They are entirely diurnal, and feed intensively in the early morning, then rest for most of the remainder of the day. They are arboreal, and probably only the gibbon exceeds this animal in agility in the trees. Their slender limbs give them diverse locomotor abilities. During travel they use both arboreal quadrapedalism and suspensory brachiation. They move bipedally in trees and can leap. During feeding they almost always hang by their tails and use all four limbs to seize and open fruit. Their social structure is like that of chimpanzees. Groups are generally large, comprising a dozen or so individuals of both sexes and all ages. During feeding the large group breaks up into smaller ones that call to one another for cohesion. Spider monkeys share their food with one another, an unusual behavior. Only rarely do they descend to the ground. When approached in the wild, they sometimes break off dead branches and drop them, attempting to hit the ground observer. They also emit terrier-like barks when approached. The most frequently heard call resembles the whinny of a horse, and is made when monkeys are separated. They will defend their territory, but prefer to make a discrete retreat.