West Indian Sloths and Two-Toed Tree Sloths: Megalonychidae

The family Megalonychidae consists of one living genus (JEE-nus), Choloepus, the two-toed tree sloths. A genus is a group of animals within a family that have some similar characteristics. Megalonychidae also includes eleven or more extinct genera ( JEN-uh-rah; the plural of genus). Since the last of the West Indian sloths is dead, scientists have learned about them by studying fossils. From skeletons found in Haiti, researchers determined that the lesser Haitian ground sloth weighed about 50 pounds (23 kilograms) and was as large as a medium-sized dog. It lived on the ground, and probably also spent time in trees.

The lesser Haitian sloth, like the living Choloepus species, had long limbs, long claws, and a broad body. While tree sloths have tiny tails or none at all, this extinct sloth had a long tail that touched the ground. The ground sloth could balance with its tail and then stand on two feet to reach into trees.

Within Choloepus are two living species, Hoffmann's two-toed sloth and Linné's two-toed sloth (also called the southern two-toed sloth). Both use their limbs to hang upside down in trees. Front limbs are slightly longer than back limbs.

Two-toed sloths have small heads and shaggy fur ranging in color from brown to gray. There is a green tint to sloth fur. The color comes from algae (AL-jee), tiny water plants growing in the sloth's hair. The algae, along with the sloth's natural fur color, camouflage (KAM-uh-flaj) the tree-dweller and keep it hidden from predators, animals that hunt it for food. When hungry, sloths may lick the algae on their fur.

The head and body length of two-toed tree sloths ranges from 2 to 3 feet (60 to 90 centimeters). They weigh from 9 to 18 pounds (4 to 8 kilograms). Sloths have eighteen teeth and 3-inch
(7.5-centimeter) claws on each digit of their feet. There are two digits, or toes, on the front feet and three on the back feet. Sloths use their hook-shaped claws to hang from trees and to move.

Sloths may have as many as eight neck vertebrae, or bone segments. Other mammals, including humans, have seven. Two-toed sloths can turn their heads 180 degrees (a half-circle), which gives the sloths a very broad view of their surroundings.