Hi I’m Elliott. This is Mental Floss on YouTube. Today I’m going to talk about some
misconceptions about sharks.
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 allow 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 twelve 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, 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, like 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 EIGHTY 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 seal­shaped 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. 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 go right into its mouth. So what should you do? First of all, some experts
claim that swimming away fast from a shark (pre­attack) will 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 pumps 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’s no evidence that it is). Sharks DO get cancer. As of December 2013, scientists
know of 23 species of sharks that can get tumors, including the great white shark. That
same year, researchers discovered a great white with a tumor under its mouth that was
1 foot long and 1 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 IV 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
nanograms/milliliter, 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. Thanks for
watching Misconceptions on Mental Floss on YouTube, which is made with the help of all
these nice people. If you have a topic for an upcoming Misconceptions episode that you’d
like to see, why don't you just leave it in comments and we'll check it out. See you next
week! Bye!