Tree-dwelling olive colobus live in groups of 5 to 20 individuals with around half containing one adult male with several females and their young (2), and the remaining groups having multiple breeding males (6). Members of a group communicate infrequently with very quiet chirping or burring calls (2). Little is known about reproduction in the olive colobus, but it is believed to be the only Old World monkey in which the female carries the newborn in her mouth while she travels for the first month of life (5) (7). As the infant grows, it may wrap itself around the female’s neck or cling to her body as she moves through the forest (2).
The olive colobus is a shy and retiring monkey that prefers to move through dense growth below ten metres, but climbs higher when feeding amongst the protection of other species (2) (7), particularly Diana monkeys (Cercopithecus diana), with which they have a strong tendency to associate (8), as well as lesser spot-nosed guenons (Cercopithecus petaurista) and king colobus (Colobus polykomos) (5). By foraging with other primate species, the olive colobus benefits from extra eyes and ears to stay alert for predators (8). Males are also believed to use these associations to obtain mating partners; the Diana monkeys are a resource which the males can expect female olive colobus to visit (6). If a predator is spotted, the primary response of the timid olive colobus is to move in to dense foliage and freeze in a hunched or crouched position (2).
The diet of the olive colobus is dominated by young leaves and flowers, with one study showing 27 percent of their diet acquired from lianas, in the tangles of which they spend much of their time (9). Fruit and seeds are also eaten, with the quantities varying seasonally (2), but mature leaves are rarely ingested (9). They eat the food directly off the plant using their mouth, and do not pick it by hand (2).