the sloth spends its life hanging upside-down from branches, and has highly specialized third and fourth digits for the purpose. they have short and squat proximal phalanges with much longer terminal phalanges. they have vestigial second and fifth metacarpals, and their palm extends to the distal interphalangeal joints. the arboreal specialization of these terminal phalanges makes it impossible for the sloth to walk on the ground where the animal has to drag its body with its claws.