A distinctive feature of this group of primates is their extreme adaptation to a leaf-based diet. They have developed a four-chambered stomach that functions similarly to the digestive system of ruminants. The first two stomach chambers operate comparable to the rumen of herbivorous ungulates. This allows Angolan colobus monkeys to access the abundant leafy vegetation of evergreen tropical forests and makes them independent of locally and seasonally irregularly available fruits. The fermentation processes that occur in the stomach, in which bacteria break down the plant matter for the monkey’s „host,“ produce considerable amounts of gas. This gas is regularly expelled through belching and is an important indicator of the proper functioning of these specialized and therefore sensitive leaf-eaters. As with ruminants, longer rest periods are necessary for digesting the high-fiber food. The genus is named for its reduced thumb, which is now only a small stump. As inhabitants of dense forests, their contrasting coloration helps maintain group cohesion, as it allows them to easily see each other over long distances. The animals reach sexual maturity at 4 (females) and 6 (males) years of age; young are weaned at 15 months. This species is very rarely kept in zoos.