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Zebras are magnificent animals, with the tall stature and long faces shared with their horse family (equid) cousins. Unlike equids, though, they’re dazzlingly striped black and white.
The functional significance of the zebra coat stripe pattern is one of the oldest questions in evolutionary biology, having troubled scientists ever since Charles Darwin and Alfred Russel Wallace first disagreed on the subject.
Well, till now, Scientists have come up with Five (05) Hypotheses that why Zebras have Stripes over the past 120 years.
Why Zebras Have Stripes?
It’s a coloration that must have evolved for a reason, yet scientists have struggled to figure it out.
Do these help Zebra to stay cool?
One of the prevailing hypotheses has been that they help zebras stay cool, a notion supported by the recent finding that zebras in warmer climates have more stripes covering their entire bodies compared to zebras in cooler regions.
The idea is that because black and white absorb and reflect heat differently, the air around the stripes moves at different rates. This, theoretically, would create tiny convection currents at the boundaries between the stripes that would serve to cool the zebra down.
The Test of the Hypothesis?
To test if this effect actually works, a team of researchers led by Gábor Horváth of the Eötvös Loránd University in Hungary used an interesting method: barrels of water, covered in animal skins.
“A zebra body was modeled by water-filled metal barrels covered with horse, cattle and zebra hides and with various black, white, grey and striped patterns,” they wrote in their paper.
“The barrels were installed in the open air for four months while their core temperature was measured continuously. Using thermography, the temperature distributions of the barrel surfaces were compared to those of living zebras.”
The sunlit zebra-striped barrels reproduced the surface temperature characteristics of actual sunlit zebras really well, which meant the team had a strong analog on their hands.
But when the researchers compared the core temperature of the barrel between the different skins, even on runs of very hot days, the zebra barrel did not significantly differ from the grey barrel.
So, it looks like stripes did not provide a thermoregulation advantage over solid grey coloration.
“The average core temperature of the barrels increased as follows: white cattle, grey cattle, real zebra, artificial zebra, grey horse, black cattle,” the researchers wrote.
Do these help Zebra Camouflage and Social Interaction?
The popular conception that zebra stripes are used for camouflaging purposes doesn’t add up, according to a new study.
“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 color 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-colored 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, the idea that the stripes produce a sort of optical illusion that “dazzles” predators when the zebras are running. It’s possible, but the jury is still out on that one.
Do stripes deter biting, bloodsucking flies?
Well, this may have something to do with polarisation. A 2012 study (in which Horváth also participated) found that the light and dark stripes reflect polarised light in a way that deters flying insects; put more simply, flies don’t like landing on zebras’ striped coats.
And this would neatly explain the higher number of stripes in warmer climates, too. The type of flies that are of biggest annoyance to zebras also prefers warmer temperatures.
And if you think the last one is correct, you’re right. Zebras have stripes to keep pesky insects at bay, researchers concluded after analyzing the distribution of tsetse flies in Africa.
The researchers explained in a news release:
“I was amazed by our results,” said lead author Tim Caro, a UC Davis professor of wildlife biology. “Again and again, there was greater striping on areas of the body in those parts of the world where there was more annoyance from biting flies.”
While the distribution of tsetse flies in Africa is well known, the researchers did not have maps of tabanids (horseflies, deer flies). Instead, they mapped locations of the best breeding conditions for tabanids, creating an environmental proxy for their distributions. They found that striping is highly associated with several consecutive months of ideal conditions for tabanid reproduction.
Why would zebras evolve to have stripes whereas other hooved mammals did not? The study found that, unlike other African hooved mammals living in the same areas as zebras, zebra hair is shorter than the mouthpart length of biting flies, so zebras may be particularly susceptible to annoyance by biting flies.
“No one knew why zebras have such striking coloration,” Caro said. “But solving evolutionary conundrums increases our knowledge of the natural world and may spark a greater commitment to conserving it.”
Still, there may be multiple reasons for the stripes. We may not know for sure until they’ve all been roundly and thoroughly field-tested.
The team’s research has been published in the journal Scientific Reports.