There is a tail in Bradypus, though it is short, stout, and blunt, having the appearance of the stump of an amputated limb. Whereas most mammals have seven neck vertebrae, Bradypus has eight or nine, thus allowing greater flexibility, a desirable feature in an animal that is so limited in its other movements. The eighth and ninth neck vertebrae sometimes bear a pair of short, movable ribs. The head is small and round, and the eyes and ears are small. As in the Megalonychidae, there are 5 upper and 4 lower teeth on each side, and thus a total of 18 teeth.
Females have two mammae in the chest region.
Three-toed sloths inhabit forests and spend practically their entire life in trees, where they hang beneath the limbs or sit in a fork. Arboreal locomotion is usually by using the claws as hooks to move along beneath a branch or to climb a tree. Sloths apparently descend to the ground about once or twice a week to urinate and defecate and on occasion to move from one tree to another. They swim readily. If attacked on the ground, they endeavor to catch their adversary with their claws, with which they can inflict severe wounds.