A monkey is a primate with long limbs and a tail (except for marmosets). Most monkey species are arboreal, spending their lives on trees. Others spend part of their time on land. Some monkeys use their tails for balance and vertical leaping; others have prehensile, or grasping, tails. Monkeys have forward-facing eyes and flat noses, as well as opposable thumbs, which give them a great deal of manual dexterity.
They are often playful, and their strong grip on branches makes them good climbers. Some species, like marmosets, live in trees all the time; they are known to hold on tightly for hours or even days! Other monkeys, like vervets and spider monkeys, are able to swing from tree limb to branch using their arms. This movement is called brachiation and it allows them to travel over wide areas of the forest. Orangutans, gibbons and some other apes also brachiate.
In the wild, monkeys usually live in groups that are called troops. Depending on the type of monkey, troops have different social structures. Some monkeys have one male who dominates the troop and has access to female monkeys for mating; this is called monogamy. Other monkeys live in all-male groups or with a mixed group of male and female monkeys. The one-male leader of a monkey troop may face competition from other males trying to overthrow him.
Scientists have found that monkeys imitate each other to learn new skills. For example, a monkey may observe a member of its group make a certain hand gesture and then imitate the same action to get food. A study of capuchin monkeys in the wild showed that a monkey’s motivation to learn a task is affected by whether or not it can be accomplished with help from another monkey. This is the first demonstration of “payoff bias” learning in a wild animal.