Why Do Zebras Have Stripes? To Keep Away Horseflies

Zebra StripesThe question has been speculated since 1870 when Charles Darwin criticized Alfred Russel Wallace and his theory about Zebra’s having stripes to help camouflage themselves in tall grass. Darwin’s argument was that Zebras like the Savannahs were the grass is short.

Scientists from Hungary and Sweden investigated previously and found that brown and black horses attracted more horseflies than white horses because of the way the light reflects of their coats. When the sun reflects off water, or off a dark horse coat, it creates a horizontal polarized light wave. Insects are drawn to that type of light because it’s a sign of water.

Susanne Akesson from Lund University, a member of the international research team that carried out the study said, “We started off studying horses with black, brown or white coats. 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.”

Zebra embryos start out with dark skin and then develop their white stripes before birth. So does this make Zebra’s less attractive for the horseflies? “In contrast, the dark and light stripes of zebras each reflect different polarizations of light, and the fact they are arranged vertically might ward off horseflies looking for smooth, horizontally polarized signals,” researcher Susanne Åkesson, an evolutionary ecologist at Lund University in Sweden explained.

Why Do Zebras Have Stripes

Researchers visited a horsefly infested from near Budapest where they set up an experiment to see which coat do the flies prefer. Dr Akesson said, “We created an experimental set-up where we painted the different patterns onto boards.” They put up a black board, a white board and several boards with different type of stripes and varying in widths. They then put insect glue on each of the boards and counted the number of flies that it attracted.  Researchers found the board that was closest to a pattern of a Zebra had the least amount of flies on it. Even less than the white board that reflects unpolarised light.

To test the horseflies reaction, they placed four life-size 3-D 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.

“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 less attractive they were to the flies.,” Dr. Akesson explained.

Scientists still need to perform experiments as to if odors or any other molecules from Zebras help fight off horseflies.

Image Source: bbc.co.uk

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