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A weekly show where we debunk common misconceptions. This week, Elliott discusses some misconceptions about sharks!

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Elliott: Hey, I'm Elliott, and this is mental_floss on YouTube.  Today, I'm going to talk about some misconceptions about sharks.

(mental_floss intro plays)

Misconception #1: Sharks have poor eyesight.  People use this myth to explain why sharks are known to attack so many different animals, including humans.  Actually, sharks have something called a tapetum lucidum in their eyes.  These are mirrored crystals behind the retina that allows sharks to see clearly in dark or murky water.  In fact, they can see ten times better than humans in clear water.  Many believe that great white sharks sometimes attack humans because they confuse people for seals.  That would explain why they typically don't eat the people they attack, which I'll get to in a minute, but shark experts maintain that a great white's behavior when attacking a seal is definitely different from when they attack a person.  They tend to attack seals with much more vigor.  They just settle for us. 

Misconception #2: Sharks eat people.  First of all, it's worth noting that there are over 400 species of sharks that we know of.  Only around 12 of those are what experts would consider dangerous to people, including the great white shark, tiger shark, and bull shark, and even still, human isn't an ideal food to them.  They prefer things like fish and seal and porpoise.  Typically, sharks that attack humans will spit them out because of their bones.  Those bones will affect a shark's digestive tract for days, so they choose their food carefully.  And that brings me to

Misconception #3: Sharks will eat anything.  This doesn't make sense if you think about it.  If you've ever been to an aquarium, you've probably seen a shark swimming around with other fish that it seems to have no interest in eating, and I just mentioned sharks won't eat things that negatively affect their digestive tracts.  There are some sharks that will eat pretty much whatever they can find, but there are also species like the basking shark and whale shark that only eat plankton.

Misconception #4: Sharks aren't very smart.  Actually, many sharks have a pretty high learning capacity.  For instance, a 1975 study determined that lemon sharks were able to learn a classical conditioning task a full 80 times faster than a cat or rabbit could.  Experts also consider the great white shark to be intelligent and curious, despite its reputation as mindless.  In a 1996 study conducted in South Australia, a researcher found that 9-31% of white sharks were more likely to investigate a floating square rather than a seal-shaped object, and experts continually find that white sharks will approach objects that are new and unique to their environment.

Misconception #5: If a shark attacks you, you should punch it in the nose.  Okay, before I talk about this, it's worth mentioning that according to the International Shark Attack File, a database on shark attacks, you have a 1 in 3.8 million chance of being attacked by a shark, so don't be worried.  Anyway, it turns out that this common advice might not be the best way to handle an attack.  One major problem is that water affects your perception of where the shark actually is, plus, punching underwater is not that effective.  You ever tried it?  You have a brother?  Me too.  While attempting this on a shark, your fist might miss and just go straight into its mouth.  So, what should you do?  First of all, some experts claim that swimming away fast from a shark pre-attack will also draw more attention to yourself, but if it's too late for that advice to be relevant, poke the shark's sensitive eyes and gills. 

Misconception #6: If a shark stops swimming, it dies.  This used to be the popular belief but it isn't anymore.  Basically, most sharks breathe by swimming fast.  Their gills allow them to absorb the oxygen in water, keeping them alive, but if they need to stay still, there's also a method of breathing known as buccal pumping.  This is when muscles near the shark's mouth pump water, accomplishing the same thing that swimming does.  There are some sharks that aren't capable of buccal pumping, but experts have seen them take a break from swimming from time to time.  They get exhausted, guys. What do you want from them?

Misconception #7: Sharks have sharp teeth.  #NotAllSharks.  Like, the Horn Shark has teeth that resemble molars.  They use them to crush up their prey, and the Basking Shark has tiny hooked teeth.

Misconception #8: Sharks do not get cancer.  This one was popularized in the 1992 book, Sharks Don't Get Cancer, plus many believe that shark cartilage is a cure for cancer, but there is no evidence that it is.  Sharks do get cancer.  As of December 2013, scientists know of 23 species of sharks that can get tumorous, including the great white shark.  That same year, researchers discovered a great white with a tumor under its mouth that was one foot long and one foot wide.  That's a big tumor.

Misconception #9: Bull sharks have more testosterone than any other animal.  This myth gained popularity thanks to Internet rumors and a character in Grand Theft Auto 4 who says he injects bull shark testosterone.  There hasn't been a big study testing testosterone levels in bull sharks.  One study did test three bull sharks and found testosterone levels of 0.1 ng/mL, 2.7 ng/mL, and one with 358 ng/mL, so one did have high levels, but the other two weren't exceptional.  To compare, a male bonnethead shark has been found with 303 ng/mL of testosterone, and male rainbow trout have similar numbers.   This one needs to be tested more, but it's safe to say that the bull shark isn't anything special compared to other animals when it comes to testosterone levels, so if you find one bragging about it, don't listen to it.

Misconception #10: Sharks don't have any predators.  They do have some.  For example, killer whales prey on some sharks, including mako sharks, and let's end this episode on a depressing note, shall we?  Humans are actually considered the shark's greatest predator.  People kill tens of millions of sharks every year for their fins. 

Thank you so much for watching Misconceptions on mental_floss on YouTube, which is made with the help of all of these nice people.  If you have a topic for an upcoming misconceptions episode that you would like to see, why don't you just leave it in the comments and we'll check it out, and I'll see you next week.  Bye.