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Why Turtles Have Shells (It’s Not What We Thought)

Science is a discipline of questions. Many of these questions have puzzled great thinkers for a long, long time. Others have been completely overlooked, because we assume we know the answers. Case in point: Nobody wondered why turtles have shells; everybody knows they evolved to keep the turtles safe. Except maybe they didn’t. That's what some researchers are saying. They propose that turtle shells originally evolved not to protect the leathery reptiles, but to help them burrow into the ground. Their report was published in the journal Current Biology. 

Turtles have been around for a very, very long time. They were here before, during, and after the dinosaurs, quietly toughing it out through drastic changes in climate and multiple extinction events. One of the earliest turtle ancestors was a long-tailed species called Eunotosaurus africanus that lived 260 million years ago in what is today South Africa.

Based on fossilized remains recovered from the Karoo Basin, scientists believe E. africanus was an evolutionary stepping stone between lizards and turtles. It had a lizard’s long tail and clawed feet, with a modern turtle’s manhole-shaped body and broad, squat ribs curving into a shallow dome.

It’s those ribs that gave researchers pause. Ribs protect the organs in our chest, but they must be big enough to allow our lungs to expand or we’ll have trouble breathing and moving. There’s not a lot of room for variation, says lead author Tyler Lyson, a curator of vertebrate paleontology at the Denver Museum of Nature and Science. 

"Ribs are generally pretty boring bones,” he said in a press statement. “The ribs of whales, snakes, dinosaurs, humans, and pretty much all other animals look the same. Turtles are the one exception, where they are highly modified to form the majority of the shell." 

But, as we said, there’s a reason most animals’ ribs look the same: It’s really hard to breathe and get around otherwise. So why would turtles evolve such a constricting body shape—and how did they survive this long with it? 

Lyson and his colleagues examined 47 different E. africanus specimens, paying close attention to their skeletal structure. The best specimen in the collection is a well-preserved, amazingly complete skeleton that was found by 8-year-old Kobus Snyman on his father’s farm in South Africa.

In looking at the specimens' ribs, the researchers noticed that the overall shape was similar to the ribs of other species. And those species had one thing in common: They were diggers. That wide, short ribcage that would be so restrictive to a person is actually freeing for burrowing animals, because it stabilizes the body and provides power to the limbs. 

This revelation makes a lot of sense, the researchers say. There are plenty of burrowing turtles, both extinct and alive today, and other studies have concluded that heading underground may have helped some species survive the extreme climate changes that killed off so many other species. 

The researchers were especially grateful to their youngest contributor, says co-author Bruce Rubidge of the University of Witwatersrand. "I want to thank Kobus Snyman and shake his hand,” Rubidge says, “because without Kobus both finding the specimen and taking it to his local museum, the Fransie Pienaar Museum in Prince Albert, this study would not have been possible.” 

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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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