[♩INTRO] For a Medieval knight, armor was a vital body
covering to block the tip of a sword. But armor is more than the unwieldy metal
suit displayed in castle museums. Broadly, it’s any protective layer. So, we still use armor now. Like, a motorcycle jacket keeps pavement from
your skin, and a face shield makes a barrier to pathogens. And other organisms use armor, too except, sometimes, their armor doesn’t look
like anything we’d expect. Sometimes, it’s fancy scales, and sometimes,
it’s a bunch of viruses. First up, though: ants! All insects have a hard body covering called
the cuticle. It’s mostly made of a big old molecule called
chitin, and since ants don’t have bones, it’s
the only rigid structure to anchor their muscles and hold their guts
together. The cuticle also serves as armor, not unlike
knight’s armor. Those layers of chitin are held together by
flexible membranes with elastic proteins, especially in places
like knee joints that need to move. And overall, those hardened layers serve as
a great first defense against a predator looking for a snack. But some insects take their armor one step
further including leaf-cutter ants, according to a
preliminary 2020 paper. If you know anything about leaf-cutter ants, it’s probably that they can hoist nearly
50 times their body weight in leaf cuttings, to take home to their colony fungus gardens. But the soldier ants in particular also have
some pretty impressive armor. These are the big-headed ants tasked with
things like protecting their colony. And the authors of the study found that these
soldiers have a magnesium-calcite coating on their cuticles to make them harder
and tougher. In the soldiers, the mineralized coating accumulates
as they mature, and it overall makes their cuticles more than
twice as hard. But, here’s the thing: There’s a tradeoff
between the strength of armor and the difficulty of maneuvering around in
it. So, why do these ant soldiers need super tough
armor even though it may slow them down? Because their job is to fight other ants! And ant wars, often over food resources, are
brutal. Battles between leaf-cutter and army ants
can last for days, with rows of soldiers locking limbs. And if an ant overpowers its foe, it will
hold them down and bite them to pieces. So, an extra-hard cuticle could be lifesaving. Next up: the Atlantic alligatorfish. These things don’t really look like alligators,
but one thing is for sure: Their armor is so good it’s… maybe a little
excessive. In a study published in July 2020, researchers looked at different species of
alligatorfish. They used data from CT scanners to measure
the amount of material in the fishes’ body coverings — basically,
their hard, armored plates. Then, they compared that to the size of their
internal skeletons. The ratios between the two really varied among
species, but the Atlantic alligatorfish was especially
well armored: Its body covering was ten times larger than
its skeleton by volume. It’s not clear why these alligatorfish are
so armored up, but it could have something to do with predators maybe animals like crabs, or possibly ratfish,
which have dangerous tooth plates. Some species of alligatorfish have even been
found with damage marks on their armor, which suggests they’ve been
in a few scuffles. For now, though, the factors selecting for
such extensive armor will likely remain a mystery until someone spends more time hanging
out in the ocean! Next: Sometimes an organism needs armor for
a specialized purpose like the whale shark. Like other sharks, they have dermal denticles
— a.k.a. skin scales. A dermal denticle is triangular and made of
dentin, a hard tissue derived from the same cells
as teeth. The denticles overlap, with all the points
facing towards a shark’s tail. So, stroke a shark from its head backward,
and it feels smooth. But, expect a bad time in the other direction. These denticles are etched with parallel ridges
that might help streamline swimming, helping water flow more
smoothly along the shark’s body. It’s still up for debate. But either way, what’s odd about whale sharks is that they also have denticles on their
eyes. A 2020 study found that each iris is ringed
by several thousand denticles, which have ridged lobes like a leaf making them different from the streamlined
skin denticles. And the authors speculate that this might
be to prevent mechanical damage. If you look at a whale shark, that kind of
makes sense: Unlike other sharks, they don’t have eyelids. And their eyes also protrude out, potentially
exposing them to hazards. Still, we didn’t used to think these animals
relied much on their vision but this study suggests otherwise. After all, it wouldn’t be worth the cost
of eye armor if seeing wasn’t important. In evolutionary terms, structures that are costly but aren’t useful
don’t normally stick around. So, apparently, they’ve got something to
see after all. And actually, this would also explain something
else the researchers noticed: When you approach a whale shark, they retract and roll their eyeballs back
into their sockets. Which is a sign they’ve got something to
defend. Speaking of specialized armor, check out mantis
shrimp famous for packing a punch. In a feeding style called smashing, they use
club-like appendages to punch prey with extraordinary force, enough to crack
the shells of mollusks and crabs. Male mantis shrimps also punch each other
on the tail as they compete for territory. But this is where their sweet armor comes
in. Species of mantis shrimp that punch have armored
up their last body segment. It’s called the telson, and it forms part
of the tail. And the result here is a long piece of abdominal
armor. And these mantis shrimps don’t fight to
