Zebra stripes not for camouflage: study

The striking black and white stripes that adorn zebras do not camouflage the animals from their predators, a new study has found, dispelling the long-held belief that the stripes protect them from being spotted.
"The most longstanding hypothesis for zebra striping is crypsis, or camouflaging, but until now the question has always been framed through human eyes," said lead author Amanda Melin, an assistant professor at the University of Calgary in Canada.
"We, instead, carried out a series of calculations through which we were able to estimate the distances at which lions and spotted hyenas, as well as zebras, can see zebra stripes under daylight, twilight, or during a moonless night," said Melin.

In earlier studies, the researchers showed that the zebra's stripes provide an evolutionary advantage by discouraging biting flies, which are natural pests of zebras.
In the new study, they found that stripes cannot be involved in allowing the zebras to blend in with the background of their environment or in breaking up the outline of the zebra, because at the point at which predators can see zebras stripes, they probably already have heard or smelled their zebra prey.
"The results from this new study provide no support at all for the idea that the zebra's stripes provide some type of anti-predator camouflaging effect," said Tim Caro, a professor at the University of California - Davis.

The researchers passed digital images taken in the field in Tanzania through spatial and colour filters that simulated how the zebras would appear to their main predators - lions and spotted hyenas - as well as to other zebras.

They also measured the stripes' widths and light contrast, or luminance, in order to estimate the maximum distance from which lions, spotted hyenas and zebras could detect stripes, using information about these animals' visual capabilities.
They found that beyond 50 metres in daylight or 30 metres at twilight, when most predators hunt, stripes can be seen by humans but are hard for zebra predators to distinguish.

On moonless nights, the stripes are particularly difficult for all species to distinguish beyond 9 metres.
This suggests that the stripes do not provide camouflage in woodland areas, where it had earlier been theorised that black stripes mimicked tree trunks and white stripes blended in with shafts of light through the trees.
In open, treeless habitats, where zebras tend to spend most of their time, the researchers found that lions could see the outline of striped zebras just as easily as they could see similar-sized prey with fairly solid-coloured hides.
In addition to discrediting the camouflaging hypothesis, the study did not yield evidence suggesting that the striping provides some type of social advantage by allowing other zebras to recognise each other at a distance.

The study was published in the journal PLOS ONE.