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South American Earthworms Build Enormous Mounds of Poop

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Just when you think nature has maxed out on weird phenomena, here come the poop-piling worms of South America. Researchers say enormous earthworms are likely responsible for the huge mounds covering tens of thousands of square miles in the wetlands of Columbia and Venezuela. The scientists published their report in the journal PLOS One.

Those mounds are called surales. These broad green lumps on the landscape can reach more than 16 feet across. There are scores of them, patterning the wetlands like huge green polka dots. There are several types of patterned landscapes, the authors note in their paper, but surales “… must be seen to be believed.” Yet somehow these mysterious mounds have lain largely overlooked and uninvestigated.

Aerial view of surales landscape. Image credit: Delphine Renard

There are two primary theories about the mounds’ formation. One is that they were built by the movements of small animals, insects, or worms; and the other is that they’re the natural product of erosion around circles of plant roots. “Why the plants are regularly spaced has not been addressed in these cases,” write the authors of the current paper. “In addition, we found no authoritative source that demonstrates this phenomenon or even offers an explanation of how it could work.” In other words, they weren't buying it.

Residents of the Orinoco plains are pretty sure worms were responsible, and their names for the mounds (tatuco, zuro, zural, and sural) all refer to piles of worm poop. This is less outrageous than it might sound. Earthworms are plentiful on the plains, and some of them are gargantuan. The researchers decided to figure out if earthworms were making the mounds, and, if so, which earthworms.

The scientists went at the question from all angles, taking samples of plants, soil, stone, and worms, and scanning the landscape using images from aerial drones and Google Earth.

The combination of micro- and macro-level analysis revealed some answers—and some surprises. "We were really impressed not only by the regularity in the size and spacing of mounds in surales landscapes, but also by their spatial extent,” co-author Anne Zangerlé of Technische Universität Braunschweig said in a press statement. “We showed that they occur throughout much of the Orinoco Llanos, in both Colombia and Venezuela, but they have hardly been noticed by ecologists. We were surprised to find that the main driver of these earth-mound landscapes appears to be a single very large earthworm species."

When Zangerlé says “very large,” she’s not kidding. Juvenile Andiorrhinus sp. earthworms can reach more than 3 feet long. There are also a lot of them: This single species makes up for 92.9 percent of local earthworm biomass. They’re kind of in charge. 

To make the mounds, these big, juicy worms (or “soil engineers,” as the researchers call them) suck in muddy wetlands water, process the soil, and then poop it out in dry, hardened towers called castings. Those towers are not just for show; the worms climb up them to get out of the water and breathe. The castings pile up, and they combine to form the once-mysterious mounds.

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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
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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.
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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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