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Why Do Brood V Cicadas Spend 17 Years Underground?

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In 1999, the cicadas of Brood V emerged in parts of Maryland, Ohio, Pennsylvania, Virginia, West Virginia, and New York, and then disappeared almost as quickly as they came, leaving only their eggs and molted exoskeletons behind. Once the eggs hatched, the new generation of cicada nymphs crawled underground, where they’ve spent the past 17 years biding their time and living off of fluid from tree roots.

Sometime this week, when the time is right and the soil is warm, they’ll emerge again to molt, enter their adult stage, mate, make a lot of noise, and lay their own eggs. (Brood V was due to emerge sooner, but scientists say the cool spring has delayed them.)

Not all cicadas play this long game of hide-and-seek. Most North American species are “annual cicadas” whose broods emerge every summer and have unsynchronized, two-to-five-year life cycles. Only a handful of species are “periodical cicadas” that have longer, synchronized life cycles, bursting forth together in huge broods every 13 or 17 years.

Seventeen years is a long time to hang out underground. Why do these cicadas spend so much time out of sight and out of mind? And why do they come out all at once?

Periodical cicadas have had scientists scratching their heads for centuries. As one team of researchers explained, “We do not know the answers to these questions but experimental evidence and mathematical models have enabled us to develop some ideas.”

One explanation for the cicadas’ long development times is that the 13- and 17-year cycles keep broods in the same region from emerging at the same time or too quickly after each other, which minimizes competition for resources and prevents interbreeding.

Another idea is that the cycles protect the cicadas from predators and parasites with shorter life cycles. Biologist Stephen Jay Gould explained it like this:

“They are large enough to exceed the life cycle of any predator, but they are also prime numbers (divisible by no integer smaller than themselves). Many potential predators have 2–5 year life cycles. Such cycles are not set by the availability of periodical cicadas (for they peak too often in years of nonemergence), but cicadas might eagerly be harvested when the cycles coincide. Consider a predator with a cycle of five years; if cicadas emerged every 15 years, each bloom would be hit by the predator. By cycling at a large prime number, cicadas minimize the number of coincidences (every 5X17, or 85 years, in this case).”

Staying out of sync with predators’ life cycles keeps the cicadas from becoming a reliable source of food. (It also prevents the predators from adapting or devising better ways of feeding on the cicadas.) This hypothesis is difficult to test because cicada emergences are so far apart, but mathematical models developed by researchers support the idea.

The long cycles might also be the result of North America’s prehistoric climate. Periodical cicadas evolved during a time when glaciers advanced and retreated over what is now the eastern U.S. Temperatures would have been unpredictably warm or cool and often too low for the insects to fly or mate. When researchers calculated the chances of survival for cicadas with different life cycles in this kind of climate, they found that the longer the insects stayed underground, the lower the chance they’d emerge during a too-cool summer. Over time, the scientists suggest, insects with shorter cycles died out, while those who by chance took longer to develop survived and reproduced.

There are other explanations for the broods’ impressive synchronization and overwhelming numbers. Some scientists suggest that high population densities are necessary to produce the deafening choruses that males use to attract mates. Another idea is that there’s safety in numbers. Cicadas don’t have much in the way of defenses, but when billions emerge at the same time, there are simply too many for predators to eat them all. Even after birds and other animals have had their fill, there are plenty of cicadas left to mate, lay eggs, and start the 17-year process yet again.

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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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