Zebras have one of the most eye-catching coats in the animal kingdom. Their black and white stripes are instantly recognizable, but why do they sport such a striking pattern? While experts have put forward numerous theories down the years, a new study in August 2020 provided a definitive answer.
Even though there’s still plenty to learn about the natural world, the mystery of the zebra’s stripes has puzzled researchers for decades. However, in all that time, they have failed to come up with a concrete explanation behind the creature’s stripes. According to the BBC Future science website, no less than 18 theories have been put forward over the years.
With that in mind, you may well be wondering why experts found it so difficult to answer the question. Tim Caro offered some insight into this particular point while talking to the BBC in December 2019. Caro is an evolutionary ecologist who has worked extensively on animal behaviour and conservation biology.
In addition to this, Caro has dedicated nearly 20 years of his career to examining zebra coats. He told BBC Future, “People have been talking about zebra stripes for over a hundred years. But it’s just a matter of really doing experiments and thinking clearly about the issue to understand it better.”
On that note, Caro looked to put his expertize to good use in a long-term study about zebra stripes. Around 10 years ago, the ecologist joined three other specialists to compile a report on the subject, hoping to solve the mystery. His colleagues — Alison Irwin, Martin How and Dunia Gonzales — were a team of researchers from both sides of the Atlantic.
During the study, the group analyzed suppositions that had been discussed previously by different experts. In addition to this, Caro and company seemed to reach a conclusion themselves after plenty of intensive research. Eventually, the report was published in the Proceedings of the Royal Society B journal in August 2020.
Before we dive into the findings of that report, though, let’s focus on some of the more popular theories regarding zebra stripes. As we mentioned earlier, plenty of them have been discussed over the years, but only a select few really caught on. To begin with, let’s shine a light on an idea that was originally raised back in 2008.
At that time, a researcher named J.C. Briand Peterson published a paper in the African Journal of Ecology. In this report, Peterson claimed that zebra stripes formed part of “an identification system” within the animal’s group. The BBC also noted that Peterson’s theory compared the zebra’s markings to a person’s fingerprints.
To expand on Peterson’s idea, the Daily Mail website went into a bit more detail in July 2018. According to the newspaper, the stripes could be particularly vital for female zebras and their offspring. The black and white patterns sported theoretically aid zebra’s “visual communication” with their foals.
Meanwhile, other scientists have claimed that zebras developed stripes as a way of cooling down. The theory posits that the animal’s black markings provide them with warmth at the start of the day. Then, as the hours progress, the white areas of their coats stop them from getting too hot in Africa’s searing temperatures.
This process is referred to as thermoregulation. And in the opinion of animal experts Stephen and Alison Cobb, it’s the perfect answer to the mystery of the zebra’s stripes. The couple went on to conduct their own experiment in 2016, before publishing their findings in the Journal of Natural History in June 2019.
The Cobbs wrote, “The temperatures of black and white stripes on two zebras and a zebra hide were measured throughout separate sunny days in Kenya. There was a 12°C [53.6] to 15°C [59°F] difference between [the] living zebras’ stripe temperatures throughout the middle seven daytime hours. The hide temperatures reached 16°C [60.8°F] higher than the living zebras.”
After noting that zebras perspire in the heat, the Cobbs put forward an interesting theory. In the last few years, it’s been revealed that equid animals harbor a molecule known as latherin. It plays a crucial role in fanning the sweat out to their hair, which causes it to dry and cool them down.
Without the latherin, the perspiration will sit on the creature’s skin and overheat them. So, the Cobbs continued, “We suggest that the abrupt temperature difference between the stripes causes chaotic air movement above the hair surface, thus enhancing evaporative heat dissipation. This cooling mechanism explains the lower temperatures of living zebra stripes than those of the inanimate hide.”
Away from that, another theory has been doing the rounds as well. Indeed, some experts have suggested that zebra stripes exist to camouflage the animals in the wild. That could explain why herds of them are referred to as a “dazzle,” as the patterns might throw off any approaching predators.
In keeping with that point, Johannes Zanker and Martin How produced a report on the idea in the Zoology journal. The study, which was published back in June 2014, recorded a test to see if zebras could essentially shield themselves via their striped coats. And the results were certainly intriguing.
How and Zanker revealed, “We simulated a biologically motivated motion detection algorithm to analyze motion signals generated by different areas on a zebra’s body during displacements of their retinal images. Our simulations demonstrate that the motion signals that these coat patterns generate could be a highly misleading source of information.”
However, while these different ideas raise interesting points, they’ve all been disproven down the years. Going back to the identification system theory, the BBC Future website noted that more experiments needed to be done on zebras to prove its worth. And as for the hypothesis that their coats prevent overheating, another expert dismissed this in July 2018.
After conducting research into the matter, professor Susanne Åkesson from Sweden’s Lund University spoke to the Daily Mail. She told the newspaper, “The stripes didn’t lower the temperature. It turns out stripes don’t actually cool zebras’ [bodies]. Zebra stripes are expected to cool the body by means of convective air eddies, induced by temperature gradients over alternating black and white stripes.”
“This hypothesis seems reasonable,” Åkesson continued. “Because in sunshine the black zebra stripes are warmer due to their stronger absorption of sunlight compared to the cooler white stripes of higher reflectance. Infrared photography of zebras showed that sunlit black stripes are warmer than sunlit white stripes, and that the difference between them increases with rising air temperature.”
