Why the zebras got its stripes

17 August 2015

Interview with

Anna Hughes, Cambridge University

While many animals use camouflage to try and blend into their environment in theirZebra quest for survival, others employ a very different technique. Zebras are the primary example of animals using so-called 'dazzle patterns' to try and confuse their predators, the same idea that was behind the 'dazzle-ships' of WWI. However, 100 years after these boats were designed, new research has called into question just how effective their patterns really are. James Farr spoke to Anna Hughes from the University of Cambridge to find out more...

James - We've just come in to the University of Cambridge's Fitzwilliam Museum and we're being led through a sort of series of galleries, all with lots of old-looking pieces of art and sculptures on the walls. We've arrived at a study room in which we're going to view some special drawings I believe. We're here to see a couple of drawings by an artist called Norman Wilkinson who was famous for drawing the dazzle ships that were used in the First World War. They've got really strong black and white striking stripes which looked very much like zebras really.

Anna - Yeah. So, it was actually inspired by the natural world. It's been thought for a long time that the stripes of animals like zebras might act to confuse a viewer. It might make it hard to judge a speed or direction of them when they're moving. This is why these war ships were painted in this way, but it was not really ever tested at that time whether it worked. And so, recent researchers asked the question, does it? Is that why zebras have stripes? That's the idea of this hypothesis which is often called 'motion dazzle hypothesis' that the stripes are an anti-predator defence.

James - It's a very different style of camouflage to what people might think of as a traditional style, more like something that tries to blend in.

Anna - I think the key thing here that's different is, if you're not moving and you match your background perfectly, it is in fact very difficult to see you. But as soon as you start moving, it's much easier to see even if it's well matched to its background because it just sort of pops out from the background. So, it's interesting to know whether there are types of patterns that might not make it difficult to detect something when it's moving, but that make it difficult to aim at.

James - Your new paper sets out to test this idea and assess whether it really does work.

Anna - Yeah. We have been looking at whether this could be a plausible explanation for the evolution of zebra stripes and possibly, stripes in other animals as well. So, we conducted a study using humans as sort of model predators and what they have to do is they play a little game basically, a touchscreen game where they have to try and catch the moving targets. We tested different types of targets with different types of patterns and we were asking, do stripes - of the sort that you might see in a zebra - offer any benefit, over a uniform grey that in our experiment was very well traditionally camouflaged against the background? We found that when the targets are presented on their own so that participants just try to catch one target at a time, the striped targets and the grey targets were pretty similar. But when we looked at increasing the number of targets to a present study one time, surprisingly, we actually found that the stripes were actually easier to catch than the camouflage grey. So, it didn't seem to be that the stripes provided extra protection when there were more targets present.

James - So in actual fact, the stripes made it easier in some sense for their predators, actual humans playing the game to spot the targets.

Anna - Yeah, this is kind of quite surprising. I mean, it is always worth remembering with this sort of study is, an artificial set up in a lot of ways. Humans are not the natural predators of zebra and lions and other predators do have different visual systems. So it could be that this is partly an artefact of the way that we set up the game. It's also the case that we use quite a simple model of how the targets moved. So, they weren't particularly moving in a herd or moving kind of individually. It would be really cool I think to test more thoroughly what it would be like if you actually tried to get the targets to move with relation to each other because there, you might see stronger effects.

James - Are there any other areas you'd like to go down in the future to extend this research?

Anna - So, I think one of the cool things about this topic is that it's probably quite likely that there are multiple functions to the zebra stripes. And there's been a few recent papers that have suggested alternative hypothesis. So for example, there's been some suggestions recently that the stripes might not actually be deterring mammalian predators but are in fact maybe deterring flies. So, biting flies that kind of irritate the zebra and maybe transmit things like malaria. So, there may well be tests that can be done using flies to see if they're repulsed by stripes and whether that acts possibly in a sort of motion dazzle manner, making it difficult for them to land accurately or something like that. There's various hypothesis and I think it's probably not necessarily just one explanation but maybe several  different things that have come together to create, really quite unique patterning in this animal.

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