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  • Thanks to Brilliant for supporting this episode of SciShow.

  • 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 beenmixed.

  • 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 ]

Thanks to Brilliant for supporting this episode of SciShow.

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B1 中級

反影。企鵝為什麼要穿燕尾服? (Countershading: Why Do Penguins Wear Tuxedos?)

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    林宜悉 發佈於 2021 年 01 月 14 日
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