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Wikimedia Commons
Wikimedia Commons

Swimming with Giant Sloths

Wikimedia Commons
Wikimedia Commons

Sloths aren’t the most impressive swimmers. When today’s two- and three-toed sloths take a dip, they bob along in an awkward doggy paddle. But there used to be sloths that evolution had specifically adapted to be at home in the water. Over the course of four million years, one lineage of giant sloth shuffled into the seas.

The classic image of a giant sloth is of a bulky mammal lazily plucking leaves in an Ice Age forest. But the swimming sloths—named Thalassocnus—lived along the beaches of Peru between 8 and 4 million years ago. There were no tree stands here for the sloths to waddle among. The sea met the desert, and it was in the waves that the sloths found their food.

Since these sloths were first described from Peru’s Pisco Formation in 1995, paleontologists have identified five Thalassocnus species that lived along the same seaside one right after the other. And, paired with the fact that they were found with marine animals, the skeletons of these sloths suggested that the herbivores were at home foraging in the shallows. From the first species to the last, Thalassocnus looked like a seaside sloth.

But how did Thalassocnus go where no sloth had gone before? In a paper published in the Proceedings of the Royal Society B, paleontologist Eli Amson and colleagues at the Muséum national d'Histoire naturelle, Paris reveal that the secret of Thalassocnus lies inside the beast’s bones.

Compared to other mammals, sloths have unusually dense bones. This was true of the extinct giant sloths, too, and the successive species of Thalassocnus took this feature much further. After cutting into the ribs and limb bones of the first four Thalassocnus species, Amson and coauthors found that the bones of these sloths became increasingly dense until, in cross section, they had almost entirely lost their hollow central cavity. This condition is called osteosclerosis. And more than that, the sloth species show a swelling of some bones called pachyostosis that allowed the mammals to pack more bone tissue on.

Paleontologists have seen these features in other amphibious mammals. The bones of the earliest whales and sea cows—mammals that were caught transitioning into a fully aquatic life—show the same characteristics. And these changes to bones were not pathological. They track the evolution of skeletons as bone ballast.

While lungs filled with air are vital to keep swimming mammals from drowning, they can also be a liability for diving. Big, air-filled lungs buoy submerged animals up. Denser, swollen bones allowed early whales, sea cows, and, yes, giant sloths to more easily achieve neutral buoyancy underwater and therefore use less energy to stay beneath the surface.

Thalassocnus was far from being the Michael Phelps of the sloth world, though. The sloth still retained the familiar, bulky body shape of its ancestors. Instead of becoming a streamlined swimmer, Thalassocnus probably favored a method used by marine iguanas today. Anchored down by hefty bones, the sloth gripped the bottom with huge claws and scooped up soft plants as it wafted in the surf. Sloths slurped seaweed by the sea shore.

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Big Questions
What's the Difference Between Gophers and Groundhogs?
Gopher or groundhog? (If you chose gopher, you're correct.)
Gopher or groundhog? (If you chose gopher, you're correct.)
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Gophers and groundhogs. Groundhogs and gophers. They're both deceptively cuddly woodland rodents that scurry through underground tunnels and chow down on plants. But whether you're a nature nerd, a Golden Gophers football fan, or planning a pre-spring trip to Punxsutawney, Pennsylvania, you might want to know the difference between groundhogs and gophers.

Despite their similar appearances and burrowing habits, groundhogs and gophers don't have a whole lot in common—they don't even belong to the same family. For example, gophers belong to the family Geomyidae, a group that includes pocket gophers (sometimes referred to as "true" gophers), kangaroo rats, and pocket mice.

Groundhogs, meanwhile, are members of the Sciuridae (meaning shadow-tail) family and belong to the genus Marmota. Marmots are diurnal ground squirrels, Daniel Blumstein, a UCLA biologist and marmot expert, tells Mental Floss. "There are 15 species of marmot, and groundhogs are one of them," he explains.

Science aside, there are plenty of other visible differences between the two animals. Gophers, for example, have hairless tails, protruding yellow or brownish teeth, and fur-lined cheek pockets for storing food—all traits that make them different from groundhogs. The feet of gophers are often pink, while groundhogs have brown or black feet. And while the tiny gopher tends to weigh around two or so pounds, groundhogs can grow to around 13 pounds.

While both types of rodent eat mostly vegetation, gophers prefer roots and tubers (much to the dismay of gardeners trying to plant new specimens), while groundhogs like vegetation and fruits. This means that the former animals rarely emerge from their burrows, while the latter are more commonly seen out and about.

Groundhogs "have burrows underground they use for safety, and they hibernate in their burrows," Blumstein says. "They're active during the day above ground, eating a variety of plants and running back to their burrows to safety. If it's too hot, they'll go back into their burrow. If the weather gets crappy, they'll go back into their burrow during the day as well."

But that doesn't necessarily mean that gophers are the more reclusive of the two, as groundhogs famously hibernate during the winter. Gophers, on the other hand, remain active—and wreck lawns—year-round.

"What's really interesting is if you go to a place where there's gophers, in the spring, what you'll see are what is called eskers," or winding mounds of soil, Blumstein says [PDF]. "Basically, they dig all winter long through the earth, but then they tunnel through snow, and they leave dirt in these snow tunnels."

If all this rodent talk has you now thinking about woodchucks and other woodland creatures, know that groundhogs have plenty of nicknames, including "whistle-pig" and "woodchuck," while the only nicknames for gophers appear to be bitter monikers coined by Wisconsin Badgers fans.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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Animals
Watch Christmas Island’s Annual Crab Migration on Google Street View
Google
Google

Every year, the 45 million or so red crabs on the remote Australian territory of Christmas Island migrate en masse from their forest burrows down to the ocean to mate, and so the female crabs can release their eggs into the sea to hatch. The migration starts during the fall, and the number of crabs on the beach often peaks in December. This year, you don’t have to be on Christmas Island to witness the spectacular crustacean event, as New Atlas reports. You can see it on Google Street View.

Watching the sheer density of crabs scuttling across roads, boardwalks, and beaches is a rare visual treat. According to the Google blog, this year’s crabtacular finale is forecasted for December 16, and Parks Australia crab expert Alasdair Grigg will be there with the Street View Trekker to capture it. That is likely to be the day when crab populations on the beaches will be at their peak, giving you the best view of the action.

Crabs scuttle across the forest floor while a man with a Google Street View Trekker walks behind them.
Google

Google Street View is already a repository for a number of armchair travel experiences. You can digitally explore remote locations in Antarctica, recreations of ancient cities, and even the International Space Station. You can essentially see the whole world without ever logging off your computer.

Sadly, because Street View isn’t live, you won’t be able to see the migration as it happens. The image collection won’t be available until sometime in early 2018. But it’ll be worth the wait, we promise. For a sneak preview, watch Parks Australia’s video of the 2012 event here.

[h/t New Atlas]

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