**If not to evade predators, why do zebras have stripes? **
Zebras’ stripes are highly unlikely to have evolved in an effort to provide camouflage, according to new research published Friday in the journal PLOS ONE.The purpose of zebras’ stripes has long been a mystery, with various hypotheses put forward, including “confusing predators, protecting against disease-carrying insects, controlling body temperature and social cohesion”.
But with the most longstanding of these, predator avoidance, now being rejected, what alternative explanations hold any credence?
The new research says that zebra stripes cannot have evolved to blend the beasts into the background, nor to break up their outline, because at the distance where the stripes become visible to predators, they have likely already seen or smelled the zebra.
“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 co-author Tim Caro, UC Davis wildlife biology professor.
“Instead, we reject this long-standing hypothesis that was debated by Charles Darwin and Alfred Russell Wallace.”
To test the hypothesis that zebra stripes do indeed provide camouflage, the researchers took digital images taken in the field in Tanzania and passed them through spatial and color filters, simulating how zebras’ main predators, lions and hyenas, as well as zebras themselves, would perceive the stripes.
In addition, they measured the stripes’ width and contrast, which, along with knowledge of various animals’ visual capabilities, allowed them to draw conclusions as to how far away lions, hyenas and zebras could be, while still detecting stripes.
“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, assistant professor of biological anthropology at the University of Calgary, 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.”
Results from the work indicate that beyond 50 meters (about 164 feet) in daylight or 30 meters (about 98 feet) at twilight, when most predators hunt, the stripes are difficult for large carnivores to distinguish. On moonless nights, the distance drops to a mere 9 meters (about 29 feet).
In addition, the research concluded that on open plains, where zebras spend most of their time, lions could see the outline of zebras just as easily as that of similar-sized prey with fairly solid shading patterns.
If the conclusions of this research are correct, and stripes confer no advantage against predation, where did the idea come from, and is there any evidence to support it?
“The idea that stripes allowed zebras to blend into a background composed of tall stem enriched grasses is an old one that emerged from casual observations,” says leading zebra expert Daniel Rubenstein of Princeton University, in an email interview with The Christian Science Monitor. “Until this study most evidence in support of this hypothesis has been anecdotal”.
I don’t think this story disproves natural selection, but a lot of biologists like to claim that all traits are the result of some genetic advantage.
Maybe zebras are striped because stripes simply didn’t cause any disadvantage.