Being a zebra is very uncool, science says
Scientists still trying to figure out why zebras have stripes, but they do know they're not to keep them cool
The British explorer Francis Galton was one of the first to speculate on the purpose of zebras’ stripes, and 167 years later scientists are still at it.
In the latest study — one of dozens since Charles Darwin criticised Alfred Russel Wallace’s 1867 theory that the stripes provided camouflage in tall grass — Hungarian scientists say they have finally disproved one of the most popular hypotheses: that they are there to cool zebras.Their experiment involved wrapping metal oil drums in animal hides, filling them with water and leaving them in the sun for four months. And their conclusion, just published in the journal Nature, is that if the stripes are there for cooling, they don’t work.
In fact, a white skin is the most efficient at cooling core temperature, leaving three main theories about zebras’ stripes standing:
• They are for camouflage;
• They facilitate social interactions; and
• They repel biting flies.Galton arrived in Cape Town in 1851 and made his way to the central and northern parts of what is now Namibia, an area inhabited by the Damaras.
In his book Narrative of an Explorer in Tropical South Africa: Being an Account of a Visit to Damaraland in 1851, Galton speculated that the zebras’ stripes were to camouflage them, particularly in the dark.
“On a bright starlight night the breathing of one may be heard close by you and yet you will be positively unable to see the animal,” he wrote.“If the black stripes were more numerous he would be seen as a black mass, if the white as a white one, but their proportion is such as exactly to match the pale tint which arid ground possesses when seen by moonlight.”
The Hungarian scientists, led by Gábor Horváth from ELTE Eötvös Loránd University in Budapest, say the hypothesis that the stripes repel flies has been corroborated by experiments, but their experiments focused on the theory of thermoregulation.
According to Horváth, the theory says rising warm air over black stripes is replaced by cooler air from adjacent white stripes.
“Infrared photography of zebras showed that sunlit black stripes are warmer than sunlit white stripes, and that the difference between them increases with rising air temperature,” he said.But his study measured core temperature instead, and found it was mainly governed by the average whiteness of the hides covering the barrels of water.
Horváth’s team did their experiments in the summer of 2017 at a horse farm in the small city of Göd, north of Budapest. They obtained horse, cattle and zebra hides from farmers and a zoo, and pickled them with salt and formic acid. They also created an artificial zebra hide by sewing together strips of black and white horse hide.
While they were still wet, they drew the hides onto barrels so they tightened as they dried. Then they put the barrels on wooden stands in full sunlight.
Their measurements showed that core temperature was always highest in the black-covered barrel, lowest in the white barrel, and similar in striped and grey barrels.“All these experimental findings provide evidence against the hypothesis of cooling effect of zebra stripes, because striped coats do not keep the core temperature of the body any cooler than homogeneous grey coats with a similar average whiteness,” said Horváth.
In 2014, University of California wildlife biologists claimed zebras’ stripes evolved to repel insects such as horseflies and tsetse flies, which tend to avoid striped surfaces.
They went as far as recommending that striped clothing could help protect holidaymakers from insect bites, the Daily Telegraph reported.
The study, published in the journal Nature Communications, mapped seven species of zebra, horse and ass. It compared the animals’ geographic reach with variables such as habitats, the range of predators, temperatures and the numbers of ectoparasites such as tsetse flies.
The scientists found that avoiding bloodsucking flies was the most consistent explanation for zebras’ marked coats.