The team placed the sticky model horses in afly-infested field
  
  
 
  
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. 
  
 
-  There are many theories about why zebras are striped 
-  Scientists have proposed that  the mass of stripes in a large herd confuses predators
-  Others have shown that stripes may help the animals regulate 
their temperature, and that zebras recognise other individuals by their 
stripes
-  Studies of zebra embryos show that, early in development, they are black and they develop their white stripes later
- Horses, donkeys and zebras videos, news and facts
 
"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.  
 Coloured images revealed how light was polarised as it bounced off a zebra's coat
    Coloured images revealed how light was polarised as it bounced off a zebra's coat
   
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.  
 Horseflies prevent the animals they bite from grazing, as well as carrying blood-borne diseases
    Horseflies prevent the animals they bite from grazing, as well as carrying blood-borne diseases
   
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.