How a zebra's stripes can leave bloodsucking flies seeing stars

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There had been four main hypotheses about the advantages zebras accrued by evolving stripes: camouflage to avoid large predators; a social function like individual recognition; thermoregulation, with stripes setting up convection currents along the animal's back; and thwarting biting fly attacks.

Researchers on Wednesday described experiments demonstrating that horse flies have a hard time landing on zebras while easily landing on uniformly colored horses.

University of Bristol biologist and study co-author Martin How said stripes may dazzle flies somehow once the insects venture close enough to see them with their low-resolution eyes. They use their tails to bat them away and, when flies do land, they don't stay long because zebras move around a lot.

Caro compared the regular horses and the ones masquerading as zebras. When the flies would bump into the zebra in a failed landing attempt, the zebras swatted at them with their tails, ruining their chances of re-attaching and feeding on their blood.

If you ever learned anything about zebras growing up, the key fact about their stripes you might believe is accurate might not be accurate at all. The team dressed the horses and zebras sequentially in black, white, and then black and white striped coats. In Africa, flies called horse flies carry diseases that may seriously harm zebras so they have developed the stripes to protect themselves.

Stripes might also be visual markers for group bonding or to direct companions to particular parts of the body for grooming.

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"There are enormous benefits to having a striped coat for a horse", Caro told The Atlantic.

In a similar way, human pilots can be dazzled when attempting to land into the sun.

The objective of zebra stripes has always been a mystery. The team tackled that question by studying both horses and zebras and tallying the number of insects that landed on and bit each animal. The zebra swished tails nearly continuously to ward off flies, while horses primarily twitch and occasionally swish tails to ward off flies.

The research is reported in the journal PLOS ONE.

Zebras are quite good at not getting bitten by flies. Only five flies landed on the horses dressed in zebra coats during a 30-minute period, whereas more than 60 touched down on those in the solid black and solid white coats in the same time period.

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