Why Do Zebras Have Stripes? It’s Their Missile Defense System

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Rayita stands with her mother at the Metropolitan Park Zoo in Santiago, Chile, 2006
Stripes can confuse the eyes in fliesCredit: Santiago Llanquin, AP
Ruth Schuster
Ruth Schuster
Ruth Schuster
Ruth Schuster

Why do zebras have stripes? There are more theories than zebras, but the latest explanation of their dichromatism is a doozy: The garish striping confuses bloodsucking horse flies, scientists at the University of Bristol reported in PNAS on Wednesday.

Stripes confuse the eyes of flies.Credit: Amir Cohen, Reuters

Instead of descending dexterously on their little feet and ramming their blood-siphon into the hapless animal, as they do with other quadrupeds and you, they tend to either miss the zebra entirely, or crash.

Bristol has no indigenous zebras but it does have horse flies – which, like our friend the rat, live everywhere in the world except the polar realms and some islands they happen not to have reached. Iceland is horse fly-free; England is not.

Horse flies look like house flies, but they’re bigger and the female sucks blood, slashing through skin with her mouth siphon armed with two pairs of blades. Like mosquitoes, horse flies can transfer deadly diseases, including anthrax and trypanosomiasis. So if there’s something that frustrates their ambitions, this is good to know. Zebra stripes seem to do just that, it turns out.

Prof. Tim Caro, Dr. Martin How and colleagues observed the behavior of horse flies around zebras and horses at a livery in North Somerset, using video analysis.

Horse flies can see the zebras perfectly well. Their predatory circling around horses and zebras was the same. Yet while making hay of the horses, the flies largely failed to land on the zebras.

“Horse flies just seem to fly over zebra stripes or bump into them, but this didn’t happen with horses,” Caro says.

When approaching a horse, the flies would decelerate like any self-respecting flying object and land properly. The upshot is that the flies experienced far fewer successful landings on zebras compared to horses.

Grevy's zebras in a battle over dominance: Now imagine trying to land on one of them.Credit: BBC 2012

But why? The answer seems to lie in their failure to adapt approach speed, because or coupled with the horse flies’ eyesight constraints.

Horse flies are proper flies and have large compound eyes, but the stripes may disrupt their visual system during their final approach, How postulates.

Zebra stripes and horse feathers

Could the difference in approach speed and/or landing ability be a matter of body odor? It seems not.

The scientists then tested fly behavior with the very same horses garbed in coats: black, white or zebra-striped. The horses wearing stripey coats experienced fewer successful horse fly landings versus when they wore single-color coats, the team wrote.

So there we have it. That may not be why the zebra evolved stripes. But it definitely shows that stripes are helpful.

One wonders if other colors would also be helpful in confusing the horse flies, but the team didn’t test horses decked out in pink and purple, for instance.

Joren Bruggink (left) and Jai Lake checking how horse flies react to horses wearing different colored coats. Dressing up a horse in stripes dismayed the insect.Credit: School of Biological Sciences, University of Bristol

The team adds that zebras were relatively proactive about evading the horse flies. They were more likely to run away and swish their tails at the flies compared with the horses. So those flies that did successfully land on zebras spent less time there versus flies who alit onto horses – sometimes not even enough to grab a meal.

Conclusion: If your horses suffer from flies, or if you do, consider painting on stripes. It can’t hurt.

As said, for some reason, people seem to be profoundly preoccupied with the zebra's stripes.

Last year, scientists in Sweden had a go at addressing the issue of zebra stripes. Publishing in Nature Scientific Reports, they debunked an intriguing if weird theory that zebras evolved stripes in order to stay cool. Stripes create a mini-climate on fur, the theory went: The black stripes would get hotter than the white ones during the blistering African day, and as heated air above the black stripes collided with cooler air above the white stripes, small vortexes would form. That hypothesis turned out to be horse feathers.

Another Swedish study, from 2012, reported that tsetse flies – the cause of sleeping sickness in Africa – prefer dark animals to white ones under lab conditions. OK, so the zebra is halfway there.

Then there is the theory, brought up in late 2013, that flies aren’t the only ones discombobulated by the stripes. So are lions and other predators, suggested one Prof. Johannes Zanker. He’s a computational neuroscientist, not a zoologist.

Really? Camouflage en large? Maybe. Just look at a rotating barbershop pole or wagon wheel and think of a whole herd of animals on the move, Zanker said. The way these objects are actually moving, and the way we see them moving, are completely different.

Or it could be all the above, except for the micro-climate. Stripes, good. More stripes, more good.

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