Scientists have proposed more than a dozen ideas to explain why zebras evolved stripes. Some say the bold patterns confuse their predators, or that they keep the animals cool. But all of these ideas have been disproved or lack strong evidence.
In 2014, researchers showed the ranges of the horsefly and tsetse fly species and the three most distinctively striped equid species (Equus burchelli, E. zebra, and E. grevyi) overlap to a remarkable degree. The scientists argued that zebras evolved the stripes to avoid these insects, which often carry fatal diseases. Now, they’re back with more proof.
The researchers recorded three captive plains zebras and nine monochromatically colored horses in adjacent fields in the United Kingdom where European tabanids (horseflies) naturally occur. They also covered the horses with three different coats, one black, one white, and one striped (pictured) much like a zebra.
The zebra stripes did not deter flies from afar; both zebras and uncovered domestic horses experienced the same rate of circling flies. But a close analysis of the flies’ final approach to the striped animals revealed the insects failed to decelerate, and instead flew over the stripes or bumped into them, the team reports today in PLOS ONE.
Indeed, the flies landed on the zebras at an average of one-fourth of the rate they landed on the horses. Further, the scientists did not see a single tabanid probe a zebra’s skin during 5.3 hours of direct observation, whereas the flies successfully did so 239 times on the uncovered horses during 11 hours of observing them. Only five flies landed on the horses dressed in zebra coats during a 30-minute period, whereas more than 60 touched down on those in the solid black and solid white coats in the same time period. The flies attacked all the horses’ uncovered heads at the same rate.
It seems the stripes affect the insects only at very close range, the scientists say, and they suggest zebra-striped coats may be a simple way to protect domestic horses from biting flies.