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article imageSure, sloths are slow, but their evolution was in a big hurry

By Megan Hamilton     Sep 12, 2014 in Science
London - Modern sloths may not be the speediest creatures on the planet but their gigantic ancestors were certainly in a hurry. At least from an evolutionary standpoint anyway, a recent study suggests.
Up until 11,000 years ago, sloths were a very diverse group. Some species shrank until they were within the 13 pound range of our modern sloths, while others grew as large as elephants, The Washington Post reports. While our living three-toed and two-toed sloths look quite similar in size and appearance, they are instead more closely related to the larger extinct sloths than they are to each other.
The evidence that the researchers found suggests that environmental conditions and competition with other species favored larger sloths before they became extinct, HNGN reports.
By examining existing models to see how sloths diversified and began shrinking down to their current size from their huge ancestors, Scientists from University College London (UCL) and University College Dublin (UCD) determined that the sloths ancestors shrunk by about 220 pounds every million years. This is one of the fastest rates of body change ever seen, HNGN reports.
"Today's sloths are really the black sheep of the sloth family. If we ignore the fossil record and limit our studies to living sloths, as previous studies have done, there's a good chance that we'll miss out on the real story and maybe underestimate the extraordinarily complex evolution that produced the species that inhabit our world," Anjali Goswami, of UCL Earth Sciences told UCL News.
Megatherium  an enormous ground sloth.
Megatherium, an enormous ground sloth.
By ДиБгд at ru.wikipedia [Public domain], from Wikimedia Commons
The study, published in BMC Evolutionary Biology, showed that the sloths that grew larger did so fairly rapidly. Some lineages may have even grown as much as 300 pounds every million years, the Post reports. This may sound slow, but it's one of the fastest growth rates that scientists have seen in mammals. For growth to have occurred so quickly, these huge sloths likely had a huge advantage over their smaller relatives. This suggests that in every generation, those that were the largest must have survived, passing on their tremendous genes. A smaller collection of sloth species were successful in shrinking in size, and it's likely that this happened so that they wouldn't have to compete with the giants for the same food.
Indeed, some giant sloths grew to truly epic proportions. Megatherium americanum weighed as much as four tons and was the size of an elephant, UCL reports, while Eremotherium eomigrans weighed as much as five tons and had claws that were a foot long.
The scientists recorded information regarding all known sloth species — living and extinct, and they tested how current evolutionary models explained the vast range in body sizes. They showed that models focusing only on living species didn't do the trick when it came to explaining the changes regarding size. Instead, they found that models which incorporated fossil species showed that they evolved extremely rapidly, and the environmental conditions at the time must have really favored larger body sizes. This may have been due to climate or competition between species, UCL reports. All of this changed again, about 11,000 years ago. Scientists aren't certain what really killed off these magnificent creatures — climate change, human hunters, disease, or perhaps a combination of all of these may have killed them off, but the smaller sloths survived.
"There are many other groups, such as hyenas, elephants and rhinos, that, like sloths, have only a few living species," Dr. John Finarelli of University College Dublin, told UCL. "But if we look into the distant past, these groups were much more diverse, and in many cases very different to their current forms." This method could be used to examine the evolutionary past of other species as well, the authors say.
Living sloths now spend the majority of their time as slow moving tree-dwellers in the tropical rain forests of Central and South America. Moving through the canopy, they cover about 40 yards per day, all the while snacking on leaves, buds, and twigs, the World Wildlife Fund reports. A sloth's metabolism works very slowly and it generally spends 15-20 hours sleeping each day. They are also good swimmers and take an occasional plunge into the water.
The WWF reports that there are two different types of sloths — the two-toed and three-toed, and out of this there are six species:
• Pygmy three-toed sloth (Bradypus pygmaeus)
• Maned sloth (Bradypus torquatus)
• Pale-throated sloth (Bradypus tridactylus)
• Brown-throated sloth (Bradypus variegatus)
• Linnaeus’s two-toed sloth (Choloepus didactylus)
• Hoffman’s two-toed sloth (Choloepus hoffmanni)
This reporter has rescued a baby two-toed sloth, and while they are adorable, they have an iron grip, and if one grabs on to your hand, it feels like its crushing walnuts. Imagine the powerful grip of their ancient ancestors.
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