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  • On the northern coast of Ireland, there's a line of cliffs made up of countless pillars

  • of volcanic rock, all crammed together.

  • And at the foot of these cliffs is the famous Giant's Causewayaround 40 thousand

  • of these columns worn down and jutting out into the sea.

  • People have marveled at these for centuries, because the columns /almost/ look like they're

  • carefully crafted into a regular, hexagonal pattern.

  • It's an incredible display of nature, but it's not the only place you can find strange,

  • regular patterns like these.

  • You can see something similar just across

  • the sea, in Scotland's Fingal's Cave, or in a Wyoming rock formation called the Devil's Tower.

  • But we also see these shapes in other places, like the cracks of drying mud or the glaze on pottery.

  • In fact, once you start to notice them, you can see that certain patterns, like hexagons,

  • spirals, and stripes, appear again and again throughout nature, even in phenomena that

  • clearly have nothing to do with each otherlike zebra stripes and sand dunes.

  • And by studying these patterns, scientists are discovering that many things in nature

  • seem to share simple, fundamental rules that play out all over the natural world.

  • Of course, we can't reduce /every/ pattern down to a few simple mechanisms.

  • But, by taking a close look at the way certain ones arise, we can start to understand how

  • a few simple principles may unite patterns across the universe.

  • First, let's get back to Giant's Causeway and those weird polygon-shaped cracks.

  • Scientists now think those regular shapes may be created by the release of stress.

  • See, Giant's Causeway formed at a volcanic fissure, where lava spilled onto Earth's surface.

  • And in general, as lava cools, its surface layer shrinks and solidifies, which puts an

  • increasing amount of tension on the rock.

  • Eventually the surface has to release that tension by cracking open.

  • Now at first, the cracks form at random, and they tend to criss-cross because perpendicular

  • lines can release the most energy from the rock.

  • But then, as the lava cools from the top down, each lower layer of lava cracks, also.

  • Usually, the new cracks form right below the existing ones, because the rock is already a little bit weaker there.

  • But instead of forming a T shape again, the new cracks literally cut corners to release more energy.

  • So, as the crack gets deeper, the T shape gets more rounded, until eventually it looks

  • like a Y with equal angles.

  • And if you have a bunch of Y-shaped intersections next to each other, you end up with hexagons.

  • So, at Giant's Causeway, cracks formed columns in the hardened lava, and as that rock eroded

  • down, it exposed those Y-shaped columns.

  • Meaning that, in the end, this grandiose natural structure was likely all a result of some

  • pretty basic physics.

  • And in other natural places where we see hexagons, like the cracks in glaze or dried mud, scientists

  • think something similar is happening.

  • It's just that, in these cases, instead of growing deeper, the original cracks are

  • closing up and then re-forming during cycles of heating and cooling or wetting and drying.

  • Each time they split open again, they tend to crack along the same lines, but just like

  • the cracks in lava, they will also cut corners until the T shapes have evolved into Ys.

  • So, even though a glazed pot and a dry lakebed don't have a lot in common, simple physics

  • may explain the strangely similar patterns on both of them.

  • Now, if you haven't noticed the hexagons in nature before, one emergent pattern you've

  • almost certainly noticed is stripes.

  • Take zebras, for instance.

  • They might be the one of the most famously striped creatures, but the pattern is far

  • from unique to them.

  • Animals like tigers, okapi, angelfish, and certain hyenas all wear stripes, too, and

  • /none/ of them are closely related in terms of evolution.

  • But they actually /might/ have something in common.

  • One idea that scientists have investigated for years is that zebra stripes form through

  • a chemical process called a reaction-diffusion system.

  • The idea is that, as a zebra's body grows, at little points throughout its skin, cells

  • start to make a protein or chemical called an activator that does a few things:

  • First, it signals to skin cells around it to start producing pigment, turning that patch

  • of skin a certain color, like black.

  • Second, it tells those cells to create more of the activator.

  • Since it essentially self-multiplies, the signal from the activator will spread and

  • get stronger over time.

  • This means that, if left alone, even a little blip of this chemical on the zebra's nose

  • would turn the whole animal black.

  • But it /doesn't/ just go on forever, because the activator also does one more thing: It

  • tells the cells to produce something called an inhibitor.

  • And an inhibitor is some chemical or protein that puts a stop to the self-multiplying behavior

  • and breaks the cycle.

  • So, the activator is sending mixed messages: It's creating one chemical that says, “Turn

  • black and make more of me,” and another, just behind it, that says, “Ignore that order.”

