Why zebras have stripes
It’s not to keep them cool.
Pondering the purpose of zebra stripes has been a popular pastime for naturalists stretching back to Charles Darwin.
As many as 18 theories have been proposed. Now a team led by Gabor Horvath at Eötvös Loránd University in Hungary, has succeeded in reducing the number by one.
Their findings published in Scientific Reports in July, demonstrate that the stripes do not help the animals stay cool beneath the African sun.
The most popular explanation for the function of zebra stripes is that they provide camouflage. But even Darwin had a hard time seeing how stripes offered protection on the open plains. And recent studies have found that zebra stripes, whose distinctness varies across African herds from north to south, do not show a strong correlation with the presence of large predators. They did however correlate with the presence of swarms of biting horse flies and tse tse flies. That favoured the view that stripes dissuade flies, a finding borne out by studies in the lab.
Stripes also correlate with climatic conditions - herds in the warmer northern regions have more distinct stripes than those in the southern cooler climes. That has favoured the theory that stripes help cool the zebra. How could stripes provide cooling? Given that the animals’ black and white bands absorb and reflect heat at different rates, the temperature gradient between the zones might create a tiny breeze that cools the animal down.
To test this theory, Horvath and colleagues used water-filled metal barrels, each covered by a pelt from either a horse, cow or a zebra. The skins covered the spectrum from solid black, solid white, grey and stripey.
The barrels were left out in the sun for four months and their core temperatures measured continually.
Alas, even after runs of several hot days in a row, the temperature of the zebrastriped barrel was not much different to that of the grey cow and grey horse stand ins.
The authors reported the average core temperature of the barrels increased as follows: white cow, grey cow, zebra, grey horse, black cow.
One theory down, 17 to go.