Like many other preschoolers, young Alison was intrigued by zebras and their striking black and white stripes. Why would an animal have such curious coloring? That question and its unsatisfying answer—that no one really knows—stuck with her into adulthood. Her affinity for zebras only grew during the several years she lived in Africa.
For her seventieth birthday, Alison Cobb’s husband, a wildlife management consultant, indulged her curiosity with a one-of-a-kind gift: a trip to the equator to study the temperatures of zebra stripes. Now 85-years old, Cobb has just published those results in the Journal of Natural History, her first science journal article. Her work provides field-gathered evidence that stripes play a key role in temperature regulation.
Over the years, scientists have presented many ideas about the evolution of zebra stripes. Some argue that the visual nature of the pattern is key, that the animals can hide from or confuse predators and disease-carrying flies via the black-white contrast. Others propose that the stripes, which are unique to each zebra, aid communication. Some argue that it’s about temperature, as infrared images of zebras show that black stripes are hotter than white stripes in the sun. There have been lots of ideas, but not a clear resolution.
On the trip to Kenya, Cobb and her husband (Stephen Cobb, co-author of the paper) measured the temperature of stripes on two zebras. The zebras were born in the wild but living on ranches, free to roam and graze in large pens. Over the course of one day, during daylight, the researchers recorded the temperature of adjacent black and white stripes every 15 minutes on the neck, flank, and rump of each animal. They made similar measurements on a dried zebra hide for comparison. Here’s what they found.
In the early morning, the black stripes on live zebras were slightly cooler than the white stripes. As the day warmed up, the average temperature of the black stripes quickly increased. The temperature of the white stripes increased too, but not as fast or as high. During the seven hottest hours of the day, the black stripes were 12-15°C higher than the white stripes.
During the hottest hours, the average temperature of the black stripes oscillated, going up and down by as much as 10°C several times throughout the day. The temperature of the white stripes oscillated too, but over a smaller range.
On the dried zebra hide, the black and white stripes started off at the same temperature. As the sun rose, both temperatures rose quickly, peaking earlier and 15°C hotter than in the live zebras. The temperature difference between the black and white stripes was about the same as in the live zebras, about 10-15°C, until both colors cooled down in the evening.
These results imply the stripes play a role in temperature regulation that goes beyond the surface. “The fact that the living zebras maintained lower temperatures at the tips of the hair (the surface that we were measuring) than those of the hide suggests that living zebras may have some mechanism that helps them to suppress heat gain,” the authors write.
Zebras live in dry, hot climates. They have inefficient digestive systems and need to spend most of the day eating; they don’t have the luxury of hiding out during the hottest parts of the day like some animals. Past research hasn’t uncovered any unique internal cooling mechanisms aside from the fact that like horses, zebras sweat. When sweat evaporates from the skin, some heat is carried away and a zebra’s core temperature cools down.
Drawing on previous work that shows zebras in hotter climates tend to be smaller and have more and brighter stripes, Cobb and Cobb revisit the idea that contrasting stripes increase the efficiency of the sweating process. Since black and white stripes heat up differently, the air near a zebra’s skin likely contains small-scale air currents and turbulence. These could act to increase the overall airflow and therefore the pace at which evaporation occurs, they say.
Furthermore, they observed an interesting behavior that they say could aid this process. It’s never been reported before, but the hair on a zebra’s black stripes can stand up on end, even while the hair on its white stripes lays flat. They propose that by doing this in sunlight, zebras might be introducing even greater turbulence in the airflow, promoting more efficient sweat evaporation, and providing additional paths by which warm air can leave the skin.
“The solution to the zebra’s heat-balance challenge is cleverer, more complex and beautiful than we’d imagined,” says Cobb in a statement. “Of course, there is much more work to be done to gather evidence and fully understand how the stripes help zebras control temperature, but I am 85 now, so that’s for others to do.”
Scientists are still straightening out the history of zebra stripes via PopSci
Zebra Stripes May Help Beat the Heat via Inside Science