Last summer, a series of rather unusual water-filled canisters were left to bake in the northern Hungarian sun. Each barrel was outfitted with a different skin: white, black, brown, gray or black-and-white stripes. From far away, they almost resembled the torsos of lazily grazing animals—only, when flies flicked thirstily onto their sides, no tail swatted them away.
Strange as it may seem, biophysicist Gábor Horváth and his colleagues had placed the barrels not for the sake of eccentric art, but to settle an age-old evolutionary debate: why do zebras have stripes?
One longstanding theory posits that the stripes are actually conducive to keeping zebras cool. Because black hair absorbs more heat than white hair, the air above black stripes may be prone to forming strong, fast microcurrents. When these hot pockets meet the comparatively sluggish air moseying over the white stripes, the clash could create little vortexes of chilled air. In keeping with this idea, one team of researchers published findings in 2015 that demonstrated the warmer the climate, the stripier the zebras.
But Horváth and colleagues from Hungary and Sweden have shown that black and white stripes are no better than uniform brown or gray at keeping the heat at bay—at least, if you’re a barrel full of water.
In their study, published last week in the journal Scientific Reports, the researchers covered metal barrels with white cattle hide, black cattle hide, brown horse hide, light gray cattle hide, real zebra hide or an artificial coat composed of stripes of black and white cattle hide as a zebra imitation. They then placed thermometers inside the canisters to measure the internal temperature of the water within.
Unsurprisingly, they found that the white cattle hide kept its barrel coolest in the afternoon sun. On the other end of the spectrum, the black cattle hide kept its canister cooking at a temperature hotter by 6 to 7 degrees Celsius. But while the black stripes were consistently hotter than the white stripes on both the real and artificial zebra hides, reliably mimicking the contrast observed on the hides of living zebras, the core temperatures in both zebra barrels were about the same as those in the brown- and gray-coated containers. The amount of whiteness in the coat seemed to be the biggest predictor of cooler internal temperatures. The experiment determined that when it comes to temperature regulation, stripes, it turns out, have the flash, but not the substance.
This is not the first time that scientists have poked holes in the “cool stripes” theory. For one, the theory makes sense only in direct sunlight: in the shade, the lack of juxtaposition between hot black air and cool white air would preclude the formation of icy vortexes. What’s more, even if these the swirls of cooling air were to form, they would be easily disrupted by any passing breeze or even the slightest twitch of a zebra flank, according to Timothy Caro, an evolutionary ecologist at the University of California at Davis. Additionally, this theory applies only to flat, horizontal surfaces parallel to the ground. Along the sides of zebras, vertical black stripes simply absorb sunlight without being tempered by this phenomenon, putting most of the zebra’s body at a thermoregulatory disadvantage. Also problematic? The conspicuous lack of other striped creatures in hot, arid environments. In fact, most mammals that brave these scorching climes come in more expected shades of white or light yellow.
Tony Barthel, a curator and zebra caretaker at the Smithsonian’s National Zoo not associated with the study, agrees that the study’s findings (literally and figuratively) hold water, but points out that even gussied up with stripes, barrels are not zebras. “It doesn’t answer whether there could be another mechanism, like different internal blood flow under the stripes, at work here,” Barthel explains. “They’re not able to address that with the way they set it up.”
However, a large body of research in the field supports several other possible evolutionary drivers for snazzy stripes. One widely accepted possibility, supported by previous work from both Horváth and Caro, speculates that stripes help repel the nasty nips of bloodsucking insects, who appear flummoxed by black and white stripes. Other more hotly contested hypotheses suppose that stripes may help with predator avoidance or as visual cues for distinguishing zebras from one another, as individuals appear to be uniquely patterned.
Of course, Barthel says that putting stock in one theory certainly doesn’t rule out the possibility of others. “Adaptations can have multiple benefits,” he says. “It doesn’t have to just be one or the other—whatever the reasons zebras have stripes, there’s probably more than one.”
What can we say? When it comes to the zebra and her stripes, the issue remains far from black and white.