monkey, in general, any of nearly 200 species of tailed primate, with the exception of lemurs, tarsiers, and lorises. The presence of a tail (even if only a tiny nub), along with their narrow-chested bodies and other features of the skeleton, distinguishes monkeys from apes. Most monkeys have a short, relatively flat face without great prominence of the muzzle, although baboons and mandrills are notable exceptions. The vast majority of species live in tropical forests, where they move on all four limbs. All but the durukuli of tropical Central and South America are active during the day, moving frequently in bands as they search for vegetation, birds’ eggs, smaller animals, and insects to eat. Monkeys are capable of sitting upright, and, consequently, their hands are freed for many manipulative tasks. Except for a few Old World forms, monkeys are predominantly arboreal, leaping from limb to limb in their travels among the trees. Their hands and feet are both used for grasping and typically have five digits, the thumb and big toe being divergent from the others. Commonly, the digits have flattened nails, but the marmosets have claws on all digits except the big toe, which bears a nail. On the ground, monkeys walk with the entire sole of the foot touching the ground but with the palm of the hand raised. They almost never walk on two legs (bipedally) and can stand erect for only short periods, if at all.

Monkeys have large brains and are known for their inquisitiveness and intelligence. Brain development, combined with the freeing of the hands and well-developed vision, allows them a great latitude of activity. Most are good at solving complex problems and learning from experience, but they do not quite reach the cognitive levels of great apes. Some, especially the capuchins (genus Cebus), spontaneously use objects as tools (e.g., stones to crack nuts). Others, such as baboons, readily learn to use sticks to obtain food. However, in strong contrast to the great apes (gorillas, chimpanzees, and orangutans), most monkeys do not appear to be very good at learning from each others’ experience—individuals more or less have to learn new behaviours for themselves. A significant exception is the Japanese macaque (Macaca fuscata). In field experiments, these monkeys were introduced to new foods such as sweet potatoes and candies wrapped in paper. Once a few individuals had solved the problems of getting at the new foods, their innovations gradually spread throughout entire troops. These experiments have had implications in redefining cultural behaviour.