Why do zebras have stripes? For more than a century, researchers and storytellers have batted around various hypotheses: the stripes might confuse predators, or keep the animal cool on hot days, or send social cues to other zebras. One theory, first proposed in 1930, is that the stripes deter biting flies. To finally answer this fabled question, a team led by UC Davis researchers decided to test all of these theories at once.
They turned to the evolutionary record, studying the distribution of 27 existing and extinct horse species (both with and without stripes) and compared them with environmental variables in each of those current and former ranges. Here's the Guardian on what makes this method unique:
They take a completely original approach, stepping back from one species of zebra and attempting to account for the differences in patterning across different species and subspecies of zebras, horses and asses. Is there anything about the habitat or ecology of these different equids that hints at the function of stripes?
Pesky flies, statistical analyses revealed, push stripes to appear. Species that evolved stripes, the team found, overlapped with the distribution of biting flies, which other studies have shown tend to avoid black and white surfaces. On the other hand, stripes had no connection with group size or mating rituals—ruling out the social hypothesis—and did not correlate with predator distribution or temperature, either. The findings "tease apart long-standing but mainly untested hypotheses," the researchers conclude, and show that "striping on equids is perfectly associated with increased presence of biting ﬂies."