Spider monkeys use their long, prehensile tail as a full limb, giving the appearance of possessing a fifth leg. Thirty-five feet (90 cm) long, this tail is capable of retaining all the animal's weight, which averages around 26 lbs (12 kilos). Devoid of hair on the last third of its underside and constantly rolled around a branch, it acts as an additional security by guaranteeing a good adhesion to the primate.