Dazzling stripes are a way of deterring flies. Photo: Getty
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How the zebra got its stripes

A method for dodging predators? A means of social interaction? Or a way of getting rid of flies?

Zebras' stripes have baffled biologists since Charles Darwin. Many hypotheses have been proposed regarding their purpose but, despite hundreds of years of study, there remains disagreement.

In an attempt to end the debate, researchers have pitted various models against each other and systematically analysed data from past studies. Their results reveal the one reason zebras have stripes: to ward off flies.

A handful of ideas regarding zebras’ stripes have found some support among biologists. One proposed that the dark and light bands change how air flows around a zebra’s body and helps in heat management, which could go a long way in the hot tropical areas that zebras live in.

Another proposed the stripes were used by zebras as a way of social interaction. They may use them to identify other zebras and for bonding as a group in the wild.

A third proposal suggested zebras used the stripes as camouflage. While stripes are clearly visible in the day, there some thought that it helped at dawn, dusk, and in the night.

All these ideas were shot down when tested rigorously. Two others, however, remained intriguing.

The first was that the stripes were used to dodge predators. It is called the “motion dazzle hypothesis”, and it suggests predators are confused by zebras' stripes and cannot understand their movement. Research published in the journal Zoology in 2013 used a simulated visual system to show that zebra stripes do interfere with visual perception. But this is a difficult hypothesis to test in the field.

Martin Stevens at the University of Exeter has researched the motion dazzle hypothesis by getting human subjects to catch moving stripy objects on a computer. “It’s an artificial experimental system,” he admitted.

The second proposal was that stripes helped keep flies at bay. Zebras are especially susceptible to biting flies due to their geographic spread. These flies, which include the tsetse fly, stomoxys stable flies, and tabanid biting flies, are particularly prevalent in areas with high temperature and humidity – exactly the areas where zebras are normally found.

Bites from these flies can be nasty and, quite literally, draining. About thirty flies feeding for six hours on just one horse can draw as much as 100mL of blood. Usually the flies can number in the hundreds around one animal.

Zebras have shorter hair than other equids – the family that includes horses, donkeys and zebras – which may also increase their susceptibility to attack. Also, four diseases which are fatal to equids have been found in Africa. This could mean that investing in anti-biting defenses such as stripes is especially important for zebras compared to non-African equids.

It is possible that the dazzle effect acts on flies, rather than larger predators, and deter them from biting. “Stripes clearly have a number of functions,” Stevens said, “and these could be interacting in zebras.”

Revealing maps

In the new research, just published in Nature Communications, Tim Caro and his colleagues at the University of California in Davis, didn’t perform experiments. Instead they used ecological and observational data on zebras' geographical locations and related factors. It is the first time that a comparative approach has been applied to find the reasons for zebras' characteristic colouration. Caro thinks his findings may have nailed the answer at last.

Caro looked at seven species of equids and scored them for number and intensity of stripes. Just to be sure, they tested all five hypotheses regarding zebra stripes' use: camouflage, predator avoidance, heat management, social interaction, and warding off flies. The extent of overlap between the geographic distribution of striped equids with each of these five measures was calculated.


E. greyvi, E. burchelli and E. zebra have stripes on all their bodies. Other equids don’t. Caro, Izzo, Reiner, Walker and Stankowich

“The results were a shock to me,” said Caro. Of these five proposals, only warding off flies had statistical support. He had not expected such a clear-cut answer to the question. As the map shows, the only places where flies and equids live together are areas that are populated by striped equids.

The exact mechanism by which stripes deter flies remains unknown, but experimental studies performed by researchers at Lund University in 2012 have found support for this proposal. They created striped surfaces and stuck glue on them. Based on the number of flies on the surfaces with different thicknesses of stripes, they concluded that these flies stayed away from stripes as thin as those found on zebras.

“As is normal in science you get a solution that asks more questions,” Caro said. It is time to hand the problem over to vector biologists, who can understand the susceptibility of horses to biting flies.

In Darwin’s days, people didn’t consider animal colouration with respect to fitness advantages. “People thought that animal colouration existed simply to please humans or was caused directly by the environment,” Caro said.

Darwin “would be delighted” that researchers are now considering animal colouration as a functional trait, he said. We might not have all the answers regarding zebra stripes – but it seems we may be looking through the right lens.

The ConversationThis article was originally published on The Conversation. Read the original article.

