Zebra stripes stop flies carrying deadly diseases from landing on them, research shows

Scientists place sheets on horses to test theory

Scientists tested their theory by putting striped coats on horses to mimic the patterns found on zebras (REUTERS)

It has puzzled them since the days of Charles Darwin, but now scientists have established why Zebras have stripes - and it's not just for looks. They also stop blood-sucking flies from landing on the animal and protect them from deadly diseases, according to new research.

To test the theory they put striped coats on horses and found that fewer flies landed on them, than when they wore single-colour coats.

“We showed that horse flies approach zebras and uniformly coloured horses at similar rates but that they fail to land on zebras – or striped horse coats – because they fail to decelerate properly,” said behavioural ecologist Tim Caro, who led the research which was published in the journal PLOS ONE.

The insects either flew past or bumped into them and bounced off, he added.

Previous theories have suggested stripes evolved in zebras because they helped with temperature regulation, camouflaged them from predators or performed a social function, such as individual recognition.

The idea that stripes help zebras thwart biting flies has previously been suggested and Mr Caro said it was the only one that "stands up to scrutiny."

He added: "Most biologists involved with research on mammal colouration accept that this is the reason that zebras have stripes."

African horse flies carry diseases such as trypanosomiasis and African horse sickess, which can cause wasting and can be fatal.

“We think there’s a real strong selective pressure against being bitten because it could be a matter of life or death,” University of Bristol biologist and the study's co-author Martin How Dr Martin How told BBC Radio 4’s Today programme.

Researchers recorded horse flies as they attempted to prey on captive zebras and domestic horses at a livery in North Somerset, England.

Although stripes did not deter flies from a distance, they landed on zebras less than a quarter as often as they did on horses.

Zebras are close cousins to horses and donkeys and their distinctive stripe patterns are unique to each individual.

The research could also be useful for humans who want to avoid being bitten by some insects.

“We found that while we were doing our research, if we wore dark black clothing we tended to get attacked far more often by the horse flies than if we wore stripes,” Dr How said.

In addition to their stripes, zebras exhibited more preventative behaviour towards horse flies, such as running away or tail swishing at a far higher rate than horses.

"Zebras are constantly swishing their tail and may run off if horse flies do land successfully, so they are also using behavioural means to prevent flies probing for blood," Mr Caro said.

The researchers are now conducting tests to see what it is exactly about stripes that stops flies, by testing coats with different patterns, contrasts and thickness.

Agencies contributed to this report