- [Host] Thank you to
Brilliant for supporting PBS. A caterpillar and a kangaroo, it's hard to imagine two
animals more different or a stingray and a seagull or a penguin and an ancient armored dinosaur. These animals last common
ancestor lived hundreds of millions of years ago. But there is one thing that all of these and many other animals share they're darker on top and
lighter on the bottom. (upbeat music) All over the world, we find
animals with this pattern, whether they're predators
or prey, whether they live on land or in the ocean or in the sky. So why did this trait
evolve over and over again in species that have almost
nothing to do with each other? (upbeat music) Hey, smart people, Joe
here, back in the 1800s, biologists were scratching their heads. There was just no good
explanation for why animals in a coral reef and a desert and a Savannah and nearly every other type of habitat on our planet all
shared this shading pattern. Then around the turn of the 20th century an answer came from an unexpected place. An American painter named
Abbott Thayer declared that he had cracked the code. According to him, this nearly universal
pattern is camouflage. As a painter, Thayer saw the
world a little bit differently than most people, he knew that to paint a three-dimensional scene on a two-dimensional canvas,
one of the keys was shadows. To give a flat picture,
the illusion of shape, areas cast in shadow were always darker than areas under direct light. But Thayer realized that in
the three-dimensional world of nature, many animals
coloration canceled out the shadows that their
bodies cast under the sun. Take this wolf, the top of
its coat is dark brownish and the colored fades to tan
and then white under the belly. So when a wolf is in the sun,
it's upper body is illuminated while it's light colored
belly is in shadow making its whole coat look
roughly the same shade. Thayer called this obliterate
shading, and he believed that by obliterating shadows, animals made themselves look flatter. And he argued that this let them blend into their backgrounds so seamlessly that they essentially disappeared. Now, the idea of animals camouflaging their shadows wasn't entirely new. Other biologists in the 19th century, had noticed this pattern
in individual species, but Thayer went much further, extended his idea to all kinds of animals. Thayer used his art skills
to actually test his idea. He once painted wooden
birds with darker tops and lighter undersides
to show how they vanished against the colors of
their natural habitats. Supposedly, there are
two ducks in this photo, one on the left that does
not have this shading pattern and one on the right that does. I promise there's two
ducks in that picture. Thayer eventually got
a little carried away and argued that almost
all animal coloration of any type was for camouflage like when he insisted
that flamingos were pink so that they would blend into
the sky at sunset and sunrise. Cute idea, but that's
not how flamingos work. His artist's side got the
best of them with that one but he was onto something
with obliterate shading. I set up this demo to show
you how bonkers this effect is in real life. So the shape on the left
really easy to pick out, it's a white cylinder. It's casting a shadow underneath it. The shape on the right, it's
much harder to pick out. It looks much flatter. It's just black on a black
backdrop, but in fact, it's counter shaded. (swoosh music) These days this effect
that they are called obliterate shading is
known as counter shading. And the idea that it works as camouflage has been
pretty widely accepted. In some ways, the explanation seems too
obvious not to be true, for instance, some animals
that hang out upside down like these caterpillars
have reverse counter shading so their bellies are darker
and their backs are lighter. It's easy to spot them
when they're right side up but when they're in their usual position the shadows disappear, which
seems like pretty good evidence that counter shading has something to do with hiding in the shadows, and that's not the only evidence. In brightly lit habitats like grasslands and deserts where shadows
are especially strong, counter shading patterns
are more pronounced, balancing out those strong shadows. Paleontologists have even
found pigments preserved in dinosaur fossils showing that some dinosaurs were counter shaded. But in science, just because an answer seems
obvious doesn't mean that it's right. Scientists have only
recently started carefully testing Thayer's century old idea in controlled experiments to see if it's really as universal as he claimed. And while some studies have
shown that counter shading does protect animals from being spotted it's still not exactly clear how it works. Even if real animals
don't completely disappear like Thayer's painted
birds, counter shading could still make them harder
to detect because many animals visual systems work by
zeroing in on contrast, the difference between
light and dark in a scene. So got shadows, and you
