Monkeys are the most common type of primate and, with humans and apes, make up one part of the anthropoid suborder or “simian” infraorder. The easiest way to tell them apart is by the presence of a tail; most monkey species have prehensile tails that can grasp branches. Like other anthropoids, they move by clinging to trees and leaping between them or by walking on all fours (quadrupedalism).
Most monkey species are diurnal—active during the day. Monkeys often live in large troops, and the members of these troops are typically closely related. In general, the troop structure is hierarchical. Dominant individuals get better access to mates, food and territory.
In the wild, monkeys are omnivorous. They eat fruits, vegetables and nuts in addition to meat. They also eat bird eggs, insects and small mammals. Some monkeys, such as the cotton-topped tamarin, use body language to communicate with each other. For example, when one monkey in a group of tamarins smiles, it is to signal that food is available. In fact, the same gestures we associate with smiling, such as pulling the lips back to show teeth, are used to show aggression or anger in tamarins.
Some monkeys are very clever, including spider monkeys and Japanese macaques, both of which can use rocks to open nuts. They also have large brain-to-body ratios, and they have a very high rate of learning by observation. This is why some monkeys can solve complex problems, such as figuring out how to open a box.