Zebra stripes may be mainly defense against flies

Protection from flies best explains the function of zebra stripes, says a new analysis.

A head-to-head test of five explanations that researchers have proposed for zebras’ distinctive patterning finds no support for some long-discussed ideas, says Tim Caro of the University of California, Davis. Looking at the ranges and ecology of various zebras and other subspecies of the genus Equus undermines, for example, the notions that stripes camouflage animals in woodlands or dazzle big predators into misjudging prey movements and flubbing an attack.

The best explanation for the function of the iconic stripes turns out to be discouraging bloodthirsty tabanid horse flies and tsetse flies, Caro and his colleagues report April 1 in Nature Communications. Other experiments have found that flies prefer landing on solid colors to contrasting stripes.

The puzzle of what showy stripes do for a zebra intrigued Charles Darwin and Alfred Russel Wallace as they laid the foundations of modern evolutionary theory. To test five broad categories of the many ideas that scientists have proposed over the decades, Caro and his colleagues looked to geographic information. The researchers determined the ranges of 20 subspecies in the Equus genus, which includes the vividly striped zebras, three wild asses with striped legs plus eight asses and a wild horse with unstriped coats. Researchers quantified stripiness, or lack thereof, of each subspecies using such measures as the number of bars on the neck or the intensity of color in leg stripes. Then the analysts looked for links between striping and ecological factors such as fraction of range covered by woodlands.

The researchers failed to find much of a link between woodsiness and stripe measures, undermining the idea that dark and light stripes benefit zebras by letting them blend in with the dark-and-light patterns in thickets of branches.

Likewise environmental links didn’t support the importance of stripes for setting up little convection currents that might cool zebra skin or for disrupting the attacks of big predators such as lions. This idea of disruption included the appealing concept called motion dazzle, in which bold, repetitive patterns twist perceptions of motion. Studies of diets among intensely monitored wild lions also debunk this idea since lions catch an abundance of zebras, Caro notes.

And, in a different kind of evidence, the size of social groups in various Equus subspecies showed no strong connection with stripes, downgrading the hypothesis that stripes might facilitate social interactions.

But stripes did correlate with geographic regions that had months of weather favoring intense fly populations. Further backing the idea is that flies in zebra zones spread four diseases lethal to the animals. Studies of domestic livestock in North America have also shown that biting flies cause substantial blood loss and lower the production of milk.

The analysis gets at what functions the stripes serve now, not what drove their original evolution in ancient climates and ecosystems, Caro says. We may never know how zebras first got their stripes.

Altogether the case is “compelling,” says Innes Cuthill of England’s University of Bristol, whose favorite hypothesis before this paper had been dazzling predators.He’s cautious about declaring such a complex problem as the function of zebra stripes absolutely resolved, but he says he now ranks Caro’s selection of flies as “the leading contender in the queue of explanations.”

This is the first test that pits stripe explanations against each other, Caro says. Now he’d like to know how the flies put so much evolutionary pressure on Equus subspecies to maintain the stripes. “I’d like to know what’s so dangerous about being a horse around biting flies,” he says.