The Encyclopedia Americana (1920)/Sloth
SLOTH, an edentate mammal of the family Bradypodidæ. (See Edentata). The best known are the unau or two-toed sloth (Cholœpus didactylus) of Brazil and the ai or three-toed sloth (Bradypus tridactylus) of northern South America. This family is distinguished by the flat short head, and the elongated legs, furnished with powerful compressed and curved claws. No incisor teeth exist, but simple columnar molars without enamel are developed. The stomach is somewhat complex. The three-toed species are unique among mammals in possessing nine cervical vertebræ. The long bones are solid, and destitute of marrow or medullary cavities. Being adapted solely for an arboreal life, the fore-limbs exhibit a much greater length than the hind-limbs, and a powerful muscular organization. The forearm possesses an unusual degree of mobility, the feet being strong and the claws very powerful and permanently flexed. The usual mode of progression of these animals is to move back downward suspended from the branches of trees, and they are known to sleep in this curious position. On the ground the sloths are entirely out of their element; the feet being jointed in an oblique manner to the limbs, the palms and soles are thus naturally turned inward, and the claws themselves are bent inward toward the soles of the feet. Three toes exist in this species, and its general color is a brownish gray, with darker tints on the face and limbs. The fur is of very coarse character, and when the animals are living in their damp native forests is tinted green from the presence upon it of a minute green plant or alga. The unau, as its specific name implies, has but two toes, and 23 pairs of ribs exist, no mammal possessing a greater number than this. Its average length is about two feet, and its color is a lighter gray than that of the ai. The tail in both species is either wanting, or at the most is rudimentary. The sloths are proverbially sluggish in their movements and are remarkably tenacious of life. They are nocturnal and feed upon leaves and shoots which are secured with the slender prehensile tongue. Generally only the two genera mentioned above are recognized, but by some zoologists the three-toed sloths are subdivided. The number of species is also doubtful. About 15 have been described, all confined to South and Central America. The Megathium and Mylodon (q.v.) were gigantic sloth-like terrestrial mammals, the remains of which are found abundantly in the Pleistocene deposits of North and South America. Consult Flower and Lydekker, ‘Mammals Living and Extinct’ (London 1891); Leidy, ‘Extinct Sloth Tribe of North America’ (Washington 1855); Bates, H. W., ‘Naturalist on the Amazon’ (1910).