You might’ve heard that  zebras are really opinionated.

And that’s true…they’re very black and white! And you definitely don’t want  to take on a zebra in a fight.

They’ve got a TON of black belts! Their famous stripes are the basis  for corny jokes, animal-print clothes, the easiest coloring book pages of all time, and of course, scientific head-scratching. Because what are these black  and white stripes for, anyway?

Researchers have spent centuries pondering, and we’ve only just started  to piece together the answer. We made a video exploring  some hypotheses back in 2014, and as it turns out, we were a little off. But in recent years, researchers  think they’ve cracked the mystery so let’s talk about both how and  why the zebra got its stripes. [intro music] Before we can get into  nuanced scientific speculation about these black-and-white patterns, let’s talk about Zebras 101.

On the mammalian family tree, the genus Equus includes three living groups of  animals, horses, zebras, and asses— and, yes, that’s their real name. It’s the group with donkeys and stuff. DON’T LAUGH!

Currently, there are three species of zebras: the plains zebra, the mountain  zebra, and the Grevy's zebra, which can be found in different habitats across the southern half of the African continent. You can tell them apart by their features, sizes, and general placement and thickness  of stripes on their bodies. But any two zebras within the same species, or the same subspecies for that matter, will have stripe patterns that are totally unique.

Zebra skin and hair is pigmented by melanin, the same family of proteins  that colors our skin and hair. One of the big roles of melanin  across living things is pigmentation, and if a cell has more melanin molecules, it’ll appear darker. If you look beneath all their  stripey hair, zebra skin is black, chock full of melanin.

There are melanocytes in all their hair follicles, which produce the eumelanin  that colors their hairs. The black stripes have a bunch of  pigment, and the white stripes have none. We don’t know the exact biological  mechanisms behind these iconic patterns, but scientists have some ideas about the basics.

Zebra embryos probably start developing  their stripe-printing mechanisms as early as the third, fourth,  or fifth week in utero. But the pigmented hairs only start  growing around the eight month mark. Genetics play a role in what  body parts will have stripes and even the shape of the stripes themselves, including some weird cases of zebras  with speckles instead of stripes.

But the exact pattern likely comes  from a mix of genetic factors and other chemicals and cells  floating around during development that influence the melanocytes. No two zebras are alike, and  even genetically identical twins don’t have the same stripes. It’s even probable that if you  took one zebra and cloned it that your clone would have  different stripes from the original.

We did, in fact, email a zebra  stripe expert to fact-check this, and he said that while there’s  no data on zebra clones, there’s enough of an environmental component to stripe growth that it’s probably true. So, you heard it here first!! And while we don’t know what genes or chemicals are directing those hair follicle  melanocytes in such precise ways, scientists have modeled what  might be going on mathematically, thanks to decades of studying other  stripes and splotches in nature.

In 1952, the British mathematician Alan Turing hypothesized that chemicals reacting with  each other and diffusing through living cells could form all kinds of biological patterns. He wasn’t concerned with any  specific molecules like melanin, because this was a generalized model. So he came up with a term morphogens to stand-in for any kind of substance  that could affect pattern development.

These reaction-diffusion systems that Turing proposed could form rings, spots, stripes, or any number of regular patterns  with just two morphogens involved. One of these chemicals causes the  production of more and more of itself, so let’s call it an activator. In a developing zebra, this activator might be  something that causes melanocytes to produce more melanin and  make a black stripe of hair.

But the key part of this system is that, even though the activator gets a head start, it also leads to the production of  an inhibitor that switches it off. In a developing zebra, that inhibitor might be something that says  “stop producing melanin!” to make a white stripe. If you tweak the activator and inhibitor so they get created, break  down, react with each other, or move through an organism at different speeds, you can make different geometric patterns.

Since this 1952 paper, scientists keep gathering bits of evidence that seems to support that  these so-called Turing patterns are a really useful foundation  to understanding animal coloring, from zebra hairs to zebrafish skin. So even though some of the details are fuzzy, the mathematical and biological answer  to “how the zebra got its stripes” comes down to melanin-producing  cells in hair follicles getting turned on and off in precise patterns while a baby zebra is still developing. But the “why” has been just as  much of a puzzle for centuries.

There are oral traditions trying to figure  out the mystery of these iconic stripes. The English naturalists Charles  Darwin and Alfred Russell Wallace both threw out guesses in the late 1800s. And we here at SciShow tried to  synthesize an answer in a 2014 video.

And we all…fell a little short. At least we were in good company. The tricky thing about figuring  out what evolutionary pressures are keeping zebra stripes  around is that the main evidence comes from observing zebras in the wild, which takes a lot of time,  patience, and sunscreen.

So to recap, there are five-ish  main categories of explanations that researchers have put forward. Why might zebras have their stripes? Well, they don’t want to be spotted.

Okay yeah, that was corny, but the joke is kind of based on one family  of stripe-explain-y hypotheses: camouflage. For instance, Alfred Russell Wallace was a camouflage hypothesis kind of guy. He thought that the stripes  could help them blend in with tall grass, trees, or shadows  and fade into the background.

Some Wallace’s contemporaries, like Darwin, thought that zebra stripes  would be too conspicuous to help them hide. But the starkness of the black and  white stripes is the foundation of a second, related bucket of  hypotheses: predator avoidance. Maybe zebra stripes don't help  them blend into the scenery, but they help individuals blend  into a herd of striped bodies.

