"Leaf it" to the sloth: Thousands of years ago, large ground sloths roamed the United States. They ranged in size from an average-size dog to that of an elephant. These ground sloths had long claws and ate plants. They became extinct about 10,000 years ago. Present-day sloths are much smaller, and live in trees. The anteater is their closest relative.

In a nutshell, sloths are slow-moving, nocturnal creatures that spend almost their entire life upside down in the trees. They eat, sleep, mate, and give birth from this position hanging high among the branches. Sloths are the only mammals whose hair grows in the opposite direction from the hair of other mammals. To accommodate their upside lifestyle, the hair parts in the middle of the belly and grows upward toward the back. The hair on the face points upward, too. This allows water to run off during rainstorms!

Each strand of sloth fur has grooves that collect algae, giving sloths a greenish tint. That helps them blend into the trees they call home. The algae also provides nutrients for the sloth when nibbling on the hair during grooming. The only time sloths descend to the forest floor is to poop, which they do about once a week. To move to a new area of trees, sloths often wait for the forest to flood and then swim to their next home.

Big forest cats like jaguars and ocelots, birds of prey such as harpy eagles, and large snakes like anacondas prey upon sloths. They defend themselves with their sharp claws and teeth. And, if needed, sloths can move quickly through the trees.

If our own body temperature changes more than five degrees, it means we are sick! The two-toed sloth is different. It has the lowest and most variable body temperature of any mammal, ranging from 74 to 92 degrees Fahrenheit (24 to 33 degrees Celsius). If there is a long period of cool, rainy weather, a nursing female can get too cold. This causes the bacteria in her stomach to stop working, so the mother can no longer digest her food. The young continues to nurse as its mother starves to death. This chilling phenomenon is called cold-weather orphan syndrome, as sometimes the youngster falls to the ground and needs to be rescued by humans.