Why is the sky blue? Check. What makes a rainbow? Check. Why do zebras have stripes? Errr … that last one has scientists stumped. Still. In fact, since Charles Darwin first espoused his theories about evolution in the nineteenth century, scientists have been trying to figure out exactly why the several species of African equids known as zebras have their distinctive black-and-white striped coats.
It’s not a black and white issue, but new research has revealed that zebras have an unexpected ability to raise the hair on their black stripes like velvet while the white stripes remain flat.
Why do zebras have stripes?
The point of zebras’ stripes has been the subject of much discussion. Scientists have mostly concentrated on the theory that zebras’ stripes keep them cool in the sun. That theory states that the black stripes get warmer than the white, creating small vortexes where hot and cold air meet that act as a fan to cool the body. What’s more, zebra stripes become remarkably more pronounced on animals living in the hottest climates, near the equator. However, the July 2018 paper ‘Experimental evidence that stripes do not cool zebras‘ published in Scientific Reports refuted that theory, but the researchers measured the changing temperatures of striped (and non-striped) barrels of water left out in the sun. No actual zebras were involved.
A hair-raising theory
The new study ‘Do zebra stripes influence thermoregulation?’, published in the Journal of Natural History by amateur naturalist and former biology technician, Alison Cobb and her zoologist husband, Dr. Stephen Cobb, uses field data from Kenya. It included two live zebras, a stallion and a mare, as well as a zebra hide draped over a clothes-horse as a control. It’s the first time zebras have been assessed in their natural habitat to investigate the role of stripes in temperature control. The research confirms the thermoregulation theory. However, as well as small-scale convection currents created between the stripes which aid evaporation of sweat, the authors argue that zebras can erect their black stripes.
It turns out that it’s not what color a zebra’s stripes are, it’s what they do with them that counts.
How they did it
The previously unrecorded ability of zebras to erect their black stripes was discovered by comparing the temperatures of living zebras to a zebra’s hide. The latter got hotter than the former by as much as 16°C/61°F. The authors propose that the raising of black hairs transfers heat from the skin to the hair surface. However, they also acknowledge that the vortexes where hot and cold air meets give credence to another theory that zebra’s stripes help it avoid blood-sucking parasites.
“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 Alison Cobb, lead author of the new paper. “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.”
“The solution to the zebra’s heat-balance challenge is cleverer, more complex and beautiful than we’d imagined.”