Scientists reveal why zebras have stripes

An 85-year-old inspired by Rudyard Kipling has had her first academic paper published explaining why the iconic zebra has stripes.

And she discovered that they help to keep the animal cool in the hot sun.

At the age of four, amateur naturalist and former biology technician Alison Cobb first read 'How the Leopard Got His Spots' in Kipling's Just So Stories.

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And it got her thinking about how the zebra got its stripes.

More than 80 years on, she finally discovered that the stripes control body temperature by creating convection currents as heat is lost from the skin.

They do this by the black hair rising as the white hair lies flat, boosting the evaporation process.

She 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.

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"My early attempts 40 years ago at testing this hypothesis involved comparing the temperatures of water in oil drums with differently coloured felt coats.

"But it seemed to me this was not a good enough experiment, and I wanted to see how the stripes behaved on live zebras."

Theories have ranged from stripes being used as a camouflage against lions and other predators to confusing biting flies. They are also said to attract mates.

Mrs Cobb and her zoologist husband Stephen assessed zebras in their natural habitat for the first time to investigate the role of stripes in temperature control.

The findings published in the Journal of Natural History are based on observations of two zebras - a stallion and a mare - in Kenya.

A zebra hide draped over a clothes-horse was used as a control.

Mrs Cobb has kept and ridden ponies, horses and donkeys since she was five.

In December 2003, as a 70th birthday present from her husband, she went with him to Kenya to carry out the groundbreaking study.

More than 15 years later the research has been published in the Journal of Natural History - her first scientific paper.

For more than a century, Rudyard Kipling's 'Just So' stories have delighted children with imaginary explanations of how animals came to look the way they do.

But while the author addressed the leopard's spots and the elephant's trunk, he never explained the zebra's stripes. The latest study fills in the void - with actual field data.

It revealed a temperature difference between the black and white stripes - that increases as the day heats up.

This stabilised on living zebras during the middle seven hours - with the black ones 12 to 15 degrees centigrade hotter than the white.

But the stripes on the lifeless zebra hide continued to warm by up to another 16 degrees centigrade - indicating the underlying mechanism that suppresses heating.

So it's the way the stripes are harnessed as part of their cooling system - rather than just their contrasting coat colour - that is key to understanding why these animals have their unique patterning, they said.

Like all species in the horse family, zebras sweat to keep cool. Recent research reveals this is facilitated by a protein called latherin.

This makes the sweat frothy, increasing its surface area and lowering its surface tension so it evaporates and prevents the animal overheating.

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.

These destabilise the air and the water vapour at the tips of the hairs, explained the researchers.

They also discovered zebras have an unexpected ability to raise the hair on their black stripes while the white ones remain flat.

During the heat of the day - when the stripes are at different temperatures - this 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 at that time.

These three components- convective air movements, latherin-aided sweating and hair-raising - work together as a mechanism.

They enable zebras to wick the sweat away from their skin so it can evaporate more efficiently - to help them cool down.

The unstable air may also play a secondary role in putting off flies from landing on zebras.

Recent research has shown the zebra stripes become remarkably more pronounced on animals living in the hottest climates near the equator.

These zebras are also smaller - providing a large surface area to volume ratio which would help the animals' ability to dissipate heat through evaporation.

Mrs Cobb added: "When we got the opportunity to collect some field data from zebras in Africa, we also noticed their ability to raise the hairs of their black stripes, while the white ones lay flat.

"It was only much more recently, when the role of latherin was discovered in helping horses sweat to keep cool, that it all began to fall into place.

"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 gather evidence and fully understand how the stripes help zebras control temperature, but I am 85 now, so that's for others to do."

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