  • For the first few cells, that second message may come a little too latethe cells are already

  • destined to become blackbut the inhibitor actually moves a little bit faster than the activator.

  • In other words, it /diffuses/ faster.

  • The reason may vary from one inhibitor to another, but, for example, maybe it's a little smaller.

  • But either way, eventually the inhibitor catches up, getting its message across in time to

  • stop the activator.

  • So it ends the stripe and leaves a patch of white.

  • If you have this scenario play out over and over across the zebra's body, you'll end up

  • with those famous stripes.

  • And with a slightly different set-up, the same mechanism can also create spotted patterns,

  • spirals, or maze-like ones too.

  • The thing is, for a long time, this mechanism was purely hypothetical.

  • It was proposed by the British mathematician Alan Turing, who put out a paper in 1952 outlining

  • how animal patterns like stripes or spots could emerge through this mechanism.

  • And chemists /had/ noticed similar, oscillating patterns in chemical reactions since 1910,

  • so it did seem possible.

  • But for decades, no one could /prove/ that chemicals like this were creating these so-called

  • Turing patterns in animals.

  • But today, scientists have done a lot more research, and they've spotted what look like Turing

  • patterns all over naturein angelfish stripes, giraffe spots, and even the arrangement of

  • feathers and hair follicles in the skin.

  • They've even taken things one step further, and in some examples, like the stripey pattern

  • of ridges inside the mouths of mice, they've been able to nail down the exact identity

  • of the proteins working as activators and inhibitors.

  • So, all over the biological world, there are patterns that /appear/ to be related to this

  • simple mathematical idea that Turing proposed in 1952… including some less obvious ones.

  • Like, in 2012, scientists even suggested that Turing patterns might be responsible for /fingers/.

  • Their study found that, in mouse embryos, fingers develop in the signature way that

  • is predicted by the Turing mechanism.

  • If they're right, that means we could think of fingers asmaybe just really weird stripes?

  • But in any case, scientists are still exploring Turing's hypothesisand the mechanism he described

  • can even be applied to patterns that /aren't/ biological.

  • For instance, sand dunes are purely physical patterns, but you can think of them as following

  • the same principle of activators and inhibitors.

  • See, sand dunes form on flat, windy land that's full of small imperfections like a boulder

  • or little ridge.

  • As the wind blows, it meets those small imperfections, which break up the air current.

  • As soon as the current is gone, any dust or sand the wind was carrying falls and builds

  • up on the downwind side of the imperfection.

  • Over time, this little pile grows, and as it gets bigger, it breaks up more wind and

  • traps more sand, which lets it get bigger, and so on.

  • So in that sense, it acts like an activator: It creates more of itself.

  • But it also plays a second role.

  • That same dune removes sand from the air so another dune /can't/ form right behind the first one.

  • In that sense, it acts like an inhibitor.

  • So, in a way, the naturally spaced-out ridges we see in dunes arise from the same principle

  • that /may/ create zebra stripes.

  • Of course, these patterns we see in dunes aren't /that/ simplethey're also

  • influenced by factors like gravity, moisture, and wind direction, which can have dramatic

  • effects on how dunes look, even though they all form out of the same basic principle.

  • But overall, this shows that, on a basic level, even patterns that /seem/ vastly different

  • can just be variations on a theme.

  • Now, it's hard to tell exactly how prevalent and important processes like Turing patterns

  • or hexagonal cracking arethe universe is a pretty complicated place.

  • So as tempting as it is to look for simple, unifying principles, it's also important to consider

  • /all/ of the factors that might be pushing an organism or landscape to evolve a certain way.

  • But overall, what looking at these processes does suggest is that biological and geological

  • patterns are all grounded in the physical world.

  • And by studying how these patterns arise, when we /see/ those patterns in other places,

  • we can infer things about what physical processes are going on there, whether it's on our

  • own planet or far beyond it.

  • So, studying how these patterns arise can give us a jumping off point to study the universeand

  • it's a reminder that seemingly small interactions can grow to create something spectacular.

  • Thank you for watching this episode of SciShow!

  • And a special thanks to this month's President of Space, Matthew Brant.

  • We couldn't make all these videos without the support of people like Matthew and the

  • rest of our patrons.

  • And if you'd like to join them and help us keep making science videos that are available for free on the

  • internet, you can find out more at patreon.com/SciShow.

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The Link Between Zebra Stripes and Sand Dunes | Natural Patterns

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    joey joey に公開 2021 年 05 月 19 日
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