Using the two zebra’s 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 thinks there’s more to it than that. The convection current is likely happening, she says, but its too small to cool off the entire animal. They think a more intricate mechanism is at play.
Zebras, like horses, sweat to cool down. As they perspire, the sweat lathers, loosening the surface tension of the sweat and allowing the stuff to evaporate more easily at the hair’s tips. 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.
Though the idea of tiny convection currents between stripes is possible, says Tim Caro a wildlife biologist at the University of California, Davis, he isn’t quite convinced. A convection current, he says, would be too easily disrupted by movement and wind. Caro, himself, believes the strongest theory behind zebra stripes is that they repel biting flies, though not all expert sagree on that either.
It was, in fact, the studies that showed zebra stripes deterred flies, that encouraged the Cobbs to take a look back at their own data more than 10 years after their trip.