Scientists Reexamine Why Zebra Stripes Mysteriously Repel Flies

For the current study, Tombak, then a PhD candidate at Princeton, and her team wanted to test stripe width to see if narrower ones might be even more repulsive to flies—a potential evolutionary advantage that would explain the difference between zebra species. They also restricted their experiment to close-range encounters to rule out the theory that the repulsion required an illusion that could only happen at a distance. Hence the plexiglass box.

An undergraduate from the lab, Lily Reisinger, built the box and set up the experiment. For each trial, the team hung two pelts with clothespins, unleashed the flies, let them circle for a minute, and then counted how many landed on each pelt. First, they tested an impala pelt vs. one from a plains zebra, which has wide stripes. Then the impala vs. a Grevy’s zebra, which has narrower stripes. Finally, they pitted the skins from the two zebra species against each other. They tested 100 rounds for each pair.

The flies chose the impala skin about four times as often as they chose either zebra skin. And over the 100 rounds, the team found…