“How did I get my stripes? It’s a long story…”Tim Caro/UC Davis By Michael MarshallWhen it comes to explaining why zebras have stripes, it’s best to remember that some issues are not black and white. Biologists have been debating the puzzle since Darwin’s time, but a study published on Wednesday offers further evidence for one…
When it comes to explaining why zebras have stripes, it’s best to remember that some issues are not black and white.
Biologists have been debating the puzzle since Darwin’s time, but a study published on Wednesday offers further evidence for one of the most promising explanations: that the stripes deter biting flies.
In the parts of Africa where zebras live, there are blood-sucking horseflies that carry lethal diseases such as trypanosomiasis. Clearly, zebras would do well to avoid being bitten. The idea is that the stripes somehow confuse the flies so that they don’t land on the zebras.
A team led by Tim Caro of the University of California, Davis tracked captive zebras and horses at a site in England. Horseflies circled round both, but they landed on horses significantly more often. Putting striped coats on the horses’ bodies meant the horseflies landed there less often – but still landed on their heads, which were uncovered. The implication is that the stripes were having a real effect.
The hypothesis backed by a lot of evidence, but does that mean it’s the only reason for a zebra’s stripes? Not necessarily. Some ideas don’t seem to stand up, notably the suggestion that the stripes help zebras cool down on hot days – if that were true, we would expect a lot more tropical animals to be stripy. But other ideas seem to have more to them.
One which at first seems ridiculous is that the stripes are a form of camouflage. Obviously, zebras are not inconspicuous. But the stripes could create “dazzle camouflage”: overwhelming the predator’s visual system and making it hard to track the zebra’s movement. Think about the experience of watching a herd of zebras all dashing in different directions, and imagine trying to pick out one of them to bring down.
The evidence here is mixed. A 2016 study suggested that the dazzle effect only really works if the stripes are parallel to the animal’s direction of travel, implying that zebra stripes don’t work this way. But this was based on tracking humans playing a computer game. A 2014 study, based on computer modelling of how moving zebras would appear to a predator, suggested that the stripes would be extremely confusing.
There is also the simple possibility that the stripes are a signal. The message may not be for other zebras: in 2017, researchers suggested that the stripes signal to other grazing animals, encouraging them to graze alongside the zebras. Such mixed-species herds offer more protection against predators. For now this is only a hypothesis.
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Perhaps the most important point is that these studies can only tell us is why zebra stripes continue to exist today, not why they arose in the first place. Evolution is good at re-purposing things, so a body part may arise, be used for one purpose, and then end up being used for something entirely different.
An obvious example is the lens of the mammalian eye, which probably arose simply as a protective cover for the retina and only later developed the ability to focus light, creating a sharper image – which is now its most “obvious” function. It may be that zebra stripes have a similarly complex history.
There is something psychologically appealing about a single, clear explanation. That instinct doesn’t mean we are wrong to seek such things – sometimes just-so stories turn out to be correct – but this is one area where our biases can work against us.
Journal reference:PLoS One, DOI: 10.1371/journal.pone.0210831
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