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.

Artie Limmer/Texas Tech University
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Meet the evangelical Christian persuading believers that climate change is real

Katharine Hayhoe's Canadian missionary parents told her science and God were compatible. Then she moved to Texas. 

During Donald Trump’s presidential campaign, alarm rose with each mention of climate change. Denial, dismissal and repeated chants of “hoax” left no doubt as to his position.

Now President Trump’s withdrawal from the Paris Agreement has been seen as a seminal moment in the fight against climate change - one which many fear could lose the battle ahead of humanity.

But one scientist has been fighting a war of her own on the ground, against those who typically doubt the facts about global warming more than most - the evangelical Christian population of America.

And to make matters even more unusual, Katharine Hayhoe herself is an evangelical Christian who lives in the indisputably "bible belt" of Lubbock, Texas.

The atmospheric scientist has been named one of Time magazine's 100 most influential people and one of Politico’s 50 thinkers transforming American politics. Now she is using her considerable heft to speak to those who are hardest to convince that there is a manmade problem that threatens the Earth’s future.

I meet her at the science and music festival Starmus in Trondheim, Norway, where she is to address the attendees on Thursday in a talk entitled "Climate Change: Facts and Fictions".

Hayhoe was born in Canada, to missionary parents. Her father, a former science educator, showed her that there was no conflict between the ideas of God and science. However, it was something of a surprise to her when she discovered her pastor husband, whom she married in 2000, did not feel the same about climate change. It took her two years to convince him.

What started as a conversation became an organised project when she moved to America's South in the mid 2000s. 

“Moving to Lubbock was a culture shock," she tells me. "When I moved there I wasn’t doing much outreach, but it moved me in that direction.

“Lubbock is very conservative. It’s small and isolated.

“I would say the majority of people in Lubbock are either dismissive or doubtful about climate change. I was surrounded by people - neighbours, parents of friends, people at church, colleagues down the hall in the university - who weren’t convinced.”

So Hayhoe, who works as an associate professor and director of the Climate Science Centre at Texas Tech University, set to work. She began to collect the responses she was seeing to the climate change discussion and prepare her counter-argument.

“When I talk to people who are doubtful, I try to connect with the values they already have," she says. “The greatest myth is the myth of complacency - that ‘it doesn’t really matter to me’.

"But I would say that the second most insidious myth is that you only care about this issue if you’re a certain type of person. If you’re a green person, or a liberal person, or a granola person."

The stereotypes mean that people outside that demographic feel "I can't be that kind of person because that's not who I am", as she puts it.

Hayhoe convinced her husband using data, but rather than repeating a formula, she tries to find out what will resonate with different people: "For many groups, faith is a core value that people share.”

Whether she’s speaking to city planners, water company managers, school kids or Bible believers, Hayhoe says her hook is not the facts, but the feelings.

“I recently talked to arborists," she says. "For them, trees and plants are important, so I connect with them on that, and say ‘because we care about trees, or because we care about water or what the Bible says then let me share with you from the heart why I can about these issues because it affects something that you already care about’.

“My angle is to show people that they don’t need to be a different person at all - exactly who they already are is the kind of person who can care about climate change.”

Hayhoe came to public attention in the United States after appearing in a Showtime series on climate change. She has appeared on panels with Barack Obama and Leonardo DiCaprio, and launched a web series. As well as plaudits, this level of fame has also earned her daily threats and online abuse. 

“My critics think they’re coming from a position of religion, but they aren’t," she says. "They’re actually coming from a very specific political ideology which believes that the government should not have control over people’s lives in any way shape or form - very libertarian, free market, free economy, Tea Party."

She believes that in the United States, faith and politics has been conflated to the point "people can no longer tell the difference". 

“Now it’s conservatism that informs religion," she elaborates. "If the two are in conflict - like the Bible says God has given us responsibility over everything on this earth - then people say ‘oh, we can’t affect something as big as this Earth, God will take care of it anyway’."

Around half of those who attack her on social media identify themselves as Christians, she notes, but almost all call themselves conservatives. 

As a scientist, she’s been preparing data herself - naturally - on her online attackers, with depressingly familiar results.

“As soon as you stick your head out of the trench, you get it. There have been papers published showing that white men disproportionately form up that small group of dismissives. They’re almost all men. When I track my social media comments, I would say that 99.5 per cent of them are white men.

“Out of 1,000 negative comments, I have maybe five from women.”

After the climate change argument moved up a gear - following the Paris withdrawal - Hayhoe admits that she and her fellow scientists are concerned, although she pays tribute to the businesses, cities and states from the US that have committed to following the Paris agreement themselves.

On the subject of the chief white male denier, Trump himself, Hayhoe says she has a discussion point which she feels may convince him to think carefully about his role in the fight against global warming’s impact on humanity.

“I would attempt to connect with the values that he has and show him how acting on this would be in his best interests," she says.

“One guess would be ‘what do you want your legacy to be? What do you want to be known as, the man who destroyed the world, or the man who saved it?’”

Katharine Hayhoe is speaking at Starmus on Thursday June 22. For more details, visit Starmus.

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

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