That’s right, bugs. More specifically, biting insects like tsetse flies, stomoxys stable flies and tabanid biting flies. The researchers found that the ranges of animals with body stripes (zebras) exactly matched the ranges of these bloodsucking insects.
In the diagram below, you can see the different species of wild equine. Some have full body stripes, some leg stripes, some neck stripes and some are not striped at all. The blue dots show species not plagued by tabanid flies. Notice that those are the animals with no stripes. There was no such correlation with other factors like the presence of certain predators or temperature.
But aren't there plenty of other animals living in those same areas that are not striped? Yes, but zebras are covered with short hairs that biting insects can penetrate with their mouth parts. Non-striped mammals living in the same areas typically have longer, thicker fur that the flies can't get through.
How do stripes help animals avoid biting insects? Apparently, many insects, including tabanids and tsetses, don’t like to land on striped surfaces. And indeed, solid colored feral horses suffer much more harassment from flies than do their striped cousins.
The idea that insect parasites drove the evolution of stripes isn’t settled science yet, but it’s a fascinating hypothesis. I can’t wait to see if further studies confirm it.