Adapting to great weight
Generally the primates have tended to evolve larger sizes, with the marmosets being an exception. Greater weight, however, places greater demands on the grasping hands and the South Americanmonkeys have developed a unique way of supplementing them. Their tails have turned into a fifth grasping limb. It is equipped with special muscles so that it can curl and twine, and at the end its inner surface has lost its hair and developed a ridged skin like that on its fingers. So powerful is it that a spider monkey (e.g. Saimiri sciureus) can hang by its tail while foraging for fruit with both hands.