Friday 19 January 2018

Stripes could protect us from biting flies, scientists claim as they explain zebra markings

Biologists believe they have unravelled the evolutionary mystery of how the zebra got its stripes claiming the markings protect them from biting flies

Zebras at Dublin Zoo. Picture: Frank Mc Grath
Zebras at Dublin Zoo. Picture: Frank Mc Grath

Miranda Prynne

Wearing striped clothing could help protect holidaymakers from mosquito bites, scientists have found after solving the mystery of why zebras are black and white.

Wearing striped clothing could help protect holidaymakers from mosquito bites, scientists have found after solving the mystery of why zebras are black and white.

It is a question which has plagued biologists for centuries but now a team from the University of California, Davis, believe zebras’ unusual monochrome markings evolved in order to repel biting insects, such as horseflies and tsetse flies, which tend to avoid striped surfaces.

And the findings could help humans cut their risk of being bitten by donning stripy T-shirts, although scientists cautioned that the type of striped surface and material used may alter its effectiveness.

“A T-shirt may help somewhat but it might not be the whole story. Certainly if you are going to buy a T-shirt make sure the stripes are thin,” Tim Caro, lead author and UC Davis professor of wildlife biology said.

“Don’t buy a striped jumper too quickly. Black and white striped surfaces reflect different sorts of visible light but they also reflect different sorts of polarised light which we can’t see but flies can.

“The extent of polarised light reflected also depends on the nature of the surface – think of gloss and matte paint – and hairs probably reflect polarised light in different ways.

“So it may be that the different hairs of the zebra’s pelt are important in preventing flies from landing on them.”

Varying explanations for zebra stripes, which have been proposed since Alfred Russel Wallace and Charles Darwin debated the problem 120 years ago, have included a form of camouflage, assisting escape from predators by visually confusing them, heat management or some kind of social function.

The study, published in the journal Nature Communications, mapped the geographic spread of seven different species of zebras, horses and asses and their subspecies and recorded the thickness, location and intensity of their stripes on several parts of the body.

It compared the animals’ geographic reach with other variables such as woodland habitats, the range of predators, temperatures and the numbers of ectoparasites such as tsetse flies.

After examining where the striped animals and variables overlapped the scientists ruled out all but one of the existing explanations, that of avoiding blood sucking flies.

"I was amazed by our results," said Prof Caro.

"Again and again, there was greater striping on areas of the body in those parts of the world where there was more annoyance from biting flies."

While the distribution of tsetse flies in Africa is well known, the researchers did not have maps of tabanids which include horseflies and deer flies so they mapped locations of the best breeding conditions for these insects.

They found that zebra striping was highly associated with several consecutive months of ideal conditions for tabanid reproduction.

Unlike other African hooved mammals living in the same areas as zebras, zebra hair is shorter than the mouthpart length of biting flies making them particularly susceptible to these insects, the team found.

"No one knew why zebras have such striking colouration," Prof Caro said.

"But solving evolutionary conundrums increases our knowledge of the natural world and may spark greater commitment to conserving it."

According to Prof Caro it is not yet known why biting flies avoid striped surfaces.

He said: “Some ideas include lateral inhibition but these are not researched as yet.”

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