Scientists claim they’ve solved a centuries-old riddle, one that puzzled even Charles Darwin and Alfred Russel Wallace. And the answer was surprising: Zebras evolved their vivid stripes as bug repellent.
People often think that zebras have stripes for crypsis, or camouflage, reasons. For color-blind predators, the stripes could make them hard to see in tall grasses. Or, that their striking black-and-white stripes are a form of disruptive coloration, working to obscure the body contour or confuse predators and parasites through optical illusions or a distracting effect called motion dazzle when they run as a herd. (Check out some illusions here, here and here: where does one end and the other begin?) Still others have suggested that it has nothing to do with predator avoidance, that the stripes help manage heat by reducing thermal loads. Well those are all great hypotheses, but no one’s ever tested all of them together.
So, pitting a bunch of ideas against each other, Tim Caro from the University of California, Davis, and colleagues examined the geographic distribution of current and extinct equid species (which include zebras, horses and asses). Among the seven species, and some subspecies, that were examined, there were both animals with stripes and without. Then they compared the geographic ranges with environmental variables, such as large predators, ectoparasite breeding conditions, and temperature.
They found that the ranges of the most distinctively striped species (Equus burchelli, E. zebra, and E. grevyi) overlap remarkably with the areas where disease-carrying blood-suckers, like horseflies (tabanids) and tsetse flies (glossinids), are active and particularly annoying. This was consistent across different types of striping -- facial, neck, flank, rump, belly, or leg stripes of varying intensity and thickness, as well as shadow striping -- and different equid species and subspecies.
"Again and again, there was greater striping on areas of the body in those parts of the world where there was more annoyance from biting flies,” Caro says in a press release. After all, certain flies are known to avoid black and white surfaces, preferring uniformly colored ones (though why that’s the case is another mystery for another day).
In contrast, they didn’t find consistent support across species for hypotheses about camouflage, predator avoidance, heat management, or some aspect of social interaction. But why zebras? According to the study, unlike other African hooved mammals living in the same areas, zebras have hair that’s shorter than the mouthpart length of biting flies, making them particularly susceptible to annoyance.
The findings were published in Nature Communications this week.
Image: Caro et al.