Why zebras have stripes has been a matter for scientific debate for decades. A lot of theories have been put forth, such as they’ve evolved for camouflage, to confuse the vision of biting flies, to bewilder predators with “motion dazzle” or that they’re a means of identification among zebras themselves.
But now, new research postulates that zebra stripes may create air flows that provide the animals with a kind of natural air-conditioning system that helps them cool off in the heat of the African sun.
In living black-and-white
In a recent study, which was published on June 13, 2019 in the Journal of Natural History, the scientific publication of the British Natural History Museum, amateur naturalist and former biology technician Alison Cobb and her husband, zoologist Dr. Stephen Cobb, measured the temperature differences between the black stripes and the white stripes on two zebras living in their natural habitats in Kenya, Africa—something that hadn’t been tried before.
The Cobbs had spent many years living in sub-Saharan Africa and were always amazed by how much time zebras spent grazing in the blazing heat of the day—for far more hours than the antelopes living in the same area. The Cobbs then moved to England; and without direct access to research animals, Alison enlisted the help of her three daughters, aged 8, 9 and 10. She made them wear rugby shirts on which she had sewed black-and-white stripes and encouraged her “experimental animals” to crawl around on their hands and knees in the sun.
Without looking, when Alison touched the different colored stripes on the girls’ backs, she could tell which ones were black and which were white just by how hot they were. But this made her wonder: if white stripes are cooler, why would zebras have the black ones at all?
By December 2003, the Cobbs were again living in Africa and got a chance to test some of their ideas about the question on a couple of captive zebras living on private ranches in Kenya. They measured the temperatures of adjacent black and white stripes on various parts of the zebras every 15 minutes, as well as taking ambient air temperatures near the animals. They also took similar measurements of a zebra hide wrapped around clothes in the shape of a horse left in the sun on the ranch.
Surprisingly, what they found was that the temperature of the black stripes and the white stripes differed greatly on the living animals and that the temperatures widened as the day heated up: the black stripes ended up being up to 27 degrees Fahrenheit hotter than the white stripes. That disparity, say the Cobbs, is enough to produce small convection currents (or air eddies) above the zebras’ skins that help to keep the animals cool by speeding up the evaporation of sweat.
The stripes on the inanimate hide had a similar difference between black and white stripes, but the highest temperatures of the black stripes could get up to another 27 degrees Fahrenheit hotter than the living animals’ coats. The living zebras’ black stripes got up to 132.8 degrees Fahrenheit, while the black stripes on the nonliving hide got up to a scorching 159.8 degrees Fahrenheit. In other words, the temperature didn’t stabilize throughout the day like it did in the living animals.
This indicates that there is an underlying mechanism to suppress heating in living zebras, and color is just one part of it. The Cobbs think they may know what another significant portion is.
While conducting their research, the Cobbs also noted that the living zebras have an unexpected ability to raise the hair on their black stripes straight up in the air (like velvet) while the white ones remain flat. And they did so at some of the hottest times of the day.
The Cobbs think that this raising of the black hairs when it’s hottest and when the temperature between the black stripes and the white stripes is at its most variant assists with the transfer of heat from the skin to the hair surface, allowing air to flow out and water to evaporate more quickly. Conversely, when the stripes are at the same temperature in the early morning and there is no air movement, the raised black hairs could help trap the cooler air and keep it in.
There is evidence from other recent studies that backs up the idea that heat control may be key to why zebras have their striking coats. It has been demonstrated that the zebra stripes become remarkably more pronounced on animals living in the hottest climates, near the equator. Zebras are also smallest near the equator, providing a large surface area to volume ratio, which assists the animals’ ability to dissipate heat through evaporation.
But Gabor Horvath, a researcher at Eotvos Lorand University in Budapest, Hungary, who has studied zebra stripes, does not believe that thermoregulation is the primary function of zebra stripes. He thinks that if the main function of zebra stripes were cooling by air eddies, then only the nearly horizontal areas on the zebras’ backs should be striped. And Tim Caro, a wildlife biologist at the University of California, Davis, said that he doesn’t think the the Cobbs’ study advances our understanding of the principal evolutionary drivers of these stripe patterns in the animals. His research and that of others indicates that the stripes could deter insects from landing.
At the same time, though, Caro is open to the idea that the stripes have thermal consequences for zebras. He, Horvath and the Cobbs all agree that likely there isn’t only one, single reason zebras have evolved stripes.
What is unequivocal from the results of this new study is that zebras are far more complex and beautiful than we previously imagined. And as global temperatures continue to rise, there is a lot more to learn about them—and from them, perhaps, in order to keep our own cool.
Here’s to finding your true places and natural habitats,