Recent study results suggest that zebra stripes are used to control body temperature, after all—and reveal a new mechanism for how this might be achieved.
The study authors argue that the special way zebras sweat to cool down and the small-scale convection currents created between the stripes aid evaporation, while zebras’ previously unrecorded ability to erect their black stripes is a further aid to heat loss. These three elements are key to understanding how the zebras’ unique patterning helps them manage their temperature in the heat, the team said.
Amateur naturalist and former biology technician Alison Cobb and her zoologist husband, Stephen Cobb, PhD, recently published their findings. Together, they have spent many years in sub-Saharan Africa, where he’s directed environmental research and development projects.
This study is the first-time zebras have been assessed in their natural habitat to investigate the role of stripes in temperature control. The researchers collected field data from two live zebras, a stallion and a mare, together with a zebra hide draped over a clothes horse as a control, in Kenya.
The data revealed a temperature difference between the black and white stripes that increases as the day heats up. While this difference stabilizes on living zebras during the middle seven hours of the day, with the black stripes 12-15°C hotter than the white, the stripes on a lifeless zebra hide continue to heat up, by as much as another 16°C. This indicates there is an underlying mechanism to suppress heating in living zebras. Thus, the way the zebra stripes are harnessed as one part of their cooling system, rather than just their contrasting coat color, is key to understanding why these animals have their unique patterning, the team said.
Like all equine species, zebras sweat to keep cool. Recent research reveals that the passage of sweat from the skin to the tips of the hairs is facilitated by a protein called latherin which is also present in zebras. This makes the sweat frothy, increasing its surface area and lowering its surface tension so it evaporates and prevents the animal from overheating.
The researchers believe the differential temperatures and air activity on the black and white stripes set up small-scale convective air movements within and just above the stripes, which destabilize air and water vapor at the tips of the hairs.
During the field research, the authors also observed—probably for the first time—that zebras have an unexpected ability to raise the hair on their black stripes (like velvet) while the white ones remain flat. The authors propose that raising black hairs during the heat of the day, when the stripes are at different temperatures, assists with the transfer of heat from the skin to the hair surface; conversely, when the stripes are at the same temperature in the early morning and there is no air movement, the raised black hairs will help trap air to reduce heat loss.
These three components—convective air movements, latherin-aided sweating, and hair-raising—work together as a mechanism to enable zebras to wick sweat away from their skin to encourage more efficient evaporation, which helps them cool down.
The authors also speculate that unstable air associated with the stripes might play a secondary role in deterring biting flies from landing on them. This insect behavior has been observed in recently published studies about zebra stripes and could confer an additional advantage to the unique coat coloring.
Evidence from other recent studies supports the idea that heat control might be key to why zebras have their striking coats. Researchers have shown that zebra stripes become more pronounced on animals living in the hottest climates, near the equator. Zebras are also smallest near the equator, which provides a large surface-area-to-volume ratio which assists the animals’ ability to dissipate heat through evaporation.
“Ever since I read How the Leopard Got His Spots in Kipling’s Just So Stories at bedtime when I was about four, I have wondered what zebra stripes are for,” said lead author Alison Cobb. “In the many years we spent living in Africa, we were always struck by how much time zebras spent grazing in the blazing heat of the day and felt the stripes might be helping them to control their temperature in some way.”
She said she started trying to test her hypothesis 40 years ago by comparing the temperatures of water in oil drums with differently colored felt coats. However, she said “this was not a good enough experiment” and she wanted to investigate stripes on live zebras.
“Steve—the man who later became my husband and co-author—teaching conservation biology in the University of Nairobi, had a student working with zebras, who said he could calm them down in their crush by brushing them with a long-handled broom,” Alison Cobb said. “This gave me courage in 1991 to ask permission to go into the animal orphanage in Nairobi National Park to see if I could tame one of the wild zebras in the paddock by brushing it with a dandy brush. Apart from its capture, it had never been touched by a human.
“To my immense pleasure it found this tickling very agreeable and as the days went by it gradually allowed me to brush it all over,” she said. “Two years later I came back to Nairobi and walked into the paddock with the brush. The same zebra mare lifted her head, looked at me hard, and walked up to me to be brushed again.”
Alison Cobb said it wasn’t until recently that she and Steve Cobb had the chance to collect field data from zebras in Africa and observed their ability to raise their black hairs.
“The solution to the zebra’s heat-balance challenge is cleverer, more complex and beautiful than we’d imagined,” she said. “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.”
The study, “Do zebra stripes influence thermoregulation?” was published in Journal of Natural History.