Life in the slow lane: for an animal that does little but sleep and eat, the sloth has proved to be a remarkable survivor, hanging out in tropical forests from Nicaragua to Paraguay.

• Author: Cohn, Jeffrey P.

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Al Gardner is a curious man. Once, while walking at night on Barro Colorado Island in Panama, the US Geological Survey wildlife biologist came across a sloth clinging to a vine growing from a tall tree. His curiosity piqued, Gardner wanted to take a closer look at the unusual critter. First, though, he had to dislodge it from the tree. He shook and shook the tree, trying to get the animal to let go, to no avail. In the end, Gardner shrugged his shoulders and gave up, then watched as the sloth climbed quickly (at least for a sloth) and disappeared into the higher branches.

Gardner's experience speaks volumes about sloths, their notorious lack of speed, and their amazing strength, as well as their peculiar lifestyle. Most people who study, care for, work with, or see sloths comment on how strange these animals are. "Everything about them is unusual," says Lynn Yakubinis, a mammal keeper at the Riverbanks Zoo in Columbia, South Carolina, and the person in charge of the sloth studbook--a comprehensive animal registry--for zoos in the United States.

For starters, sloths are found only in the Americas. Six species are known to live in the tropical forests of Central and South America, from Nicaragua down to Bolivia, Paraguay, and northern Argentina. Some people think there might be more. Scientists classify the known species into two families, based on the number of claws on their front feet. Two-toed sloths tend to be larger, with bigger eyes and longer hair than their three-toed cousins.

Today's sloths pale in comparison with some of their extinct relatives, the ground sloths, several of width reached the size of modern elephants. Ground sloths once ranged from Patagonia through Central America, the West Indies, and into the southern United States. Some even roamed as far north as modem-day Alaska. All had small, blunt teeth for browsing leaves and twigs, very large claws, thick hair, and an ability to stand on their hind legs to reach high up in trees for food. All ground sloths died out at the end of the last Ice Age, about 10,000 years ago.

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At one time, scientists classified sloths and theft" anteater and armadillo relatives as edentates, Latin for "without teeth." But, since sloths and armadillos have teeth--albeit only simple rootless molars with no canines or incisors--confusion arose, says Dennis Merritt, professor of biological sciences at DePaul University in Chicago. Most scientists now put sloths, anteaters, and armadillos into a group called xenarthra, which refers to their "foreign" or "strange" joints.

Whatever they are called, sloths have long grayish-brown or brownish-beige hair, rounded heads with flat faces, and simple teeth. There are some color differences, though. Two-toed sloths, for one, have darker brown coats than the three-toed varieties, while maned three-toed sloths have darker brown heads and necks than other three-toed sloths. Then there are male brown-throated three-toed sloths, which have a white or orange patch of fur with a black stripe between their shoulders. However they are colored, all sloths have long, curved claws they use to hang onto branches, climb trees or vines, and defend themselves against predators like harpy eagles and jaguars. If shot and killed by a human hunter, sloths often fail to fall to the ground because their claws continue to tightly grip a tree branch.

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Sloths are rarely seen in the wild by most people and are little studied by wildlife biologists. Their nocturnal, arboreal lifestyle, coloration, and tendency to stay put make sloths difficult to...