How Zebras Got Their Stripes

How did the zebra get its stripes? It sounds like the theme for a “Just So” story that Rudyard Kipling never got around to writing. You would think that someone would have come up with the definitive answer by now, but, in fact, the reason zebras have stripes remains a biological mystery. The laws of evolution suggest that the random emergence of stripes on the ancestors of modern zebras must have had some sort of advantage that allowed them to reproduce more prolifically than their unstriped brethren. The striped animals would have become more common with each generation, ultimately outlasting the ones without stripes.

But scientists have puzzled for years over what that advantage might have been. The problem isn’t that they have no good ideas—it’s that they have too many good ideas. Maybe stripes let zebras blend in with vegetation, so that predators can’t see them. Maybe they make it hard for predators to judge a zebra’s speed and distance when it’s running on the open savannah. (This idea inspired a form of camouflage used by ships during the First World War.) Maybe stripes discourage disease-carrying flies from biting. Maybe they attract mates, much as elaborate plumage does for birds. Maybe individual stripe patterns serve a social function, allowing herd members to recognize each other more easily. Maybe stripes somehow protect against excessive heat.

These are all plausible explanations for a zebra’s stripes, and until recently it’s been hard to choose between them. But now a paper, published in the April 1st issue of Nature Communications, offers the best evidence to date in favor of one thesis: that the stripes repel insects. “The fly hypothesis has proven to be the best one so far,” Daniel Rubenstein, a Princeton biologist and zebra expert, who wasn’t involved in the research, said. “And this new paper adds new support to the idea.” Discouraging bites from flies is obviously useful, since the insects often carry fatal diseases. Also, while a single bite from blood-eating flies extracts just a tiny droplet of blood, thousands of bites per day can add up to significant blood loss.

Before the recent paper, there was already experimental evidence that biting flies avoid landing on striped surfaces. “We know they don’t like it, although we don’t know why,” Tim Caro, a biologist at the University of California Davis, and the lead author of the new study, told me. But previous laboratory experiments used artificial surfaces like flypaper, not living zebras—understandable, since getting a zebra to stand around in a lab would be tough. If scientists could get a zebra to coöperate, they might show that their stripes do, in fact, deter flies, but this would do nothing to eliminate the competing hypotheses. (Maybe avoiding lions is an even more important reason to have stripes, for example, and testing that in a lab would be even tougher.)

So Caro and his colleagues tried a different approach. They took all twenty known species and subspecies of wild equids, including zebras, horses, and wild asses, and looked at how much striping each group has and where on the body it appears. Then they matched the range of the animals to the various factors that have been suggested as evolutionary reasons stripes might have appeared—the presence of large predators, for example, climate, or the kind of vegetation that is prevalent where zebras live.

Almost none of these factors correlated strongly with whether a species or subspecies was boldly striped, subtly striped, or stripeless—except for the prevalence of biting flies. (Actually, there aren’t good maps of fly concentrations in many parts of the world, so the scientists used a proxy: hot and humid conditions, which flies love. It’s not a perfect solution, since heat and humidity could, in theory, have some effect on striping that has nothing to do with flies, but that’s widely considered to be unlikely.) Despite hints from the earlier research, Caro and his colleagues were struck by how clear the correlation turned out to be. “I was rather surprised,” he said. “We found again and again that many stripes or intense striping is associated with areas that tend to have many biting flies over the course of the year.”

Caro is relieved that this maddeningly straightforward question appears to have been answered at last. “We’ve finally gotten to the stage where we can stop asking the question ‘Why stripes?’ and start asking ‘What prevents flies from landing on stripes?’ ” he said. Caro is also interested in the question of whether it’s primarily disease or blood loss that makes fly bites such a problem. “That’s what happens in science,” he said. “You answer one question and it leads to six more.”

Photograph: Valerie Shaff/Getty