“Why do zebras have stripes?” It’s a question that has been debated since the time of Darwin, and now a Calgary researcher is one step closer to answering this age old puzzle. We have always thought of a zebra’s stripes as offering some type of camouflaging protection against predators. But the scientists at the University of Calgary and UC Davis suggest something different.
Their study of zebras from Tanzania shows that the animals’ bold black patterns of stripes are not disguising them from lions or other predators after all.
“The most longstanding hypothesis for zebra striping is crypsis, or camouflaging, but until now the question has always been framed through human eyes,” Amanda Melin at University of Calgary, and lead author of the study, said in a release.
Unlike humans, animals don’t see very well — including lions, hyenas and other zebra predators.
“Knowing that most mammals have pretty poor visual acuity, I thought ‘I bet they can’t even see these stripes very far, especially at night’,” Melin said. And since animals can’t see the stripes, predators have probably already heard or smelled their zebra prey before they catch sight of them, thus ruling out any camouflage protection.
In this new study, scientists 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,” Caro said. “Instead, we reject this long-standing hypothesis that was debated by Charles Darwin and Alfred Russell Wallace.”
To test the hypothesis that stripes camouflage the zebras against the backdrop of their natural environment, the researchers passed digital images taken in the field in Tanzania through spatial and color filters that simulated how the zebras would appear to their main predators — lions and spotted hyenas — as well as to other zebras. They also saw how the striped animals would look to other members of their own species.
“We 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 researchers learned that beyond about 164 feet in daylight or 98 feet at dusk, during top hunting times, stripes are visible to humans but more difficult for predators to discern. On dark nights without a moon, most species would have trouble seeing the stripes at about 29 feet.
In areas without trees, where zebras live for most of their lives, the findings show that lions could discern a zebra outline just as well as they could pick out prey with a similar size and relatively solid-colored fur or hide.
Melin conducted the study with University of California, Davis professor Tim Caro, who has spent his entire career studying zebra stripes. In earlier studies, Caro and other colleagues have provided evidence suggesting that the zebra’s stripes provide an evolutionary advantage by discouraging biting flies, which are natural pests of zebras. Findings from their study will be published on Friday, January 22, 2016 in the journal PLOS ONE.