Are zebras black with white stripes or white with black stripes? asks Brielle Walquist, a student in Brookville. How do stripes help zebras? asks another reader.All striped zebras, scientists say, evolved from a common horse ancestor, which was probably a solid dark color. One clue: A now-extinct zebra called the quagga had a solid-colored dark rump. And we know that a zebra developing in its mother's womb starts out dark (black or brown). But as the little zebra grows, he or she develops light stripes.
Scientists say stripes appear where genes suppress the production of dark pigments. The skin pigments are inhibited in a pattern determined by species (grevy's, mountain or plains) and by the zebra's parents.
So by the time zebra babies are born, they are covered in stripes. And scientists have suggested a number of ways in which a zebra's bold stripes make good survival sense.
Surprisingly, camouflage is the oldest argument for the stripes' usefulness. A herd of milling zebras creates one big, dizzying pattern. (Even still photos hint at the visual confusion. See for yourself at sciencephoto.com/media/388826/
Even still photos hint at the visual confusion. See for yourself at sciencephoto.com/media/388826/enlargeResult: The optical interference of moving stripes may make it harder for predators, including hungry lions, to pick out and chase down individual animals.
Next, there's zebra social lives. A zebra's stripes help other zebras identify it both by species (the number, width and general pattern) and as a unique and attractive individual.
And then there's the fly theory. Yes, zebras are indeed stylin' horses. But here "fly" refers to the nippy insects that plague horses, from tsetse flies, which carry the encephalitis virus, to horseflies. Researchers say that biting flies are especially attracted to large, dark-colored animals. Stripes, some think, might act as a kind of fly repellent.
Earlier this year, the fly theory got a boost from a report published in the Journal of Experimental Biology. Researchers knew that bloodsucking female horseflies are drawn to polarized light (light that vibrates in a flat plane). Dark-colored horses, they say, reflect horizontally polarized light, and flies find them very attractive. On the other hand, flies land less often on all-white horses.
The scientists devised experiments using black, white or striped boards, and also brown, black, white, and striped plastic horses, setting them up on a fly-ridden horse farm in Hungary. They found that horseflies were even less attracted to black-and-white stripes than to solid white surfaces. And the more narrow and zebralike the stripes, the more flies took a pass.
Why? Black and white stripes, according to the researchers, are good at reflecting light in alternating polarized then non-polarized patterns, making zebras visually confusing to divebombing horseflies, as well as to prowling lions.