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6 Common Myths About Sharks, Debunked

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Sharks are mysterious creatures. Even the origins of the word "shark" are unknown (though it might come from the Mayan word xok). Maybe that's what makes these 400 million year old denizens of the deep so captivating—and why they sometimes fill us with dread. Thanks to that general air of mystery, many myths and untruths about sharks have spread. Here are six for you to sink your jaws into. 

1. Sharks eat humans.

Negative public perception of sharks has proliferated their image as man-eaters since before Steven Spielberg’s summer blockbuster classic Jaws made you afraid to go into the water. But technically speaking, sharks don't seek out humans for food. When an attack does occur, it's most likely that the territorially-minded shark has mistaken the human for its actual prey (a seal, for example). In fact, most of the time, shark bites are actually “exploratory bites” in which a curious shark tries to determine if what it's biting is food. But despite the relatively slim chance of being attacked by a shark, there are some exceptions.

Of the hundreds of known species of sharks, only about a dozen are considered dangerous—including the great white, tiger shark, and bull shark—and are responsible for the most human attacks. The United States has the most recorded attacks, according to the International Shark Attack File; 1,022 total have been recorded between 1670 and 2012. Though Australia is second in total attacks, it has the most reported fatalities (144 as of 2012).

According to the Global Shark Attack File (GSAF), a similarly-named resource that seeks to “provide current and historical data on shark/human interactions” to the public, shark attack indices are divided into five separate categories. The most common categories are “Provoked”—in which “the shark was speared, hooked, captured or in which a human drew ‘first blood’”—and “Unprovoked,” which results when “a shark perceives a human as a threat or competitor for a food source.”

2. Sharks may not hunt and eat humans, but they're all vicious predators.

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There are over 400 different species, so there's no such thing as a typical shark. Yet popular opinion tends to veer towards the great white or the hammerhead out of sheer fascination and fear, thus propagating the myth that all sharks are dangerous and bloodthirsty hunters. While those species—and others such as the blue shark or the mako shark—are apex predators that reside on the top of the food chain, there are plenty of other species of sharks that go against the misconception that all sharks are predatory.

Take the dwarf lanternshark, for instance. This little guy, found off the coast of Venezuela and Columbia, is possibly the smallest shark in the world and can fit in the palm of your hand, reaching a maximum length of 21 centimeters. It’s so little of a threat, in fact, that even fishermen discard them if caught because they’re too small. On the opposite end of the spectrum, the whale shark (above)—the largest fish and shark of them all—reaches lengths up to 40 feet or more, and is a migratory filter feeder whose diet consists of mostly plankton. Though they are still carnivores, these swimming school bus-sized behemoths are so low key that they sometimes allow swimmers to hitch a ride on their dorsal fins.

3. If a shark stops moving, it will die.

Most sharks don't have to constantly swim to breathe or to stay alive. The majority of species use a process called “buccal pumping,” named after the cheek muscles they use to physically filter water into their mouths and over their gills, and can alternate periods of activity and rest.

But about two dozen species—including the great white, the whale shark, and the mako shark—are known as “obligate ram ventilators,” meaning it is mostly essential for them to keep moving to stay alive. Instead of breathing via buccal pumping, obligate ram ventilators pass water through their opened mouths and over the gills while in constant swimming motion so as not to asphyxiate. It's actually easier for these particular species of sharks to keep moving than to stay still, but it is possible for them to catch a break every once in awhile to rest up for a moment before swimming off again.

4. Sharks have endless rows of teeth.

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Sharks don't come out of the womb outfitted with endless rows of teeth. Technically, the serrated and pointy pearly whites just regenerate as needed.

A human tooth rests in a socket and falls out once during adolescence. But a shark’s teeth are attached by soft tissue to the skin covering the jaw, and easily fall out if they wear out or break. The teeth in a shark’s mouth are arranged in progressive rows, and should a tooth fall out, the next one behind it moves up to take its spot, like a rotating dental Rolodex. Some sharks can produce up to eight rows of teeth at once, and it takes a shark as little as 24 hours to produce a replacement tooth. With an average lifespan of 20 to 30 years, a shark can use thousands of teeth over its lifetime. Take the spiny dogfish shark, which holds the record for the longest lifespan at 100 years, and we’re looking at quite a lot of teeth!

