A year later, a spatial modelling study of plains zebras – the most numerous species that range from eastern to southern African – led by Brenda Larison of University of California, Los Angeles, found stronger striping patterns in areas that are warmer or receive more intense sunlight. This seemingly straightforward logic has, however, received mixed support.Ĭaro and his team found only a weak spatial overlap between striping patterns and maximum temperatures. The basic idea is that black stripes would absorb heat in the morning and warm up zebras, whereas white stripes reflect light more and could thus help cool zebras as they graze for hours in the blazing sun. Thermoregulation has long been suggested by scientists as the function of zebra stripes. “Every zebra must avoid getting hot, and biting flies will come at certain places, and certain times of the year, but they are by no means as definite or frequent a threat as overheating,” says Cobb. While Alison agrees with Caro’s findings, she thinks biting flies “seems too unimportant an effect” to have driven the evolution of zebra stripes. They believe that zebra stripes aid primarily in thermoregulation. However, other zebra researchers, like retired animal lab technician Alison Cobb and zoologist Stephen Cobb of Oxford in the UK, aren’t convinced by the parasite-deterring explanation. And in Princeton University, evolutionary biologist Daniel Rubenstein and his collaborators are tackling the question using “fly vision in virtual reality”. “It looks as if they cannot recognise that black and white surface as a good landing spot,” says Caro.Ĭaro says his team is working with “lots of unpublished data” from videos of flies approaching different patterns to learn how the stripes mess up a fly’s landing. The flies would try to land on the stripes, but then fail to decelerate as they normally would approaching a non-striped surface, and bounce off. The horseflies hovered around zebras and horses in similar amounts, but far fewer flies landed on zebras – or horses with striped coats. They observed horseflies around zebras and horses some horses had black, white or striped coats placed on them. The research at Hill Livery earlier this year, shed new light for Caro’s team. “We also found absolutely no support for the other hypotheses.” “ really showed something really remarkable to us,” says Caro. While scientists still debate the exact origins and functions of zebra stripes, their recent efforts have focused more closely on three possibilities protection from biting flies, thermoregulation and protection from predators. These striping differences, coupled with the challenges that zebras face in their environment, have guided our understanding of how the stripes might work. Striping patterns and intensity vary across species as well as location. The three living species of zebras that roam eastern and southern Africa with their coat of dark hair broken by stripes of white, unpigmented hair, are the only striped equids. Zebras, together with horses and asses, are members of the Equus genus. But, for a long time new theories were introduced without rigorous tests. Scientists have put forward at least 18 reasons why, from camouflage or warning colours, to more creative explanations like unique markers that help to identify individuals like a human fingerprint. How and why zebras evolved to sport black and white stripes are questions that have tested scientists for over a century. “People have been talking about zebra stripes for over a hundred years, but it's just a matter of really doing experiments and thinking clearly about the issue to understand it better,” he says. For Tim Caro, an ecologist from the University of St Andrews who has been studying zebra stripes for almost two decades, the livery yard’s relatively tame zebras provided a rare opportunity to stand within metres of them and observe them.
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