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Go to Brilliant.org/SciShow to learn more. [ intro ]. In 1896, painter and naturalist Abbott Thayer published a scientific paper with lots of illustrations of dead birds.

But in one of those pictures, he painted the bird's belly the same colors as the rest of its body. And it stuck out like a sore wing. It was way easier to spot in its natural habitat.

Thayer wasn't just a weirdo. He wanted to answer a question that applies to a huge number of animals:. Why do they have light-colored bellies and darker backs?

But 123 years after his paper, we still don't actually know for sure. This pattern is called countershading, and you see it in everything from dogs to nautiluses. Thayer thought there must be something about countershading that helps animals survive; it's just too widespread to be coincidence.

And he settled on two related ideas. First, sunlight makes dark backs look lighter than they are -- so an animal's back ends up looking a lot like its lighter front. Thayer thought that makes it harder to figure out where parts of an animal are just by looking at it -- like, if you're a predator who wants to go for the throat.

Second, he proposed that white bellies look like a sunlit sky, while dark backs look more like shadowy ground. So from above and below, countershaded animals blend into the background. Naturalists spent decades cataloguing examples that seemed to confirm Thayer's ideas.

They saw countershading pretty much everywhere -- any place sunlight creates a lot of contrast. They didn't really find it in places with less sunlight -- like caves or the deep ocean. Seems pretty open and shut!

But over the last thirty years or so, biologists have pointed out that listing examples of countershading is different from proving why it happens. They've been testing Thayer's ideas, like scientists are supposed to do. And their results have been… mixed.

One strategy is to present birds with fake insects, some countershaded and some uniformly colored. In some experiments, birds eat fewer countershaded insects. But in others, birds eat them all the same.

And some researchers find that countershaded mammals don't blend into all environments better than they would without countershading, like Thayer originally proposed. Others say that in computer models, countershaded animals do blend in. And that's pretty much how it's gone.

Whenever Thayer's ideas seem vindicated, they get called into question again. Meanwhile, people have proposed other hypotheses for how countershading can help animals survive. Dark pigments tend to be better at blocking cell-damaging ultraviolet radiation, so maybe animals evolved with more pigment where there's more sunlight.

But nobody's shown that countershaded animals suffer from increased UV exposure as a result of countershading going away -- like if they're albino or something. Dark pigments may also make some materials more wear-resistant, but the evidence for that is thin as well. So none of these hypotheses seem to cover it.

Or maybe it has more to do with how dark pigments interact with heat. It could help animals stay warm. Dark colors absorb more heat, and penguins turn their dark backs to the sun when they're cold.

So maybe that's where their little tuxedos come from. Or it could help some animals stay cool under the right conditions. Experiments have shown that dark pigments are better at dispersing heat via the wind than light pigments.

There's even some evidence that this surface-level warmth thins air and water around countershaded animals, making swimming or flying easier. In the end, it's possible that there's no one reason so many different animals have evolved countershading. Some animals likely had different evolutionary drivers than others -- even if their bodies ended up at the same solution.

So unfortunately for Abbott Thayer, and just-so explanations, countershading is probably a little more complex than general-purpose camouflage. If complex answers to interesting questions are the sort of thing you enjoy, you might like the Daily Challenges over on Brilliant. They post multiple new challenge questions every day, covering everything from statistics to electricity to computer science.

Each challenge question gives you all the tools you need to solve it -- and if it really gets your wheels turning, each question also ties back to a whole interactive course you can take. Premium members can access the whole archive of challenge questions -- and it just so happens that the first 200 people to sign up at Brilliant.org/SciShow will get 20% off an annual Premium subscription. So check it out if you're interested, and thanks for your support. [ outro ].