5. A shark is just a dumb animal with a brain the size of a walnut.

Maybe it’s because of species like the tiger shark—which might seem dumb because they’re like swimming vacuums, eating almost anything in their paths—that this myth has spread far and wide. But the truth is, a shark’s brain is a complex organ belonging to a large and sophisticated animal.

A full-grown great white shark brain measures about 2 feet long and is a linear Y-shaped string of millions of neurons that arranges its functions into hind-, mid-, and fore-brain groups (as opposed to a human brain, which is folded into a compact, circular cluster). Almost two-thirds of the shark’s brain is devoted to its olfactory organs, highlighting how important it is for a shark to have an acute sense of smell. It’s so huge because certain olfactory stimuli—such as being able to identify prey, recognize aquatic territorial markers, or to find potential mates—are of the utmost importance for the shark’s essential wellbeing.

Any way you look at it, sharks are intelligent creatures that are astutely aware of their environment—so intelligent, in fact, they can even be trained!

6. Sharks don't get cancer.

You’ve heard of disingenuous snake-oil salesman, right? Well how about the shark cartilage salesman? Certain alternative health and nutrition stores sell shark cartilage as a means to ward off cancerous diseases based on the anecdotal evidence that sharks don’t get cancer. There are even books that promise a cure.

But it's a myth—you can read a little about how it got started here. The truth is that there are hundreds of cases of benign and cancerous tumors in sharks that have been reported in science and medical journals. Researchers, including John C. Harshbarger and Gary Ostrander, also proved the myth false by providing evidence of a shark’s ability to get cancer in a presentation of 40 separate cases of cancerous tumors in sharks at the American Association of Cancer Research in June 2000.

This myth isn't just dangerous to people suffering from cancer—because guess what? Shark cartilage doesn't cure cancer—but to sharks, too: It's led to a multi-million dollar industry and decimated shark populations. "North American populations of sharks have  decreased by up to 80 percent in the past decade, as cartilage companies harvest up to 200,000 sharks every month in US waters to create their products," writes Christie Wilcox in a 2011 Scientific American post.

The bottom line? Sharks do get cancer. Anyone who says otherwise is uninformed—or selling something.

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How Does Catnip Work?
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If you have a cat, you probably keep a supply of catnip at home. Many cats are irresistibly drawn to the herb, and respond excitedly to its scent, rubbing against it, rolling around on the floor, and otherwise going nuts. There are few things that can get felines quite as riled up as a whiff of catnip—not even the most delicious treats. But why does catnip, as opposed to any other plant, have such a profound effect on our feline friends?

Catnip, or Nepeta cataria, is a member of the mint family. It contains a compound called nepetalactone, which is what causes the characteristic catnip reaction. Contrary to what you might expect, the reaction isn’t pheromone related—even though pheromones are the smelly chemicals we usually associate with a change in behavior. While pheromones bind to a set of specialized receptors in what’s known as a vomeronasal organ, located in the roof of a cat's mouth (which is why they sometimes open their mouths to detect pheromones), nepetalactone binds to olfactory receptors at the olfactory epithelium, or the tissue that lines the mucus membranes inside a cat’s nose and is linked to smell.

Scientists know the basics of the chemical structure of nepetalactone, but how it causes excitement in cats is less clear. “We don’t know the full mechanisms of how the binding of these compounds to the receptors in the nose ultimately changes their behavior,” as Bruce Kornreich, associate director of the Cornell Feline Health Center, tells Mental Floss. Sadly, sticking a bunch of cats in an MRI machine with catnip and analyzing their brain activity isn’t really feasible, either from a practical or a financial standpoint, so it’s hard to determine which parts of a cat’s brain are reacting to the chemical as they frolic and play.

Though it may look like they’re getting high, catnip doesn’t appear to be harmful or addictive to cats. The euphoric period only lasts for a short time before cats become temporarily immune to its charms, meaning that it’s hard for them to overdo it.

“Cats do seem to limit themselves," Michael Topper, president of the American Veterinary Medical Association, tells Mental Floss. "Their stimulation lasts for about 10 minutes, then it sort of goes away.” While you may not want to turn your house into a greenhouse for catnip and let your feline friend run loose, it’s a useful way to keep indoor cats—whose environment isn’t always the most thrilling—stimulated and happy. (If you need proof of just how much cats love this herb, we suggest checking out Cats on Catnip, a new book of photography from professional cat photographer Andrew Martilla featuring dozens of images of cats playing around with catnip.)

