But they are not found in mangroves and swamp forests, where other specialists displace them. They are faster and more agile than mangabeys and savannah guenons. They specialise in habitats with infrequent supplies of fruit, being able to survive the lean periods by eating more leaves and insects. They search slowly and intensively for insect food, prying open bark and lichen. Insect prey comprise mainly caterpillars, ants and spiders.
Blue monkeys have cheek pouches for more efficient foraging.
One male often accompanies the group and is usually replaced in 1-2 years by one of the solitary males that constantly hover near such groups. Females exclude other females from their territory, while males focus on keeping other males away from the females. Blue monkeys associate with other primates for a better early warning system for predators. They may even gang up with another primate species to oust other primate species. Blue monkeys are hunted as a pest, they sometimes debark young trees in softwood plantations.