For Tim Caro, it was surprisingly easy to dress horses like zebras. Several vendors were already selling coats with black-and-white stripes, often as fun gimmicks. But, as Caro learned, such coverings have an unexpectedly serious effect. “There are enormous benefits to having a striped coat for a horse,” he told me.
Caro, a biologist at the University of California at Davis, has spent years thinking about why zebras are striped, and has even written a book about this mystery. In his latest bid to get clear answers, he and his colleagues traveled to Hill Livery, a stable in southwest England that keeps several captive zebras alongside domestic horses. By comparing these two species, as well as horses that were comically cloaked in zebra-striped coats, the team found fresh evidence for what Caro thinks is the only plausible explanation for the striking stripes: They evolved to deter bloodsucking flies.
Scientists have been puzzling over the role of zebra stripes for more than 150 years. But, one by one, the most commonly proposed explanations have all been refuted. Some researchers have suggested that the stripes act as camouflage—they break up zebras’ outlines or resemble fields of tree trunks. But that can’t be true: Amanda Melin of the University of Calgary recently showed that lions and hyenas can’t even make out the stripes unless they get very close. Another hypothesis says that the black stripes heat up faster than the white ones, setting up circulating air currents that cool the zebras. But a recent study showed that water drums cloaked in zebra pelts heat up just as much as those covered in normal horse skins.