Alison and Stephen Cobb of Oxford reveal why zebras have stripes

Now, a husband and wife team of scientists from Oxford think they have answered the question.

Alison and Stephen Cobb, who live in Binsey, have spent their lives studying the iconic black-and-white animals on the plains of sub-Saharan Africa.

Last week, they published a paper in the Journal of Natural History in which they answered the question of how the zebra got his stripes – and it is all to do with sweat.

Scientists had long thought that the stripes may have evolved as a way to control body temperature, because sweat would evaporate at different rates off the black and white stripes in the hot African sun.

However no-one ever put forward a satisfactory mechanism for how exactly this would be achieved.

The Cobbs think they have finally found that mechanism.

Wild Grant’s zebras in Laikipia District, Kenya. Picture: Alison Cobb

The couple argue that it is the tiny convection currents created between the stripes which aid evaporation of sweat, but they have also discovered a previously unrecorded ability in zebras to erect the hairs in their black stripes to further aid heat loss.

These combined elements, the Cobbs say, are key to understanding how zebras’ unique stripey patterning helps them manage their temperature in the heat.

Alison Cobb, an amateur naturalist and former biology technician, and her zoologist husband Dr Stephen Cobb, published their findings in the Journal of Natural History, the scientific publication of the British Natural History Museum.

The study was the first-time zebras had been assessed in their natural habitat to investigate the role of stripes in temperature control.

Working in the hot Kenyan sun, the Cobbs collected field data from two live zebras, a stallion and a mare, and used a zebra hide draped over a clothes horse as a control.

The data revealed a temperature difference between the black and white stripes that increased as the day heats up.

While the difference stabilised on living zebras during the middle seven hours of the day, with the black stripes 12 to 15 degrees hotter than the white, the stripes on a lifeless zebra hide continued to heat up, by as much as another 16 degrees.

That subtle difference hinted at an underlying mechanism to suppress heating in living zebras.

The Cobbs concluded that it was the specific way in which zebra stripes are harnessed as one part of their cooling system, rather than just their contrasting colours, that was key to understanding why these animals have their unique patterning.

Like all species in the horse family, zebras sweat to keep cool.

Raised hair in the black stripes of an African zebra. Picture: Alison Cobb

Recent research revealed that the passage of sweat in horses 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 overheating.

The Cobbs proposed that 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 destabilise the air and the water vapour at the tips of the hairs.

During their field research, the couple 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 proposed that the raising of 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 and 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 at that time.

These three components– convective air movements, latherin-aided sweating and hair-raising – work together as a mechanism to enable zebras to wick the sweat away from their skin so it can evaporate more efficiently, to help them cool down.

Commenting on their findings, Mrs Cobb said: "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.

"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. Of course, there is much more work to be done to fully understand how the stripes help zebras control temperature, but I am 85 now, so that’s for others to do.”