Secret of the Stripes

Why have zebras evolved those brilliant, even showy stripes? It's as if they are blatantly advertising their presence, which makes no sense on the African savanna, where many lions look to make a meal of them. Their stripes cannot be interpreted as camouflage, since they stand out so strikingly. So what's going on? There's nothing subtle about their alternating bands of black and white. They stand out as if their coloring announced their presence with a trumpet blare.

Researchers have posited a few explanations of why evolution gave the zebra such a gaudy decoration. What are some of their postulates for zebra stripes? One prominent idea is that, although the stripes are conspicuous, they are illusory. All those random stripes in a herd of zebras can confuse a predator—it makes it difficult for predators to figure out an individual zebra's size, speed, or trajectory. Secondly, the zebra stripes may serve a social function for other zebras—either as part of the process of sexual selection, or a means to identify each individual zebra. Then a third possibility is for thermal regulation: the black stripes soak up more of the sun's energy, thus causing warm air to rise over them, which pulls in air from the adjacent white stripes, creating an airflow over the zebra's body, to cool it a bit, on a hot African afternoon.

Then there's a fourth hypothesis: those stripes could be an evolved defense against biting flies. Zebras, unlike horses, evolved in parts of the world inhabited by nasty, biting flies that carry diseases. That possibility gained credence recently, when researchers compared solid-colored horses to zebras—as to which equine was more susceptible to bites. What they found was that solid-brown horses had flies land on them much more often than on nearby zebras. One could ask, however, if horses and zebras might have a different smell—maybe horse odors are more to the liking of biting flies? The researchers had that possibility covered, when they threw zebra-striped blankets over the horses and noted that flies continued to land on their heads, but not on the stripes.

So the difference must be visual—something like what happens with lions. As the researchers watched, they noted that when the flies zeroed in on a zebra, they did not slow down and land, as they did for the horses. The guess is that flies' low-resolution vision caused them to view horses and zebras the same from a distance, but when a fly got close it suddenly saw stripes and got confused, and veered off without landing.

Now here's the part that caught my eye: the researchers suggest that when we humans venture outdoors where biting flies may be looking for us, wear patterned clothes, or try applying striped body paint. I have written before on this blog about being pestered by large horseflies that can inflict a painful bite. My most vulnerable time is when I'm naked, headed to or from my outdoor tub. Maybe I should get a zebra-striped robe to wear? Or a more permanent solution: get tattooed with black and white stripes? I could join the circus and be featured in the freak show, or maybe go on safari, and not worry about being attacked by a lion (or biting fly).