WHY ZEBRAS DEVELOPED STRIPES?
©Edgar Angelone, Smithsonian Magazine Photo Contest
Researchers from Hungary and Sweden claim to have solved the mystery of zebra stripes. The stripes, they say, came about to keep away blood-sucking flies.
"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," making dark-coloured horses very attractive to flies. 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. 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 scatter along any plane, and are much less attractive to flies. As a result, white-coated horses are much less troubled by horseflies than the dark colored horses.
Having discovered the flies’ preference for dark coats, the team then became interested in zebras. What kind of light would bounce off the striped body of a zebra?
We painted different patterns onto boards,” then placed a blackboard, a whiteboard, and several boards with stripes of varying widths into 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.”
The striped board that most closely match to the pattern of a zebra’s coat attracted the fewest flies, “even less than the white boards”. “That was a surprise because, in a striped pattern, you still have these dark areas that are reflecting attractive horizontally polarised light.
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 “zebra-striped”. The researchers collected the trapped flies every two days, and found that the zebra-striped horse model attracted the fewest. Source
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