the death. Instead, they do a ritualized fight, punching
each other’s telsons. The ritualized fights probably exist for the
mantis shrimps to learn about each other basically, to size each other up and see if
it’s worth fighting or standing down. So they’re not trying to kill each other. But the telsons do still have to absorb quite
the impact. And 2010 research on the Caribbean rock mantis
shrimp showed exactly how they do it. Instead of setting up a mantis shrimp fight
club, they did… something even weirder. They glued dead mantis shrimps to plexiglass and dropped steel balls on their telsons. They found that the telson is like a punching
bag, absorbing and dissipating impacts. Specifically, the telson’s center, a bull’s
eye to a mantis shrimp opponent, is reinforced with minerals. And other research has shown that they’re
arranged in a layered, spiral-like structures that absorbs energy. Meanwhile, the area surrounding the telson
cushions the blow like a boxer’s punching bag. Curved ridges running lengthwise stiffen it
along the central axis, but also allow the telson to flex inward when
smacked. Oh, and in case you’re wondering: The shrimps do molt, losing their super hard
shells as they grow out of them. And just like a soft-shell crab, a newly-molted mantis shrimp is soft until
its shell hardens after a few weeks. So, if one of these guys encounters another
male… they’ll just flee, since both their clubs
and their telson shields are useless. The animals we’ve talked about so far have
basically had one kind of armor: one adaptation that helps them out in various
situations. But that’s not true for everyone. Some organisms have adaptable armor — like
a genus called Daphnia. Sometimes called water fleas, these are tiny
crustaceans common in freshwater. And they’re eaten by a ton of small organisms, including tadpoles, insects, salamanders,
and minnows. Daphnia is therefore in need of some serious
defenses. The problem is that their wide range of habitats
makes for a wide range of predators. And every predator is different. So, Daphnia employs what’s called induced
body armor, which appears as-needed. Depending on which predators are around, juvenile Daphnia develop different features
as they grow. For instance, Daphnia grow neck spines when
they grow up in a habitat with larval midges. In contrast, Daphnia that grow up around tadpole
shrimps get tail spines and more angular heads. And there are more options than that, too including hard helmets, spiny head lobes,
and jagged crests. What’s happening here is that chemicals
emitted by the predators are actually cueing the water flea’s development. Like, when researchers exposed Daphnia to
a chemical naturally emitted by those midges, that triggered their bodies to start developing those neck spines. Researchers think these customized responses
interfere with how each predator captures and processes prey. And honestly, it’s a really clever strategy
for a group of creatures that live in so many different
environments. Finally, armor, in all its variations, can
even be life-like. Just ask some bacteria. It’s odd to think of bacteria as needing
to protect themselves. But, like any organism, a bacterium faces
threats from its environment. One of the main ones is from viruses called
bacteriophages, which attack, infect, and kill bacteria. Bacteriophages are called phages for short. And after hijacking their hosts’ machinery
to copy their own DNA, they often free themselves by killing the
bacteria specifically, by releasing enzymes that destroy the bacterial cell membranes
or wall. But, some phages have formed a sort of truce
with bacteria. They’re called filamentous bacteriophages, and they’re long, spaghetti-like viruses. These phages do a lot of their business as
normal, replicating their genetic material inside
the bacterium. But when they’re done, they bind to special
receptors on the bacteria, and are transported out of the cell using
special cell machinery. So, the phage gets to replicate and ride around
in the bacterium, but the bacterium survives. And somehow, that’s not even the best part. The phage also secretes liquid crystalline
droplets that form a coating around the bacterium. And this biofilm directly helps the bacteria
by keeping it from drying out, and making it stickier so it can attach to
surfaces more easily. But the biofilm also protects the bacterium from the damaging effects of antibiotics! That’s great for them, but kind of a problem
for us, since that natural hazmat suit gives bacteria
resistance to our medications. And an estimated 80% of our infections have to do with bacteria encased in biofilms. The medical community continues to agonize
over how to get at biofilmed bacteria — such
as those that make plaque on our teeth. New approaches range from photosensitizing
chemicals that make toxic molecules to anti-adhesion coatings
that resist the sticky biofilms. It’s ultimately a work in progress, but
still: watch out, phages! So, overall, the armor worn by knights was
great and everything, but it didn’t outshine the startling array
of armor in nature. Natural armor evolves again and again to defend
organisms against abrasion, competition, predation,
and pathogens. And it does so in some pretty clever ways. If you liked this episode and are a fan of
sharing cool things about the world… Well, you might also be interested in SciShow
Kids! It’s our science channel for early elementary
learners, and we’re releasing new episodes over there
every Monday. If you have a kid or teacher in your life,
we think they’ll really enjoy it. We’ve even got an episode on mantis shrimp! You can find it over at youtube.com/scishowkids. [♩OUTRO]