might jump out to a predator but smooth your contrast
with counter shading and you may be harder to pick out even if you don't completely disappear. Counter shading might also work
by erasing depth perception. So instead of looking
like a round juicy snack, a counter shaded
caterpillar might just look like some flat boring leaf. This is another case where
it's important to remember that not all animals
see the way that we do. Counter shading might make an
animal just barely disappear to our very awesome human vision. But to a predator that
can't see the colors and details that we can, it
might be enough to not get eaten and good enough is all
that matters in evolution. We still don't know which
explanation is right or if maybe counter shading
serves a different function for different animals
and for some species, counter shading might not have
to do with shadows at all. The thing is, if you're a
land animal, it makes sense that you'd want to hide your
shadows because predators and prey mostly see each
other from the side. But many swimming animals
don't have the luxury of just concealing
themselves from one angle. They can be spotted from above, below, or just about anywhere. Plus, hiding shadows
underwater isn't an easy task because underwater sunlight gets scattered by the water and the
particles floating in it. So light comes from above,
below and the sides. So it could be that
counter shading evolved in swimming species for a
completely different reason. To help these animals
blend in when they're seen from different angles. Say that you're a predator
looking at a fish from over here so you're seeing it against
a fairly bright background, the fish's light belly will conceal it against that bright background,
but if you glide up here you'll see it against darker deep water. Now the fish's darker top
side is absorbing most of the strong light shining from above so it's concealed against
this background too. Essentially, no matter
where you look at it from the fish's body reflects about the same amount of
light as its background making it nearly invisible,
at least near the surface, 'cause once you get a few
hundred meters underwater the only light is coming from above. And no matter how white a fish's belly is, it will cast a shadow
against the bright sky. That's why some sea
creatures have little organs on their undersides that produce light. This is called counter illumination and a bunch of fish, squid, crustaceans and even sharks have adopted
this trick as a way to hide their silhouettes when
they're seen from below. The bottom line is that
we don't know exactly how counter shading
works for every animal. In some cases, it may be
even more than camouflage. Consider the penguin, they're
about as counter shaded as animals come. It might help them blend in
with different backgrounds but penguins also seem to use their black and white coats to
regulate their temperature. At least one study found that
penguins turn their backs to the sun when they need to
warm up since the dark part of their coat absorbs more sunlight. What we do know is that both predators and prey are always crafting
visual tricks to try and gain an advantage over the other and evolutionist fashioned all sorts of color patterns to
mess with the visual cues that many species use to
make sense of the world. Whether counter shading
helps animals blend into the background,
erase their own shadows or make three dimensions look like two, it's worked well enough that nature has produced this
pattern over and over again all over earth for at least
tens of millions of years. And that is something that any artist or scientist should be impressed by. Stay curious and scientifically speaking that is why corgis have
such cute white bellies. Oh, hey, I didn't see you there. Thanks for sticking on
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for supporting PBS. Brilliant is an online learning platform for STEM with hands-on
interactive lessons. Brilliant is for curious learners both young and old,
professional and inexperienced. Brilliant courses teach you how to think via interactive lessons, problem solving activities and exercises. And they also teach you
how to solve problems with interactive lessons in STEM. For example, brilliant.org offers a course called computational biology. Computational biology merges
the algorithmic thinking of the computer scientist with
the problem solving approach of the physicist to address
the problems of biology. Since 2000, an ocean of
sequencing data has emerged that allows researchers and
students to ask new questions. Here students develop an intuition for selecting foundational problems in computational biology, like
how do I reconstruct genomes? Sequence alignment, building
phylogenetic trees to look at evolutionary relationships. The course also addresses
some physical chemical issues of molecular biology like RNA folding. To learn more about brilliant,
go to brilliant.org/besmart. This pattern, whether
they're predators or prey whether they live in
the land or in the land. Earthworms in the land. Okay
