The zebra has one of the fanciest coats in the animal kingdom. And yet, the evolutionary underpinnings of the animal's black and white stripes remain something of a biological mystery.
At least 18 theories attempt to explain the function of zebra stripes, one of the most popular suggesting they provide some kind of external cooling system. Unfortunately, a study recently published in Scientific Reports has shot it down, showing that if a stripey coat really is meant to cool the zebra down, it doesn't do a very good job of it.
The idea assumes the black markings will get hotter than the white markings, causing little vortices of air to form above the fur where warm air over the black areas comes into contact with cool air above the white areas. This, it continues to assume, will create a neat system of air currents (or eddies) that effectively work like a fan, cooling the zebra as it wanders the African savanna.
Only it doesn't. Not according to new research carried out by biologists at Lund University in Sweden. The team put the theory to the test by placing several metal barrels filled with water and covered in a zebra skin imitation out in the Sun, measuring the temperature of the water before and after the experiment. Each barrel was wrapped in a different colored skin: black, white, brown, gray, or black and white striped.
As you might expect, the black-coated barrel was the warmest and the white-coated barrel was the coolest. However, the differences between the striped and gray barrels were minimal and did not decrease – if anything, the gray barrel did marginally better than both real zebra skin and artificial zebra skin. The precise order from coolest to warmest went white cattle, gray cattle, real zebra, artificial zebra, gray horse, black cattle.
The lack of noticeable difference between the stripey barrels and the gray barrels was found regardless of outside temperature and wind speed.
"The stripes didn't lower the temperature," Susanne Åkesson, a biologist at Lund University, said in a statement. "It turns out stripes don't actually cool zebras."
However, it might not be time to put the theory in the gutter quite yet. Previous research used temperature models to successfully predict stripe patterns, contradicting this particular study. Of course, that could be a coincidence or an indirect correlation involving a third factor.
That third factor could be horseflies and other insects. In an older study, Åkesson and colleagues from Hungary and Spain put forward an alternate theory – a zebra's stripes act like a visual insect repellant, providing an optical illusion that guards against blood-thirsty flies. Horseflies, for example, are guided by polarizing light, which puts animals with darker coats at greater risk. A combination of white and black, the theory goes, could be confusing.
So there you have it. The story of why the zebra got its stripes is still a mystery.