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Zebra stripes aren't used for camouflage or mate selection, study finds

Nothing to see here.

PETER DOCKRILL
25 JAN 2016
 

The popular conception that zebra stripes are used for camouflaging purposes doesn't add up, according to a new study.

Scientists conducted a visual simulation of zebras as they would appear to the eyes of their predators and found that the distinctive black-and-white markings that humans perceive easily are often difficult for other species to make out. As the stripes aren't as visually apparent to animals as they are to humans, the researchers argue they wouldn't offer the camouflaging effects we assumed they provided, casting doubt on one of the animal kingdom's more enduring beliefs.

 

"The most longstanding hypothesis for zebra striping is crypsis, or camouflaging, but until now the question has always been framed through human eyes," said Amanda Melin, a biological anthropologist 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."

The findings, published in PLOS ONE, suggest that stripes can't be involved in helping zebras blend in to their environment, because by the time predators are close enough to actually register the stripes – and supposedly be tricked by them – they would have already caught the zebra's scent or heard their prey's movements.

"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, one of the researchers from the University of California, Davis. "Instead, we reject this long-standing hypothesis that was debated by Charles Darwin and Alfred Russell Wallace."

To simulate how zebras would appear to their predators, the researchers took digital images of zebras taken in the field in Tanzania and applied spatial and colour filters to them.

They also measured the stripes' width and luminance to estimate the maximum distance at which lions, spotted hyenas, and zebras could detect the markings, in accordance with what's known about these animals' powers of sight.

It turns out that beyond about 50 metres (164 feet) in daylight or 30 metres (98 feet) at twilight, predators have difficulty distinguishing the stripes, which would render them immune to any purported camouflaging effects – although they can still be seen by humans.

On moonless nights the visibility drops even further, with all species struggling to make the markings out beyond 9 metres (29 feet).

As such, the hypothesis that zebra stripes may mimic tree trunks (black stripes) and shafts of light (white stripes) in woodland areas doesn't make sense, the researchers say.

They also believe that stripes don't provide camouflage for zebras out in the open in treeless habitats – which is where the animals spend the majority of their time. From the simulations, lions would be able to detect striped zebras in the open just as easily as animals with uniform, solid-coloured hides, such as waterbuck, topi, and the smaller impala.

In addition to the claims with regards to predators, the researchers also suggest zebra stripes aren't used for social purposes among zebras, noting that other species closely related to zebra are highly social without the assistance of such markings.

So why do zebras have stripes then? According to the authors of the study, the answer may lie in some of their previous research, which suggested that the black-and-white markings are actually there to deter annoying insect bites.

Perhaps not as cool as natural camouflage to trick one's predators, but still undeniably useful!

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