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Pupils need internet classes? Here are 41 lessons they should learn

Forget privacy and security, here's what to do when a black and blue dress looks white and gold. 

It is imperative that children are taught how to survive and thrive on the internet, claims a new House of Lords report. According to the Lords Communication Committee, pupils need to learn how to stay safe, avoid addictive games, and become “digitally literate”.

It’s hard to argue with the report, which is a great step forward in acknowledging that the internet now basically = life. Yet although it is crucial that children learn how to stay private and secure online, there are also some equally crucial and not-at-all-flippant pieces of information that the youth urgently need to know. Here are the first 41 lessons in that curriculum.

  1. To figure out how much to donate towards your mate’s charity half-marathon, half X OR double Y, where X is the amount paid by their mum and Y is the amount donated by your closest rival, Becky
  2. Don’t mention that it’s snowing
  3. If – for some reason – you talk about bombs in a Facebook message, follow this up with “Hi Theresa May” in case Theresa May is looking, and then Theresa May will think you are just joking
  4. If you are on a train and you are annoyed about the train, do not tweet @ the social media manager who runs the account for the train, because they are not, in fact, the train
  5. If a Facebook meme starts “Only 10 per cent of people can get this puzzle right” – know that lies are its captain
  6. It’s not pronounced me-me
  7. Never say me-me nor meem, for they should not be discussed out loud
  8. People can tell if you’ve watched their Instagram stories
  9. People can’t tell if you’ve waded back through their Zante 2008 album and viewed all 108 photos
  10. People can tell if you’ve waded back through their Zante 2008 album and viewed all 108 photos if you accidentally Like one – in this circumstance, burn yourself alive
  11. Jet fuel can melt steel beams
  12. If a dog-walking photo is taken in the woods and no one uploads it; did it even happen?
  13. Google it before you share it
  14. Know that Khloe Kardashian does not look that way because of a FitTea wrap
  15. Do not seek solace in #MondayMotivation – it is a desolate place
  16. Respect JK Rowling
  17. Please read an article before you comment about a point that the article specifically rebutted in great detail in order to prepare for such comments that alas, inevitably came
  18. Don’t be racist, ok?
  19. Never, under any circumstances, wade into the Facebook comment section under an article about Jeremy Corbyn
  20.  If a dress looks white and gold to some people and black and blue to some others, please just go outside
  21. Open 200 tabs until you are crippled with anxiety. Close none of the tabs
  22. Despite the fact it should make you cringe, “smol puppers” is the purest evolution of language. Respect that
  23. Take selfies, no matter what anyone says
  24. Watch Zoella ironically until the lines of irony blur and you realise that the 20 minutes you immerse yourself into her rose-gold life are the only minutes of peace in your agonising day but also, what’s wrong with her pug? I hope her pug is ok
  25. Nazi Furries are a thing. Avoid
  26. Use Facebook’s birthday reminder to remember that people exist and delete them from your Friends list
  27. When a person you deleted from your Friends list inexplicably comes up to you IRL and says “Why?” pretend that your little cousin Jeff got into your account
  28. Don’t let your little cousin Jeff into your account
  29. “Like” the fact your friend got engaged even if you don’t actually like the fact she is reminding you of the gradual ebbing away of your youth
  30. No one cares about your political opinion and if they act like they do then I regret to inform you, they want to have sex with you
  31. Please don’t leave a banterous comment on your local Nando’s Facebook page, for it is not 2009
  32. Accept that the viral Gods choose you, you do not choose them
  33. Joke about your mental health via a relatable meme that is actually an agonising scream into the void
  34. Share texts from your mum and mock them with internet strangers because even though she pushed you out of her vagina and gave up her entire life to help you thrive as a person, she can’t correctly use emojis
  35. Follow DJ Khaled
  36. Decide that “Best wishes” is too blah and “Sincerely” is too formal and instead sign off your important email with “Happy bonfire night”” even though that is not a thing people say
  37. If someone from primary school adds you as Friend in 15 years, accept them but never speak again
  38. The mute button is God’s greatest gift
  39. Do not tell me a clown will kill me after midnight if I don’t like your comment because that is not a promise you can keep
  40. Don’t steal photos of other people’s pets
  41. Accept that incorrect "your"s and "you’re"s are not going anywhere and save yourself the time 

Amelia Tait is a technology and digital culture writer at the New Statesman.