Living on the island of Borneo, alongside other interesting primates such as the orangutan and the proboscis monkey, is the red (or maroon) leaf monkey (or langur). This handsome, colorful resident is less-well-known than the other, bigger primates. It eats almost only leaves because it has a very delicate and specially designed stomach. Its stomach has multiple chambers like that of a cow that enables it to break down the fibrous food of its diet. The favorite food is the young leaves of vines and some of the rarer trees found on Borneo. When those are scarce though, it will resort to eating a few seeds. As a last resort it may eat some types of fruits (ones that are not sweet) but will not be coerced by any means into eating certain types of fruit. Eating a banana like a typical monkey is a no no because it would give the monkey severe indigestion by altering the acid balance of its stomach and could even be fatal. They do sample a wide variety of plants and can eat more than a hundred different species of plants. To aid its digestion, and perhaps to supplement its diet with minerals, it regularly eats the clay of termite mounds. To do this, it carefully breaks away the outer layers of the mounds in order to find and eat the uncontaminated (and possibly tastier) interior.

Maroon leaf monkeys live in large family groups of up to 13 members with one dominant male. The family consists of the male, as many females as he can collect and their offspring. Each morning they disperse into the forest to look for tasty leaves, then by afternoon they gather for nap time. They rarely if ever come down from the trees and walk on land except in search of termite mounds.

Males are vocal and can be heard calling loudly in order to warn any mating rivals or individuals intruding on its territory. They are medium-sized monkeys weighing around 13 lbs (6 kg) and standing up to about 22 inches (57 cm) excluding the tail.