Popular Science thinks they know – and the answer is literally cool:
Now the 85-year-old amateur scientist [Allison Cobb] has published her first scientific study [in the Journal of Natural History], co-authored by her husband Steven Cobb, a zoologist and environmental consultant. The study used data from the couple’s trip to Kenya in 2003. With the help of a local, the Cobbs gained access to two semi-tame zebras, one of which was an “extremely naughty” house pet allowed to drink Coca-Cola, she says.
Using the two zebras as her subjects and a zebra hide stretched out over a clothing rack as their control (to see if striping had a different effect on the live animal) they began to collect field data. With a laser infrared thermometer, Cobb measured six individual stripes on each of the two animals and the zebra hide: two adjacent black and white stripes at the neck, two in the middle, and two at the zebra’s rump. She continued to record temperatures at these sights every fifteen minutes during daylight hours.
Over the course of the day the temperature difference between the black and white stripes increased, with the black stripes becoming 18 to 27 degrees Fahrenheit hotter. Other studies before Cobb have suggested that this temperature difference creates a convection current—a flow of air created by the shift in heat that cools the animal down.
But the Cobbs think there’s more to it than that.
Cobb suggests that the tiny air current created by the differential temperature in stripes creates a “chaotic air movement above the hair surface” that further speeds up the evaporation process.
Over the course of the field work, Cobb also became the first to note that a zebra can erect its black hairs—to look and feel almost like velvet—while its white hairs remain flat. This trait may also help air flow through the hide and aid in evaporation. These findings would also help explain why the zebra hide control was as much as 29 degrees F hotter than the live zebras. The animal’s cooling is much more complex than the stripes alone.
“We do not think it is a big enough reason for stripes to have evolved,” Cobb says about the fly-repeller theory. A look at different African zebra populations reveals that the majority—though not all—striped zebras are concentrated at the equator and the farther the species get into milder climates, the fewer stripes they flaunt. A trend, she says, suggests stripes could have evolved foremost in response to temperature. But there is a way the fly-repelling theory and her own could be linked.
Perhaps the tiny currents of air produced by the differently tempered stripes go unnoticed by us but make it a very awkward and turbulent environment for these flies to land; similar to how a small plane struggles to land amidst gusting winds. Earlier this year a study by Caro showed that in the milliseconds before landing, flies attempting to land on zebras lose control, colliding with and ricocheting off their intended lunch.