Thursday, 9 February 2012

How the zebra got his stripes?

 Most animals in the savanna come in one shade of brown or another, except for zebra. Zebra, as everyone knows, are stripey. Black with white stripes, at that; or are they white with black stripes? Anyway, why they're stripey has puzzled many people for a very long time: even Wallace and Darwin debated whether zebra stripes make them conspicuous or not! For stripes to have evolved there must be some evolutionary advantage, but what, exactly is it? There are a huge number of theories out there (many reviewed here), from the rather obvious to the some more ingenious ideas too:

Lion's vision of zebra?
In fact I've been told by Tim Caro, who's thought a lot about this problem (here and here for good examples), that there are at least 17 different hypotheses going around, but none of them really have good experimental evidence. The one in the news today is the fly story, thanks to a new paper that describes experiments involving different coloured model animals. In the paper (available here, but probably not free I'm afraid) Ádám Egri and colleagues set up a series of experiments in Hungary (not the first place you might think of studying zebra colours, but why not!). They started with sticky trays with different degrees of a cross pattern and set them out to see how many tabanid flies (commonly known as horseflies) were attracted (and stuck) to each tray (click here for photos). Not immediately related to zebra stripes, I know, but they gradually increased the complexity of the experiment, making stripes and comparing black to white alone, etc., until they ended up with sticky model 'horses' of white, black, brown and zebra stripes (there's a nice picture on the BBC site!). And they counted the horseflies that got stuck to each one.

Results, well, black 'horses' attracted 562 horseflies, the brown one 334 the white model 22 and the zebra striped one only 8. So stripes certainly do protect these animals against horseflies. Why this is was the subject of many of the other experiments in the paper, and the authors suggest it comes down to the way light is reflected or not off the animals, resulting in polarisation (i.e. all the waves of light are parallel) which the flies are rather good at detecting (horsefly larvae are dependant on water, which is a good source of polarised light: if you want to find water, looking for polarised light is a good technique). Now white animals do not polarise any light, but striped animals do (from the black bits), so why the striped animals should still be less preferred than white ones isn't clear. The authors suggest it might be something to do with the resolution of the compound eye in the flies. Essentially, they argue that thanks to the width of zebra stripes combined with the resolution of the compound eyes in the horseflies, zebra are invisible to the flies until they're almost on the animal anyway.

Zebras are harder to see than wildebeest? Seronera, Nov 2010
Now, is this the answer to why zebras are stripey? Well, obviously the authors haven't tested any of the other theories, so can't rule those out. What's more, I need convincing that horseflies are equivalent to tsetse. Horseflies are dependent on water to complete their life cycle, and are consequently rather scarce in many savannahs. Does such a rare species really have a large enough influence to overcome the risks associated with stripes, if stripes make them conspicuous to predators? Tsetse, on the other hand, might be a serious selective force - they can certainly appear in such numbers that they kill animals, and are vectors for Trypanosoma  (the animal causing sleeping sickness) that can be fatal to horses. So are the results for Hungarian horseflies going to be the same here with tsetse flies? If you've been in the bush here you'll certainly know that wearing dark colour clothes will give you lots more tsetse bites than wearing light ones (and you'll probably have seen the black and blue tsetse traps in many places - coloured for maximum attraction to the flies). But I still think the work needs to be done - movement alone seems very important for attracting tsetse, and stripes don't seem to hide movement. Similarly, if the resolution of tsetse compound eyes is different to that of horseflies, the hiding won't work. And, of course, tsetse have a very unusual breeding strategy, which, thanks to internal development of  single young at a time in the female, means they're not reliant on water - so do they really have the same attraction to polarised light? I think we need a few more answers before I'm quite convinced yet.

And in fact, I spoke to Tim Caro back in December at the TAWIRI conference about his research in this area, as he's been busy doing experiments down in Katavi to test all the hypotheses he could find (involving such crazy things as dressing up in zebra skins and walking about in the savanna! That is seriously brave science - it's just asking for trouble!), including the biting flies option, and his answer so far is that he still doesn't know. Nothing conclusive. So until he's finished and written up all these experiments I'm not going to be convinced by a single study that only tested one hypothesis. I wouldn't be surprised if it was part of the story though - nor would I be surprised if there are multiple processes at work here, but I'm not going to just jump on one theory just yet.

Whatever makes they stripey, zebras are cool!
There's a more solid answer to that other zebra puzzle, by the way: they're black animals that evolved white stripes. Or at least that's the way the mechanism in the development pathways works - the melanism producing pathways have a default that is switched on, a pathway that is inhibited in the white stripes and not the other way around. Moreover, most other Equuids are grey / brown or blackish, suggesting that the common ancestor of zebras and other horses was probably dark coloured too - colouration that would need switching off, not switching on. I'll leave you to ponder that whilst I go out to paint my landrover stripey...


Egri, A., Blaho, M., Kriska, G., Farkas, R., Gyurkovszky, M., Akesson, S., & Horvath, G. (2012). Polarotactic tabanids find striped patterns with brightness and/or polarization modulation least attractive: an advantage of zebra stripes Journal of Experimental Biology, 215 (5), 736-745 DOI: 10.1242/jeb.065540

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