It’s sort of like those warships  painted with dazzle camouflage so you couldn’t tell where they were headed. The idea was that the mass of stripes could make it harder for a predator to  pick out any one zebra from the herd. However, there hasn't been much evidence  in favor of these hiding hypotheses, and plenty of evidence against them.

One study compared zebras to another  famously stripey animal, tigers. We know that tigers do use their stripes  to blend into the dappled light in forests, and this study found that the shape  and distance between tiger stripes matches up with the bands of  sunlight hitting the forest floor. Meanwhile, zebra stripes aren’t similar  to really anything in their habitats, so they wouldn’t really help a zebra  blend in the way that tigers can.

And even if humans may be dazzled by stripes predators like lions and spotted hyenas don’t seem to have a hard  time seeing through them, and can spot a large ungulate just  as easily if it’s striped or not. Another study has shown that lions  consistently and successfully hunt zebras, which means that their disguise  isn’t all that effective anyway. If stripes don't help zebras survive being hunted, then that can’t be the selective pressure  keeping them around as a signature look.

So, combined, those are a pretty big nail  in the coffin for these two camo hypotheses. On the other hand, Darwin also suggested that  these flamboyant stripes are for recognizing other zebras for  social reasons, like mating or grooming. If everyone’s got a unique look, you can tell who’s who in the crowd and develop  stronger social relationships between individuals.

But this idea doesn’t stand up to scrutiny either, because the odd zebras that  are born without stripes don’t suffer any social  consequences from their unique look. Plus, other non-stripey equids  like horses have no problem telling individuals apart just  by sight, sound, and smell, so there’s no reason to think  that zebras needed a cheat code for recognizing their bros. The fourth kind of hypothesis  is about thermoregulation.

The thought was that these  black and white patterns somehow help keep zebras  cool under the African sun. The specific idea is that black stripes of  hair absorb more UV radiation from the sun and heat up more than white stripes, which reflect heat. So the air temperatures right next to  those stripes would be slightly different, meaning there’s different  densities of air particles, and mini-wind gets created to help  with that evaporative cooling.

But just like the mate recognition idea, this is a hypothesis that solves a  problem the zebras didn’t seem to have. See, equids like zebras actually  sweat to lower their temperature, sort of like humans! And along with that water, they pump out a soap-like protein called latherin which helps spread the  moisture throughout their fur, which helps it evaporate faster  and cool them more efficiently.

And while scientists have  recorded temperature differences between the black and white stripes  on both zebra hides and living zebras, they were only surface-level, and didn’t correlate to any  differences to the core temperature, which is what really matters. Plus, this mini-wind idea would only  work while zebras are standing still, because the windy effect would be too  small to happen when they’re moving. So, if these are the hypotheses that we went  through in our old video that don’t hold water, what are those zebra stripes for?

Well, hold your horses, I’ll  tell you after this ad break. Thanks to our Presidents of Science for supporting this SciShow video! Charlie Stanley, McLaren Stanley, and TJ Steyn help bring you knowledge through  their support on patreon.

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Our best hypothesis for explaining  zebra stripes required thinking small. Like, tiny little bug, small. It turns out that these stripes are  really good at deterring ectoparasites.

Specifically, the stripes  seem to discourage flies. And since those buggers can  transmit some gnarly diseases, that’s a pretty big deal. Researchers tested this in a 2011 paper, where they looked at how horseflies  responded to different patterns and colors.

They found that striped patterns  attracted fewer horseflies, and even got a clue as to why. See, the stripes seemed to  polarize light differently, meaning that the waves of light were  oriented differently from each other when bouncing off black or white surfaces. So the idea is that these bands  of differently polarized light confused the flies and kept them from landing.

It’s supported in a 2019 paper, where  researchers compared the behavior of horseflies that tried to fly around and  land on domestic horses, zebras, and horses wearing striped zebra costumes. Which I am sure inspired many more jokes. What’s black and white and striped all over?

A horse in a zebra costume. These researchers found that even though the flies approached all three groups equally, the striped surfaces made it harder for  horseflies to land on both the zebras and the disguised horses, meaning they got fewer bites. And this also makes sense when  you look at the big picture.

Specifically, the big map of  zebra habitats across Africa. If you compare home ranges of different  zebra species with those of biting pests, it shows that the more fly activity there is, the denser the zebra stripes. And the stripes even tend to be concentrated  on body parts that flies like to snack on, like the neck and the legs.

And it turns out that this isn’t the first  time humans have discovered this hack either. A 2019 study that looked at Indigenous  bodypainting traditions across continents like Africa, Australia, and North America noted that many communities  paint white or other bright, pale stripes on their darker-skinned bodies. The researchers trapped horseflies  that landed on plastic mannequins that were light beige, dark brown, and  dark brown with painted white stripes, and they found that the model with  stripes was the least attractive to flies.

So this could be an example of a cultural  practice with extremely practical origins. And, humanity’s first topical bug deterrent. As always, there’s still more  questions to ask and answers to find, but we’re pretty sure we’ve got a better  handle on zebra stripes than we did in 2014.

Sorry it took us so long, but we hope  that owning up to our mistakes helped us… earn our stripes. [ OUTRO ]