Unlike their Old World counterparts, these monkeys are superbly adapted to an exclusively high life in the treetops. Living at such heights, they feed almost entirely on leaves and fruit. An arboreal existence is less complicated than life on the ground. There is ample food and few large predators, aside from larger snakes and some of birds of prey. Thus, unlike the more terrestrial Old World monkeys, they did not develop marked size differences between the genders.
Males are also far less aggressive.
They do not have cheek pouches as they can eat at leisure. They have long-raking limbs and some have prehensile tails. Some of them have reduced thumbs, so their elongated fingers act as hooks as they swing and leap through the trees. In those with thumbs, these are not opposable. The largest and most varied group of primates, Old World monkeys, are adapted to a more terrestrial lifestyle.