The Mystery Of The Zebra's Stripes


Gone But Not Forgotten
Aug 7, 2001
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Zebra stripes evolved to keep biting flies at bay
By Victoria Gill, Science reporter, BBC Nature

Why zebras evolved their characteristic black-and-white stripes has been the subject of decades of debate among scientists.
Now researchers from Hungary and Sweden claim to have solved the mystery.
The stripes, they say, came about to keep away blood-sucking flies.

They report in the Journal of Experimental Biology that this pattern of narrow stripes makes zebras "unattractive" to the flies.
They key to this effect is in how the striped patterns reflect light.

"We started off studying horses with black, brown or white coats," explained Susanne Akesson from Lund University, a member of the international research team that carried out the study.
"We found that in the black and brown horses, we get horizontally polarised light." This effect made the dark-coloured horses very attractive to flies.

It means that the light that bounces off the horse's dark coat - and travels in waves to the eyes of a hungry fly - moves along a horizontal plane, like a snake slithering along with its body flat to the floor.
Dr Akesson and her colleagues found that horseflies, or tabanids, were very attracted by these "flat" waves of light.

"From a white coat, you get unpolarised light [reflected]," she explained. Unpolarised light waves travel along any and every plane, and are much less attractive to flies. As a result, white-coated horses are much less troubled by horseflies than their dark-coloured relatives.

Having discovered the flies' preference for dark coats, the team then became interested in zebras. They wanted to know what kind of light would bounce off the striped body of a zebra, and how this would affect the biting flies that are a horse's most irritating enemy.

"We created an experimental set-up where we painted the different patterns onto boards," Dr Akesson told BBC Nature.
She and her colleagues placed a blackboard, a whiteboard, and several boards with stripes of varying widths into one of the fields of a horse farm in rural Hungary.
"We put insect glue on the boards and counted the number of flies that each one attracted," she explained.

The striped board that was the closest match to the actual pattern of a zebra's coat attracted by far the fewest flies, "even less than the white boards that were reflecting unpolarised light," Dr Akesson said.
"That was a surprise because, in a striped pattern, you still have these dark areas that are reflecting horizontally polarised light.
"But the narrower (and more zebra-like) the stripes, the less attractive they were to the flies."

To test horseflies' reaction to a more realistic 3-D target, the team put four life-size "sticky horse models " into the field - one brown, one black, one white and one black-and-white striped, like a zebra.
The researchers collected the trapped flies every two days, and found that the zebra-striped horse model attracted the fewest.

Prof Matthew Cobb, an evolutionary biologist from the University of Manchester pointed out that the experiment was "rigorous and fascinating" but did not exclude the other hypotheses about the origin of zebras' stripes.
"Above all, for this explanation to be true, the authors would have to show that tabanid fly bites are a major selection pressure on zebras, but not on horses and donkeys found elsewhere in the world... none of which are stripy," he told BBC Nature.
"[They] recognise this in their study, and my hunch is that there is not a single explanation and that many factors are involved in the zebra's stripes.
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I knew the job was dangerous when I took it ...
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Jul 19, 2004
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Out of Bounds
Why zebras evolved their characteristic black-and-white stripes has been the subject of decades of debate among scientists.
Now researchers from Hungary and Sweden claim to have solved the mystery.
The stripes, they say, came about to keep away blood-sucking flies.
New research supports the notion a zebra's stripes are primarily useful in warding off flies, but the stripes are effective at this by confusing flies trying to land rather than anything to do with polarized light reflection.
The Mystery of Why Zebras Have Their Stripes Has Baffled Scientists – Now a Dazzling Answer

The mystery of why zebras have their characteristic stripes has perplexed researchers for over a century.

Over the last decade, Professor Tim Caro at the University of Bristol’s School of Biological Sciences has examined and discredited many popular theories such as their use as camouflage from predators, a cooling mechanism through the formation of convection currents and a role in social interactions.

Stripes acting to confuse predators is another common explanation, but it too is flawed when looking at the scientific data. Instead, mounting evidence suggests that it is parasitic flies that are confounded by the zebra’s distinctive patterning.

In a new paper published today in the journal Proceedings of the Royal Society B, Bristol scientists have now provided significant depth to this hypothesis by narrowing down the possible mechanism. ...

Essentially, stripes dazzled the flies, forcing them to collide with the skin or fly away altogether. In their new study they explored a potential mechanism explaining how the stripes lead to this outcome: the aperture effect.

Lead author Dr. Martin How, also from Bristol’s School of Biological Sciences, said: “The aperture effect is a well-known optical illusion that, in human vision, is also known as the barber-pole effect. Moving stripes, such as those on the rotating barber-pole signs outside barbershops, appear to move at right angles to the stripe, rather than in their true direction, so the pole appears to move upwards, rather than around its axle.

“We set out to see if this illusion also takes place in the eyes of biting flies as they come to land on striped hosts." ...