Be Smart

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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.

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And thank you to Brilliant 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.

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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.

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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.

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