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Stripes do not cool zebras

Susanne Åkesson, biologist at Lund University, refutes the theory that zebras have striped fur to stay cool when the African sun is burning. That hypothesis is wrong she and her colleagues show in a new study.
Barrels covered with fur imitations in different colours.
To conduct the experiment the researchers used barrels covered with imitations of fur in different colours. Photo: Gábor Horváth

One of several theories is that the zebra has got its stripes in order to stay cool in the sunshine. The black stripes get warmer than the white areas, and the theory states that this creates small vortexes when the hotter air above the dark fur meets the cooler air above the white fur. According to the theory these vortexes works as a fan to cool the body.

The problem is that the theory is wrong, as Susanne Åkesson and her colleagues show in a study recently published in Scientific Reports.

The researchers filled big metal barrels with water and covered them with skin imitations in different colours: black and white stripes, black, white, brown and grey. Then they placed the barrels in the open air under the sun and later measured the temperature in each barrel. Not surprisingly the black one was the hottest and the white one the coolest. The striped and grey barrels were similar, in these the temperature did not go down.

”The stripes don’t lower the temperature. Stripes don’t cool the zebra”, says Susanne Åkesson.

Eight years ago she and her colleagues from Hungary and Spain presented another theory, where they claim that bright fur works as an optical protection against blood-sucking horseflies and other insects that bite. Horseflies are attracted by polarised light, the sort of light that appears when sunbeams are reflected on a dark surface. If the sunbeams are reflected on a white surface there will be no polarised light. Hence the protection.

Two years ago Susanne Åkesson, the Hungarian physicist Gábor Horváth and their colleauges were awarded with the Ig Nobel Prize for their research on polarised light, horseflies and why these blood-sucking insects bother dark horses a lot more than they bother white horses.

Jan Olsson

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