They live in large groups, called troops, composed of males, females and babies. These troops are partly territorial, keeping within their own area at night, but scattering over a wide range in search of food during the day.
Spider monkeys have no thumbs, so their grasping is done with four fingers.
A thumb would almost be an encumbrance in brachiating, and scientists believe that, as the fingers elongated, the thumb gradually disappeared. This allowed the hands to form hooks for branch swinging. This tail functions as a fifth arm. The underside of the tail has no hair and is lined with ridges much like our own fingertips. This gives the monkey a good grip and keeps him from slipping.