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.

Lifestage
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Everything that is wrong with the app Facebook doesn't want over 21s to download

Facebook's new teen-only offering, Lifestage, is just like your mum: it's trying too hard to relate and it doesn't care for your privacy.

Do you know the exact moment Facebook became uncool? Designed as a site to connect college students in 2004, the social network enjoyed nearly a decade of rapid, unrivalled growth before one day your mum – yes, your mum – using the same AOL email address she’s had since her dial-up days, logged on. And then she posted a Minion meme about drinking wine.

Facebook knows it’s uncool. It had a decline in active users in 2014, and in 2015 a survey of 4,485 teens discovered it came seventh in a ranking of ten social apps in terms of coolness. In fact, only 8 per cent of its users are aged between 13 and 19. This is the main reason the corporation have now created Lifestage, an app specifically for under-21s to “share a visual profile of who [they] are with [their] school network”.

Here’s how it works. After signing up and selecting their school, users are prompted to create a series of short videos – of their facial expressions, things they like, and things they dislike – that make up their profile. Once 20 people from any school sign up, that school is unlocked, meaning everyone within it can access one another’s profiles as well as those from nearby schools. Unlike Snapchat (and truly, this is the only thing that is unlike Snapchat) there is no chat function, but teens can put in their phone number and Instagram handles in order to talk. Don’t worry, though, there are still vomit-rainbows.

But with this new development, rather than hosting your mum, Facebook has become her. Lifestage is not only an embarrassing attempt to be Down With The Kids via the medium of poop emoji, it is also an invasive attempt to pry into their personal lives. Who’s your best friend? What do you like? What’s not cool? These are all questions the app wants teens to answer, in its madcap attempt to both appeal to children and analyse them.

“Post what you are into right now – and replace the video in that field whenever you want,” reads the app description on the iTunes store. “It's not just about the happy moments – build a video profile of the things you like, but also things you don’t like.” They might as well have written: “Tell us what’s cool. Please.”

Yet this is more than an innocent endeavour to hashtag relate, and is a very real attempt, like Facebook’s many others, to collect as much data on users as possible. Teens – no matter how many hot pink splashes and cartoon toilet rolls are used to infantilise them – are smart enough to have figured this out, with one of the 16 reviews of the app on the iTunes store titled “Kinda Sorta Creepy”, and another, by a user called Lolzeka, reading:

“I don't like how much information you have to give out. I don't want my phone number to be known nor do I want everyone to know my Instagram and Snapchat. I could not figure out how to take a picture or why my school was needed. Like I said, I don't want all my information out there.”

But Facebook already knows everything about everyone ever, and it’s not this data-mining that is the most concerning element of the app. It is the fact that – on an app specifically designed for children as young as 13 to share videos of themselves – there is no user verification process. “We can't confirm that people who claim to go to a certain school actually go to that school,” Facebook readily admits.

Although the USP of this app is that those over the age of 21 can only create a profile and aren’t allowed to view others, there isn’t a failsafe way to determine a user’s age. There is nothing to stop anyone faking both their age and the school they go to in order to view videos of, and connect with, teens.

Yet even without anyone suspicious lurking in the shadows, the app’s privacy settings have already come under scrutiny. The disclaimer says all videos uploaded to Lifestage are “fully public content” and “there is no way to limit the audience of your videos”. Despite the fact it is designed to connect users within schools, videos can be seen anyone, regardless of their school, and are “viewable by everyone”.

Of course none of this matters if teens don’t actually bother to use the app, which is currently only available in the US. Lifestage’s creator, 19-year-old Michael Sayman, designed it as a “way to take Facebook from 2004 and bring it to 2016”. Although he has the successful app 4Snaps under his belt, there is no guarantee Lifestage will succeed where Facebook’s other app attempts (Notify, Facebook Gifts, Poke) have not.

There are a few tricks Facebook has put in place to prompt the app to succeed, including the fact that users are ranked by how active they are, and those who don’t post enough updates will be labelled with a frowning or (here we go again) poop emoji. Still, this hardly seems enough for an app whose distinguishing feature is “Privacy? Nah.” 

Only time will tell whether the app will appeal to teens, but one thing is certain: if it does, your mum is totally downloading it.

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