How slow are sloths, generally considered Earth's slowest mammal? Distance moved in a day: often just a few yards. Time at rest: up to 20 hours out of each 24. Metabolism: so slow that the tree-dwelling herbivores climb down to defecate only about once a week. That's for the best, because their ungainliness on the ground makes them vulnerable to cars, humans, and other animals.

The sloth skeleton is suited for reclining or hanging upside down in trees. That's how sloths eat, sleep, give birth—and mate. Though the rain forest exhibit at Baltimore's National Aquarium has welcomed four sloth babies, the staff has never seen a sloth birth or copulation, says curator Ken Howell: "I think of them as