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Environment

Killer Sloth

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The lethargic two-foot-long tree-hugging sloths of the South and Central American tropics are small fry compared with one of their extinct relatives. Megatherium americanum, a ground sloth, stood 12 feet tall, had 7-inch claws, and weighed close to 9,000 pounds. It roamed the arid steppes of southern South America about a million years ago. And it may have been the largest hunting mammal ever to walk the Earth.

Because all existing sloths are vegetarians and because Megatherium lacked the sharp killing teeth typical of carnivores, paleontologists have assumed that it, too, was an herbivore. Richard Fariña, a paleontologist at the University of the Republic in Montevideo, Uruguay, disagrees.

Fariña once shared the majority view on Megatherium, but over the past few years, as he thought about the world in which Megatherium lived, the conventional view of the giant sloth as a lumbering leaf eater made less and less sense to him. The fossil record shows that Megatherium lived on the South American steppes with a surprisingly large number of herbivores and relatively few carnivores. Given the dryness of the climate, Fariña doubted that the steppes could have supported so many plant eaters. Perhaps Megatherium, with its long, knifelike fingers topped with deadly claws, wasn’t an herbivore at all. The sloth’s claws, which most paleontologists thought were used to strip bark from trees, would have made great stabbing weapons.

To test his idea, Fariña analyzed bones in the sloth’s forearm. He examined the olecranon process, the bony part of the elbow where the triceps muscle attaches to the forearm. The triceps extends the forearm, and the speed at which the forearm moves depends on the length of the olecranon. It’s the lever principle, Fariña explains. If you have a long lever arm, you have a strong movement. A digger, like an armadillo, has a very long olecranon process. If the arm is short, you have a fast movement. In predators the structure is invariably shorter. If you wish to capture prey, usually your prey is not very cooperative, so you need to be fast, he says.

Fariña calculated that for an animal of Megatherium’s proportions to achieve maximum speed in a stabbing motion, the olecranon should be from three to five inches long--a close match to its actual length of about 4.75 inches. To Fariña, this is good evidence that Megatherium was a stabber.

But what about the sloth’s peglike teeth, so ill-suited for killing? Fariña says that Megatherium’s teeth were actually very primitive and weren’t specialized for a diet of either meat or plants. So the favorite prey of this omnivore might have been glyptodonts, herbivores that resembled giant armadillos. Megatherium could have tipped them over and stabbed their unprotected underbelly.

Eventually this formidable predator became extinct when another hunter evolved a fast forearm and used it to throw stones and spears. Humans, says Fariña, probably wiped out Megatherium some 8,000 years ago.

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