- For many (and despite Darwin), it's obviously camouflage - just as the stripes on a tiger disguise the outline of this animal and patches on army vehicles hide them, so too, do zebra stripes hide them from lion, at least at a distance.
- It's a temperature regulation thing - black areas warm up fast and create tiny areas of hot rising air that move cool air over the white bits, to create a series of tiny eddies to cool the animal.
- It's simply so zebra can see their conspecifics easily in the distance, an advantage as if attached they'll be able to get back into a herd quickly.
- Whilst they might be conspicuous by day, in low light, and importantly, through a lion's eyes (don't forget they have different visual systems to us) zebras are rendered invisible at night.
- It makes then invisible to tsetse flies
- Moving stripes dazzle and confuse predators
- Etc., Etc.
|Lion's vision of zebra?|
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|
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!|
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