What is the unique difference between the three-toed sloths and the two-toed ones?
Sloths are a group of arboreal Neotropical xenarthran (placental) mammals, constituting the suborder "Folivora". Noted for their slowness of movement, they spend most of their lives hanging upside down in the trees of the tropical rainforests of South and Central America. Sloths are so named because of their very low metabolism and deliberate movements.
Sloths can be 60 to 80 cm (24 to 31 in) long and, depending on the species, weigh from 3.6 to 7.7 kg (7.9 to 17.0 lb). Two-toed sloths are slightly larger. Sloths have long limbs and rounded heads with tiny ears. Three-toed sloths also have stubby tails about 5 to 6 cm (2.0 to 2.4 in) long. Sloths are unusual among mammals in not having seven cervical vertebrae. Two-toed sloths have five to seven, while three-toed sloths have eight or nine. These extra neck vertebrae allow the three-toed to turn their heads some 270 degrees.