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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The moonwalkers: what it's like to belong to the world's most exclusive club

"The blue and the white and the brown just hung in the blackness of space."

It’s been almost 50 years since man first walked on the moon – and there were only a grand total of six missions.

From 1969 until 1972, as humanity reached out into space, these men – and they were all men – were at the forefront of scientific research and discovery.

But in 2017, the six survivors – now with a combined age of 505 – are the rare members of an exclusive club. The other six moonwalkers have already passed away.

Astronaut Buzz Aldrin was on Apollo 11, Charlie Duke was on Apollo 16 and Harrison "Jack" Schmitt was an Apollo 17 moonwalker. For the first time, at the Starmus festival in Trondheim, Norway, the three have come together to discuss their experience.

The three share “a special relationship, no question about it”, according to Duke. He tells me: “Our experiences are different but they’re the same in so many respects.”

Aldrin – unable to appear in person due to doctor’s orders – quips on camera from his home in Florida that President Dwight Eisenhower was advised that they should send a philosopher or maybe a poet up. His response, possibly apocryphal, came: “No, no - I want success."

As a result, it is up to these scientists to find the words to describe the off-Earth adventure which is the defining event of a moonwalker's life. 

A poetic description comes from Texan resident Duke. First and foremost a test pilot, his interest in space was piqued by the launch of Sputnik, the first artificial satellite, in 1957. He joined Nasa in 1966.

Now 81, Duke served as mission control support throughout many Apollo missions, most notably as the voice of Capsule Communicator when Neil Armstrong and Aldrin landed on the Moon in 1969.

He tells me: “Once we left Earth’s orbit, we turned our spaceship around and there was the whole Earth 40,000km away.

“The blue and the white and the brown just hung in the blackness of space. That contrast between the vivid blackness and the bright Earth – this jewel of Earth I like to call it - was right there.”

Aldrin started his career as a mechanical engineer, before joining up as a jet fighter in the US Air Force during the Korean War.

His gung-ho spirit and enthusiasm for space have not deserted him even at the age of 87 – he appears onscreen at the festival wearing a "Get your ass to Mars" T-shirt.

The most memorable experience for him came when he congratulated Neil Armstrong, the first of the team to walk on the moon (he died in 2012). But in the lunarscape, memories get confused – the men remember the moment differently.

“After the landing, I looked over at Neil, and we smiled. I remember patting him on the back and he remembers shaking hands. So here were two first-hand witnesses and we couldn’t agree on what actually happened when we got there.”

For Aldrin, the significance of the moonwalk was looking at the moon’s surface from close-up – the lunar soil, or regolith – and what happened when an astronaut's boot stepped onto it. 

“It was so remarkable, the way that it retained its exact form,” he marvels, 48 years on.

Aldrin's fascination with the moon's surface was shared by Schmitt, the 12th, and so far, last, man to walk on the Moon. A trained geologist, he was also the first scientist to do so.

In Schmitt's case, the rocky surface of the moon was enough to draw him into lunar research, which he still conducts at the age of 81.

“The commander told me as soon as I got out I had to look up and see the Earth," he recalls. "I said ‘Well, chief, you’ve seen one Earth, you’ve seen them all’."

In truth, having spent three days looking at the Earth from his craft, Schmitt’s priority was in looking down at the new surface under his feet.

After landing in a valley deeper than the Grand Canyon, his chief concern was just getting to work collecting samples in a lifesize laboratory.

While the moment on the moon may be the initiation into an elevated celebrity, it is followed quite literally a fall back to earth. 

In his post-Moon life, Duke found God.

“A lot of us have a letdown [afterwards]," he admits. Duke was 36 when he landed on the moon in April 1972. By December, the Apollo programme was over. "In January ’73, the thought occurred to me, ‘what am I going to do now?’"

Achieving his life's ambition before hitting middle age turned out not to be as satisfying as he expected. "Because you’d climbed the top, you got to the ultimate high when you were still a young man - and the drive that took you to the ultimate high was still there," he says. "That was a struggle."

In the years since, Duke has looked at his experience as a religious one. Yet he insists God wasn’t present for him when he touched down on the lunar body.

“The Moon flight was not a spiritual experience," he says. "I didn’t understand the wonder of God’s universe. I was enjoying the beauty and the excitement of this mission.”

The three men agree that Mars is the next step for the future of humanity, but there are safety and speed concerns.

“There is potential important work to be done in better physiological understanding of human exposure to long duration space flight which is going to happen whenever we go to Mars,” says Schmitt.

“Anything we do as human beings that’s productive and worthwhile carries risk, either physical of psychological. Radiation, physiological exposure to weightlessness for long durations, and the danger of landing on a distant planet where the atmosphere is not going to be much help - but you do accept the risk that it might end up as a one-way trip.”

But after all of that - the life, the death, the heartache - Duke says he would go back up there if he could.

“At my age now I wouldn’t volunteer to go to Mars - but I would volunteer for a round-trip to the Moon again.”

Starmus Festival runs in Trondheim until Friday June 23. For more information, visit Starmus.

Kirstie McCrum is a freelance journalist. Follow her @kirstiemccrum.

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