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Stripes! Hexagons! They're everywhere! These patterns in nature might seem like aesthetic coincidences, but they are actually the result of physical process that show up again and again, even in otherwise unrelated phenomena.

Hosted by: Stefan Chin

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Image Sources:
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https://www.istockphoto.com/photo/zebras-gm511949893-46746926
https://www.istockphoto.com/photo/bengal-tiger-isolated-on-white-background-gm1135303169-301974719
https://www.istockphoto.com/photo/two-fresh-water-angel-fish-pterophyllum-scalare-close-up-gm1181621284-331482876
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[ ♪ INTRO ].

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 Causeway — around 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 other — like 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 late — the cells are already destined to become black — but 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 nature—in 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 as… maybe just really weird stripes? But in any case, scientists are still exploring Turing's hypothesis — and 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/ simple — they'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 are—the 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 universe—and 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. [ ♪ OUTRO ].