Monkeys are highly inquisitive and intelligent animals. Their well-developed brains, free hands, and good vision give them the ability to solve problems. Their curiosity, combined with their ability to learn from experience, sometimes gets them into trouble. Like other primates, monkeys are social animals and live in groups called troops, usually including several females with young and either a single male (as in hamadryas baboons, mandrills, most guenons, and langurs) or several adult males that are unrelated to each other and compete for the right to lead the troop (as in savanna baboons and macaques). They communicate with gestures and vocalizations that may include grunts, squeaks, whistles, howls, barks, and roars.
Most monkeys are arboreal, spending most of their time in the trees. Their long, flexible arms are adapted for grasping branches and leaves. They can move from tree to tree by clinging and swinging on their arms, or by leaping between branches. Their feet, like those of all primates, are adapted for walking on all fours (quadrupedalism). Their digits usually have claws except the big toe, which is often flattened.
Scientists classify monkeys into two groups, New World and Old World monkeys. They think that the ancestors of New World monkeys separated from their Old World relatives during the Eocene epoch, when Africa and South America were much closer together. Some New World monkeys, such as marmosets and tamarins, have tails that are prehensile, or capable of holding on to things. Others, such as the New World capuchins and squirrel monkeys, have tails that are non-prehensile.