Well, I can’t give a definitive answer to that question, but I can start by telling you that zebras are not white with black stripes, but black with white stripes. The ground color in embryos is black, and the white stripes appear later in development in areas where the deposition of melanin pigment is inhibited.
But of course you’re wondering why they have stripes at all. Various hypotheses have been suggested, the most famous being camouflage, confusion of predators, or even thermoregulation. None of these have been experimentally supported, for it’s hard to test them. (The camouflage story, though, doesn’t hold up since it depends on zebras hiding in tall grass, but they live on the savanna where tall grass is almost nonexistent.)
There is, however, a new hypothesis that has garnered some experimental support: it’s presented and tested in a new paper in the Journal of Experimental Biology by Ádám Egri et al. This is a rather long and complicated paper, and perhaps I’d best refer you to the BBC report of the findings, which contains a quote from our own Matthew Cobb.
In short, experiments on Hungarian horse farms using model horses and other targets for flies suggest that the striped pattern reduces the kind of polarized light that attracts blood-sucking tabanid flies (“horseflies”). Those flies are attracted to horizontally polarized light, presumably because that’s a sign of water, and water is where female tabanids lay their eggs. Model horses and pans of fly-attracting oil painted with a dark color are very attractive to flies; white models less so. Surprisingly, striped models attracted the fewest flies, and the stripe width that was the most deterrent was that actually present on the heads and legs of all three existing species of zebras.
The authors theorize that the adaptive significance of the stripes is to deter tabanids in Africa, because fly bites can reduce grazing, and hence survival and fertility.
It’s a reasonable theory, but has a few problems. First, there’s some special pleading about the width of stripes: as I said, the most deterrent widths are those found on the zebra’s heads and legs, not on their backs, where the stripes are wider (see below). To explain this, the authors argue that the heads are important for survival because they contain vital sensory organs, and the legs are critical because they “are indispensable to escape from predators.” (The authors also claim the blood vessels are closest to the skin in these places.) But that doesn’t explain why the stripes aren’t the same fly-deterring width all over the body.
Here’s a Grant’s Zebra (Equus quagga boehmii, a subspecies of the Plains zebra). Note that the stripes are narrowest on the head and legs:
Matthew Cobb, quoted in the BBC piece, raises another problem:
Prof Matthew Cobb, an evolutionary biologist from the University of Manchester pointed out that the experiment was “rigorous and fascinating” but did not exclude the other hypotheses about the origin of zebras’ stripes.
“Above all, for this explanation to be true, the authors would have to show that tabanid fly bites are a major selection pressure on zebras, but not on horses and donkeys found elsewhere in the world… none of which are stripy,” he told BBC Nature.
Indeed! To add to that, there’s another fly that could be a serious pest on zebras in Africa: the tsetse fly, although the authors claim that tsetses don’t bite zebras as often as they do other mammals. But zebras are susceptible to the trypanosome parasite that causes human sleeping sickness (and makes horses chronically ill), and the authors didn’t test whether stripes deterred tsetse flies. (The experiments were, after all, done in Hungary where the tsetse doesn’t live.)
I find the authors’ hypothesis intriguing and the data somewhat convincing, but there could be other reasons for stripes that we just don’t understand. And, as Matthew noted above, we also need to explain why most species of equids lost their stripes, because it’s pretty clear that the ancestral equid was striped. (Darwin noted this in The Origin when he pointed to the existence of occasional stripes in domesticated horses as evidence for the occasional re-expression of an ancestral trait). Do unstriped equids not encounter horseflies? Or was a stripey pattern too conspicuous to predators?
And someone needs to investigate this problem in felids.
Egri, A. M. B., G/ Kriska, R. Farkas, M. Gyurkovszky, S. Åkesson and, and G. Horváth. 2012. Polarotactic tabanids find striped patterns with brightness and/or polarization modulation least attractive: an advantage of zebra stripes. J. Experimental Biol. 215:736-745.