Zebra stripes mystery 'explained'

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Zebras' bold stripes protect the animals by masking their movements, according to a study.

The conspicuous colours do not blend in to the background and scientists have theorised they developed to dazzle predators.

Using computer models, researchers confirmed the markings create optical illusions when the animals move.

They suggest this confusion helps to protect the animals from both big cats and tiny insects.

The research, conducted by researchers from the University of Queensland, Australia, is published in the journal Zoology.

"Zebra stripes have long confused evolutionary biologists, right back to Darwin and Wallace," said lead author Dr Martin How.

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"Previous theories for the function of these stripes include social communication signals, camouflage at dusk or dawn in grassy habitats, and the so-called 'dazzle' effect when being pursued by predators or blood sucking insects."

To test this latest theory that the patterns confuse predators and pests, Dr How and colleague Prof Johannes Zanker from Royal Holloway, University of London, analysed photographs and video footage of zebras.

Computer models that tracked the appearance of the patterns revealed that they worked as optical illusions to provide "misleading information".

Humans and a wide range of animals share what scientists refer to as "motion detection mechanisms": neural circuits which process the direction something is moving based on how its contours appear.

One of the best known examples of an illusion that confuses this mechanism is the barber-pole effect, where the spiral of stripes on a vertical pole appears to move upwards when the pole spins.

The wagon-wheel effect is a further example, where the spokes of a wagon wheel turn clockwise but when a particular speed is reached, the wheel appears to turn anti-clockwise.

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The results also might help us understand how similar camouflage might function in man-made situations”

End Quote Dr Martin How University of Queensland, Australia

According to Dr How, zebra stripes capitalise on this type of illusion to help protect the animals.

He explained that the broad diagonal stripes on a zebra's flank and the narrower vertical stripes on its back and neck give unexpected motion signals that confuse viewers, particularly in a herd of zebras.

"We suggest that these illusions cause pests and predators to mistake the zebra's movement direction, causing biting insects to abort their landing manoeuvres and chasing predators to mistime their attacks," said Dr How.

"The results have implications for the study of patterning in animals - there are many other species such as humbug damselfish or banded snakes that use apparently conspicuous black and white stripe body patterns.

The results also might help us understand how similar camouflage might function in man-made situations, such as the large-scale 'dazzle' camouflage patterns used on battleships."

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