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This Crab's Secret to a Happy Relationship Is a Turtle's Butt

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BMC Ecology via Wikimedia Commons // CC BY 2.0

Columbus crabs (Planes minutus) are tiny and can’t swim very far, but they manage to get around—in more ways than one. They make their homes on anything that floats—logs, ship hulls, plastic junk, and even sea turtles—and ride the waves around. On many of these mobile “islands,” the crabs are plentiful and have an “inclusive” mating system, say biologists Joseph Pfaller and Michael Gil. Sometimes, the crabs like to play the field, readily switching among multiple mating partners. At other times, though, they have more monogamous love lives, sharing their territory with just one other crab, with whom they form a mating pair. The difference? The size of the place where they shack up, say the researchers. They recently published their findings on the crabs' love life in Biology Letters.

Pfaller and Gil were curious about what made the crabs switch between these two mating styles. What was it about a particular turtle or piece of debris that pushed them toward swinging, or into settling down? For many crustaceans that live on or in other animals, it’s the size of the host that dictates how many critters call it home and how they mate. The biologists wondered if the same was true for Columbus crabs, who thrive in both living and non-living “hosts.”

To find out, they took a boat out into the North Pacific Subtropical Gyre—site of the Great Pacific Garbage Patch—and collected dozens of pieces of plastic debris, from soda bottles to boat bumpers. Back in the lab, they measured the size of the debris and recorded the number of crabs, and each crab’s life stage (adult or juvenile) and sex. To get data on sea turtles, they dove into scientific journals instead of the ocean, reading through 270 records of sea turtle–Columbus crab associations, and recording the same information they did with the debris.

Pfaller and Gil found that even though Columbus crabs can travel far and wide on a hunk of plastic or a sea turtle, they still seek out the cozy confines of a burrow-like refuge on their floating homes. On pieces of debris, they usually take up residence in stalks of barnacles. On sea turtles, the crabs tuck themselves into the supracaudal space—the small area between a turtle’s tail and shell. The researchers say that it’s the size of these refuges, and not the overall size of the host, that matters most when it comes to the size and demographics of a crab community—and thus how they mate.

Bigger refuges, of course, support more crabs. A bigger space with more neighbors makes it harder for any one crab to defend a chunk of real estate or a mate, so they opt for a more open and free mating system. A smaller refuge, on the other hand, can only hold so many crabs and is easier to monopolize, leading the crabs to form monogamous pairs and fend off rivals. This was almost always the case for crabs living on sea turtles, where Pfaller and Gil found lone male–female pairs far more frequently than expected by chance. Because a typical turtle’s supracaudal space is the perfect size for two crabs, the researchers say, “sea turtle symbiosis specifically promotes social monogamy.” Who would have thought that living on a turtle’s tush could provide the key to a happy, stable relationship?

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Animals
Watch a School of Humpback Whales 'Fish' Using Nets Made of Bubbles 

Just like humans, humpback whales catch many fish at once by using nets—but instead of being woven from fibers, their nets are made of bubbles.

Unique to humpbacks, the behavior known as bubble-net feeding was recently captured in a dramatic drone video that was created by GoPro and spotted by Smithsonian. The footage features a school of whales swimming off Maskelyne Island in British Columbia, Canada, in pursuit of food. The whales dive down, and a large circle of bubbles forms on the water's surface. Then, the marine mammals burst into the air, like circus animals jumping through a ring, and appear to swallow their meal.

The video offers a phenomenal aerial view of the feeding whales, but it only captures part of the underwater ritual. It begins with the group's leader, who locates schools of fish and krill and homes in on them. Then, it spirals to the water's surface while expelling air from its blowhole. This action creates the bubble ring, which works like a net to contain the prey.

Another whale emits a loud "trumpeting feeding call," which may stun and frighten the fish into forming tighter schools. Then, the rest of the whales herd the fish upwards and burst forth from the water, their mouths open wide to receive the fruits of their labor.

Watch the intricate—and beautiful—feeding process below:

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Big Questions
Why Do Dogs Love to Dig?
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Dog owners with green thumbs beware: It's likely just a matter of time before Fido turns your azalea bed into a graveyard of forgotten chew toys. When dogs aren't digging up your prized garden, they can be found digging elsewhere in your yard, at the beach, and even between your couch cushions at home. But what exactly is behind your dog's drive to turn every soft surface he or she sees into an excavation site?

According to Dr. Emma Grigg, an animal behaviorist and co-author of The Science Behind a Happy Dog, this behavior is completely normal. "When people say 'why do dogs dig,' the first thing that always comes to mind is 'well, because they're dogs,'" she tells Mental Floss. The instinct first appeared in dogs' wolf ancestors, then it was amplified in certain breeds through artificial selection. That's why dogs that were bred to hunt rodents, like beagles and terriers, are especially compelled to dig in places where such animals might make their homes.

But this tendency isn't limited to just a few specific breeds. No matter their original roles, dogs of all breeds have been known to kick up some dirt on occasion. Beyond predatory urges, Dr. Grigg says there are two main reasons a dog may want to dig. The first is to cool off on a hot day. When stuck on an open lawn with little to no shade, unearthing a fresh layer of dirt untouched by the sun is a quick way to beat the heat.

The second reason is to stash away goodies. Imagine your dog gets bored with chewing his favorite bone but knows he wants to come back for it later. Instead of leaving it out in the open where anyone can snatch it up, he decides to bury it in a secret place where only he'll be able to find it. Whether or not he'll actually go back for it is a different story. "There's a disconnect with modern dogs: They know the burying part but they don't always know to dig it up," Dr. Grigg says.

Because digging is part of a dog's DNA, punishing your pet for doing so isn't super effective. But that doesn't mean you should stand idly by as your yard gets turned inside-out. When faced with this behavior in your own dog, one option is to redirect it. This can mean allowing him to dig in a designated corner of the yard while keeping other parts off-limits, or setting up a raised flowerbed or sandbox especially to satisfy that urge. "You can get him interested in the area by burying a couple bones or some interesting things in there for him to dig," Dr. Grigg says. "I like the idea of buried treasure."

If your dog's motive for digging is more destructive than practical, he may have an energy problem. Dogs require a certain amount of stimulation each day, and when their humans don't provide it for them they find their own ways to occupy themselves. Sometimes it's by chewing up shoes, toppling trash cans, or digging ditches the perfect size for twisting ankles. Fortunately, this is nothing more walks and playtime can't improve.

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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