斑马为什么有条纹？

Scientists have provided new evidence to help solve one of the world's oldest mysteries: why does a zebra have stripes? It appears that all those lines cause problems for insects and makes it very hard for them to land on the animal.

Researchers recently described experiments showing that horse flies have a difficult time landing on zebras while easily landing on horses of a single color.

In one experiment, the researchers put blankets with stripes on horses and saw that fewer flies landed on them. They "fly past them or ...bump into them and bounce off," said Tim Caro of the University of California-Davis. He was the lead writer of a report on the study. The report was published in the journal PLOS ONE.

Closely related to horses, the world's three zebra species are known for their black-and-white striped bodies.

Zebras are native to Africa's savannas, or grasslands. The design and appearance of the stripes is different from one zebra to the next. No two zebras are alike.

There had been four main theories about the reasons zebras developed stripes. The first theory is to avoid attack by meat-eating animals. Another is so that zebras have an easier time recognizing other zebras. A third idea is thermoregulation. That is the ability of a creature to control its body temperature, even when the surrounding temperature is different. The fourth theory is to prevent attacks by biting flies.

"Only the last stands up..." Caro said. "Most biologists involved with research on mammal coloration accept that this is the reason that zebras have stripes."

African horse flies carry diseases, such as trypanosomiasis. The sickness can cause wasting and be deadly.

The researchers recorded videos of horse flies as they tried to bite zebras and horses at a farm in North Somerset, England. Stripes did not stop flies from a distance, as they circled horses and zebras at similar rates. But the flies were 25 percent less likely to land on zebras.

University of Bristol biologist Martin How worked with Caro on the study. He said stripes may confuse flies when they get too close to zebras.

"In addition to stripes that prevent controlled landings by horse flies, zebras are always moving their tail and may frighten off the horse flies if they do land successfully," How said. He added that zebras are also using behavioral means to prevent flies from biting them.

I'm Susan Shand.

The Associated Press reported this story. Susan Shand adapted the AP reports for VOA Learning English. George Grow was the editor.

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