From there, Åkesson then made her final point to the newspaper. The academic concluded, “At night, however, [the] temperature differences are reversed, with black stripes being cooler than white ones.” Meanwhile, the camouflage theory was dissected by Tim Caro back in 2016, as he published a book simply titled Zebra Stripes.
In that publication, Caro pointed out that zebras don’t conceal themselves in the wild. Instead, dazzles usually stand in large spaces with nothing around to block a predator’s view. Due to that, the ecologist noted that the black and white stripes would make them even more visible to hungry animals.
Caro suggested that the black and white stripes might be more effective if zebras gathered in woodland areas, but that’s not the case. In addition, the animals are also prone to fleeing when faced with danger. In those instances, the expert claimed that the so-called camouflage becomes fairly meaningless.
Furthermore, Caro said that lions are seemingly unaffected by the zebras’ stripes, as they continue to feed on the latter. Past figures would also suggest that the predators have never found it hard to spot them. For instance, from 1966 to 1968, Kruger National Park revealed that roughly 20 percent of the big cats’ prey were zebras.
On that note, let’s switch our focus back to the study that was published in August 2020. As we mentioned earlier, both Caro and How were involved in the 10-year project, alongside another two specialists. And according to them, they’ve uncovered the real reason why zebras sport striped coats.
As it turns out, the black and white patterns are incredibly effective at throwing off hungry insects. The BBC reported that scientists have studied this idea on and off for nearly 100 years, as certain bugs pose a real danger to African animals. Tsetse flies and horseflies are particularly problematic in that respect.
Those two bug species feed on the blood of wildlife, and subsequently carry several harmful ailments. As per the BBC website, tsetse flies and horseflies spread African horse sickness and sleeping sickness. If that wasn’t enough, they can pass on equine influenza as well, which could kill the affected animal.
Yet in previous studies, researchers noted that tsetse flies didn’t have any zebra plasma in their systems. Further reports also indicated that different flying insects avoided dropping onto areas with striped patterns. So Caro and company focused on this idea during their work together, before presenting their findings to the world.
The Proceedings of the Royal Society B report read, “Of all [the] hypotheses advanced for why zebras have stripes, avoidance of biting fly attack receives by far the most support. Yet the mechanisms by which stripes thwart landings are not yet understood. A logical and popular hypothesis is that stripes interfere with optic flow patterns needed by flying insects to execute controlled landings.”
Following on from this, Caro and his colleagues went on to suggest that the “aperture effect” could be responsible. To shed more light on this, How spoke to the iNews website in August 2020 following the report’s publication. And he revealed that you might already be familiar with the concept, even if you don’t recognize the term.
How told the website, “The aperture effect is a well-known optical illusion that, in human vision, is also known as the barber-pole effect. Moving stripes, such as those on the rotating barber-pole signs outside barbershops, appear to move at right angles to the stripe, rather than in their true direction. So the pole appears to move upwards, rather than around its axle.”
With that in mind, the four researchers outlined their plan in the study. To learn more about the optical effects, they conducted an interesting experiment. The paper read, “By recording and reconstructing tabanid fly behavior around horses wearing differently patterned rugs, we could tease out these hypotheses using realistic target stimuli.”
The test was held back in February 2019 at the Hill Livery facility in Bristol, England. As the researchers noted, a number of horses were selected to take part, but that’s not all. In fact, the proprietor Terri Hill had a herd of zebras too, so they were also used.
Horseflies were the final component of the experiment, as Caro and his colleagues looked to see how they would react. So with everything in place, the horses were then decked out in three different attires. A few of them wore plain-colored covers in either black or white, while the rest sported zebra patterns.
From there, the horses and zebras were left to graze on their own, with the horseflies taking an interest. However, the researchers noticed an obvious pattern as time went on. The flies were drawn to the animals in the plain attire, just as they would have been under normal circumstances.
However, the remaining horses and zebras weren’t bothered as much. While some of the horseflies touched down on their bodies, most of them struggled to find their targets. The BBC website reported that the insects couldn’t slow down as they got closer, which caused them to ricochet off the animals.
Caro summed up the situation at the time of the experiment. He said, “It looks as if [the horseflies] cannot recognize that black and white surface as a good landing spot.” To add to that, How broke down why the insects were having so much trouble with the striped patterns during his chat with iNews.
How explained, “As any fly approaches a landing surface, it will adjust its speed according to how quickly the surface expands across its vision, enabling a slowed and controlled landing. Stripes however disrupt this ‘optic flow’ through the aperture effect, leading the fly to believe the landing surface is further away than reality. This means the fly fails to slow down or land successfully.”
So after decades of debate, the true purpose behind the zebras’ black and white markings has finally been revealed. Unsurprisingly, Caro was delighted with the findings, but his thoughts immediately sprang to another animal once the report got published. According to the professor, horses could benefit from this information in the long run.
Speaking to iNews in August 2020, Caro concluded, “Not only do these exciting studies bring us closer to understanding one of the world’s most iconic and photogenic species, they will be of great interest to farmers attempting to reduce the damage caused by fly bites. And even general horse-wear companies [will be interested].”