Spider monkeys have an exceptionally long tail and legs in relation to body length, a prehensile and extremely flexible tail, and thumbs that are poorly developed or lacking. The head is small, and the muzzle prominent. The clitoris of the female is greatly elongated, so that it is often mistaken for the penis of the male. Spider monkeys are found in rain and montane forests and tend to occupy small branches of the high strata of the canopy. Probably only the gibbons exceed Ateles in agility in trees.
Spider monkeys move swiftly through trees and use their tails as a fifth arm or leg.
They pick up objects with the tail and can hang by a single hand or foot or by the tail alone. The normal movement through trees is along the upper surfaces of limbs, with the tail arched over the back. Only rarely do they descend to the ground. When approached in the wild, spider monkeys sometimes break off dead branches weighing nearly 5 kg and drop them, attempting to hit the observer. They also emit terrierlike barks when approached.