amount of invertebrates. Fruit eating occupies about 65 percent of feeding time and young leaf eating about 35 percent of feeding time. They move and feed mainly in the upper and middle levels of the canopy and almost never come down to the ground. Families often feed together in the trees.