Bats and sloths are the only known vertebrates that do not develop osteoarthritis.

The reason these mammals are “immune” to the condition is simply because of the amount of time they spend just “hanging out”...upside down, that is! Osteoarthritis, the degeneration of joints, has even appeared in the fossils of dinosaurs, mammoths and extinct fish! We're not sure exactly what causes osteoarthritis, though it is usually associated with aging. You can learn more about Osteoarthritis and how it works here.

Oddly enough, ground sloths, the extinct cousins of modern sloths, did get osteoarthritis. Unlike today's sloths, who hang in trees, ground sloths walked around on the ground. Preserved ground sloth remains discovered in tar pits have shown that these prehistoric sloths were susceptible to osteoarthritis in their neck vertebra. Click here for a picture of prehistoric sloth osteoarthritis.