douc, (genus Pygathrix), any of three colourful species of langur monkeys found in the tropical forests of central and southern Vietnam, southern Laos, and northeastern Cambodia. Doucs are among the most strikingly coloured primates. The head is brownish, but the body appears blue-gray owing to black-and-white bands on each hair. The tail and long cheek whiskers are white, as are the rump patch and the throat, which is bordered by a necklacelike red line. The vivid orange-yellow face has slanting, almond-shaped eyes. Doucs grow to more than a half metre (20 inches) long, not including the tail, which is slightly longer than the body. Males weigh 11 kg (24 pounds), females about 8.5 kg.
The red-legged douc (P. nemaeus) lives from 14° to 20° N latitude. The legs are maroon below the knees, and the forearms are white. The black-shanked douc (P. nigripes) is found south of 14° N and has black legs, gray arms, and a darker face. The ranges of the two species overlap, apparently with very little interbreeding, in the Southern Highlands of Vietnam. In the 1990s a third species, the gray-legged douc (P. cinerea), was discovered in Vietnam in a few isolated forests around 14° N.
Doucs have been only briefly studied in the wild. Their troops vary in size from 4 to 50 or more, with up to a dozen males and twice as many females. They eat mostly young leaves but also consume fruit, flowers, and seeds. Because of extensive deforestation in Vietnam and Laos, all species, especially the gray-legged, are in danger of extinction. Doucs and other langurs are primates classified in the family Cercopithecidae (Old World monkeys).