That said, not all cats respond to catnip. According to Topper, an estimated 70 percent of cats react to catnip, and it appears to have a genetic basis. Topper compares it to the genetic variation that causes some individuals to smell asparagus pee while others don’t. Even if a cat will eventually love the smell of catnip, it doesn’t come out of the womb yearning for a sniff. Young kittens don’t show any behavioral response to it, and may not develop one until several months after birth [PDF].

But some researchers contend that more cats may respond to catnip than we actually realize. In one 2017 study, a group of researchers in Mexico examined how cats might subtly respond to catnip in ways that aren’t always as obvious as rolling around on the floor with their tongue hanging out. It found that 80 percent of cats responded to catnip in a passive way, showing decreased motor activity and sitting in the “sphinx” position, an indicator of a relaxed state.

There are also other plants that have similar effects on cats, some of which may appeal to a wider variety of felines than regular old catnip. In a 2017 study in the journal BMC Veterinary Research, researchers tested feline responses to not just catnip, but several other plants containing compounds similar in structure to nepetalactone, like valerian root, Tatarian honeysuckle, and silver vine. They found that 94 percent of cats responded to at least one of the plants, if not more than one. The majority of the cats that didn’t respond to catnip itself did respond to silver vine, suggesting that plant might be a potential alternative for cats that seem immune to catnip’s charms.

Despite the name, domestic cats aren’t the only species that love catnip. Many other feline species enjoy it, too, including lions and jaguars, though tigers are largely indifferent to it. The scent of the plant also attracts butterflies. (However, no matter what you’ve heard, humans can’t get high off it. When made into a tea, though, it reportedly has mild sedative effects.)

The reason Nepeta cataria releases nepetalactone doesn’t necessarily have to do with giving your cat a buzz. The fact that it gives cats that little charge of euphoria may be purely coincidental. The chemical is an insect repellant that the plant emits as a defense mechanism against pests like aphids. According to the American Chemical Society, nepetalactone attracts wasps and other insect predators that eat aphids, calling in protective reinforcements when the plant is in aphid-related distress. That it brings all the cats to the yard is just a side effect.

Because of this, catnip may have even more uses in the future beyond sending cats into a delighted frenzy. Rutgers University has spent more than a decade breeding a more potent version of catnip, called CR9, which produces more nepetalactone. It’s not just a matter of selling better cat toys; since catnip releases the compound to ward off insects, it’s also a great mosquito repellant, one that scientists hope can one day be adapted for human use. In that case, you might be as excited about catnip as your cat is.

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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Good News, Dog Parents: You Can Teach Puppies as Well as Their Canine Moms Can
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If you’ve ever adopted a puppy, you probably know how frustrating it can be to teach your new family member the basic tenets of common decency, like not to pee on the carpet or tear up a whole roll of toilet paper.

In other areas, though, pups are rather impressive learners, capable of mimicking some human behaviors. In fact, for some tasks, they learn just as effectively from watching people as they do from watching other dogs, including their own mothers, a new study in Nature revealed.

Researchers from Hungary and the UK took 48 young puppies of various breeds and studied the conditions under which they can be taught to open a puzzle box containing food. The experiment revealed that the puppies were able to learn how to open the box regardless of whether the task was first demonstrated by a person, their mother, or an unfamiliar dog. In other words, not only are puppies capable of social learning, but they're able to learn tasks from humans they don't know—in this case, the experimenter.

However, researchers were surprised to learn that the puppies were more likely to learn how to open the box by watching an unfamiliar dog than by watching their own mothers. That may be because puppies spend more time looking at—and thus, learning from—an unfamiliar dog that intrigues them. This differs from other species such as kittens, which “learn to press a lever for food more rapidly from their mother than from an unfamiliar adult,” the study notes.

In addition, the puppies were able to perform the task again after a one-hour break, indicating that they had retained some memory of the learning experience.

The ability of dogs to learn from humans has been recorded in previous research. A 2015 study revealed that dogs learn better by demonstration (or the “do as I do” method) than training techniques that involve a system of punishments and rewards. The "do as I do" approach probably isn't the most practical method of teaching your pup to do its business outside, but if you already have an adult dog at home, your new puppy can follow the older dog's lead and learn by example.

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