Forget lions, leopards and warthogs for now – brown isn’t my colour. Monochrome has always been more on trend, and zebras are the height of patterned fashion. Often dismissed as just elaborate horses, they stand out in their stripes but receive a lack of attention, somehow becoming part of the landscape. But if you pause for a moment, and consider what it is you are looking at, they become a marvel in their own right. Not elusive, camouflaged creatures, but standing boldly right in front of you. The wonder of them is plain to see – but have you really ever thought about it? How did the zebra become so fashionable, black on white on black on white, repeatedly striped in barcode chic? Scientists since Darwin and Wallace have pondered this question, and scientists since Darwin and Wallace have unravelled some of the most intricate mysteries, but has any scientist ever come up with an answer? Many have tried, including a study released in recent weeks. You might think that the various findings have all led to a specific consensus, but you’d be wrong.
Consider appearances in the natural world. Birds can be an explosion of bright colours and highly contrasting markings. In contrast, mammals tend to be drab and cryptic, with stealth taking precedence over sexiness. Yet zebras are different, and go against the grain for mammals. “The highly-contrasting black and white stripes of zebras are an exception and a mystery” says Tim Caro, a wildlife biologist who has been working on the evolution of colouration in animals, including the stripes of zebras. Zebras evolved from un-striped wild horse ancestors about 2 million years ago, so evolutionarily, there should be a selective advantage behind their contrasting markings. “Understanding their unique colouration has captured the attention of many groups that are now trying to solve the puzzle,” says Caro.
Although strikingly different in colouration to many other mammals, perhaps the reasons behind the zebras’ markings aren’t so different after all, and are still a form of camouflage. One hypothesis suggests that the vertical stripes of zebras may help them to hide in long grasses by disrupting their outline – a little like the stripes of tigers. Alternatively, instead of the stripes helping them to blend into their environment, the very thing that we think makes them stand out from the crowd of other mammals, could help them blend in with one another during movement. This is something known as ‘motion dazzle’, in which a herd of zebra on the move may appear as one mass of rapidly flickering stripes, making it difficult for a predator to pick out a target and judge any one individual’s speed and distance. In recent years, Martin How has been investigating this theory. Coming from an animal vision background, his natural instinct was to assume that these stripes perform some kind of visual function. “I was particularly struck by how similar zebras are in appearance to well-known optical illusions, such as the stripe paintings of Bridget Riley,” explained How. It’s difficult to analyse the motion dazzle theory in the lab or the field, so How tried analysing zebra stripes using computer models of how animals perceive movement, and found that parts of the zebra can generate signatures that indicate to an observer that the zebra is moving in a way that it is not. “In theory, this could produce a form of motion camouflage, providing the zebra with a few extra seconds advantage in the heat of a chase.” How admits that this is still just a theory, and evidence would be very difficult to find, but he’s reluctant to entirely rule it out, believing it may be a part of the puzzle, if not the full explanation.
How thinks it’s possible that this motion dazzle may play a part in the findings of Tim Caro and his team, who believe that the zebra’s stripes don’t protect against predators, but parasites. Tsetse fly, stomoxys stable flies, and tabanid biting flies are prevalent in the hot and humid environments zebras inhabit. About thirty flies feeding for six hours on just one horse can draw as much as 100ml of blood – zebras can be host to hundreds of flies throughout the day. Caro didn’t need to collect experimental data, but he set many variables against one another. He looked at 20 species and subspecies of wild equids, including horses, zebras and wild asses, and recorded their level of striping, and then looked for relationships between striping and environmental factors such as the presence of large carnivores, climate and vegetation.
Caro found that only their proxy of fly abundance (hot and humid environments in which flies are found) correlated with the level of striping. “I was rather surprised,” he said. “We found again and again that many stripes or intense striping is associated with areas that tend to have many biting flies over the course of the year.” Flies can carry disease, and there are four diseases that have been found in Africa that can be fatal to equids. Being adorned with stripes could therefore be adaptive for African equids, if it reduces the contraction of fly-bite transmitted diseases. In 2012, a Hungarian study showed that zebra-striped models were less attractive to tabanid horse flies than models without stripes. Apparently, the flies are attracted to ‘linearly polarised light’ and the black and white stripes disrupt this attraction. This is most effective with narrow stripes, such as those found on zebras. It is in the flies’ perception of these stripes that How thinks his motion camouflage theory could be useful. “Insects, as they approach a landing site, use the flow of objects across their vision to judge speed and distance. The stripes of the zebra are likely to disrupt this ‘optic flow’ by generating optical illusions, thereby confusing the flies and causing them to abort their landing attempt. Again, this is purely theory which needs hard evidence to back it up.”
By combining these two theories, could we be closer to reaching a conclusion? Edging nearer, perhaps, but answers to the question are ever forthcoming, and the latest research is suggesting that we shouldn’t discount another theory – that the stripes allow the zebras to regulate their body temperature. Brenda Larison and her team from the University of California examined the plains zebra, the most common of the three zebra species. The plains zebra has a variety of stripe patterns, and Larison wished to investigate whether the variation was correlated with up to 29 environmental variables, including predator presence, climate, and fly abundance, at 16 sites across Africa. Larison found that zebras in warmer climates have stripes that are bold and cover the entire body. Zebras living in areas of the continent with colder climates had fewer, lighter and narrower stripes. This is interesting, as zebras in colder climates have more white coverage. Most of us assume that black absorbs heat and white reflects it, so wouldn’t these zebras be colder? The hypothesis is that air may move more quickly over black stripes, and more slowly over white stripes, creating convection currents of air around zebras. Essentially the contrasting stripes create small breezes across the body, that would cool them down. “I think that the next step would be to use thermal imaging cameras to look in detail at the air-flow patterns around animals in their natural environment to see if the stripes really do help zebras to deal with the heat,” suggested How.
Preliminary observations have already shown that zebras maintain a significantly lower surface body temperature than similarly sized, nearby herbivores, and it may affect how often they seek shade from the blasting sun. When Larison looked into whether predicted fly distribution was correlated with striping, unlike Caro, she found no relationship. Interestingly, she notes that some infections spread by biting flies are temperature dependent, and she suggests that the distribution of the infection and diseases may be more relevant than the distribution of the fly carriers themselves. Caro’s results about biting flies were based on a proxy, drawn from temperature and humidity, so Larison suggests that these variables themselves could be what affects the striping, rather than the flies they predict would have been found in those climates.
“Our research does not close the book on this question.” Larison tells us, but she’s excited that their results add support for the notion of temperature regulation, something that has previously been ignored. Like Larison, How thinks that all of the answers may have merit, and there could be a complex interplay between them, “it’s certainly time that someone cracked the mystery once and for all. The problem is, there may be multiple reasons why stripes evolved in zebras, making in difficult to pin down exactly what’s going on, so perhaps we’ll never know.” Perhaps we won’t. The first ancestral horse to display striped patterning may have benefitted from multiple resulting selective advantages, so there may not be just one answer. But Caro, soon to publish a book dedicated to the mystery of the stripes, is more positive: “It makes me optimistic and hopeful that we’ll get a definitive answer while I’m still alive!” One thing is for certain: in the case of the zebra, the answer is not simply black and white.
March 27, 2015