Zebras of all stripes repel biting flies at close range

The best-supported hypothesis for why zebras have stripes is that stripes
repel biting flies. While this effect is well-established, the mechanism
behind it remains elusive. Myriad hypotheses have been suggested, but few
experiments have helped narrow the field of possible explanations. In
addition, the complex visual features of real zebra pelage and the natural
range of stripe widths have been largely left out of experimental designs.
In paired-choice field experiments in a Kenyan savannah, we found that
hungry Stomoxys flies released in an enclosure strongly preferred to land
on uniform tan impala pelts over striped zebra pelts but exhibited no
preference between the pelts of the zebra species with the widest stripes
and the narrowest stripes. Our findings confirm that zebra stripes repel
biting flies under naturalistic conditions and do so at close range
(suggesting that several of the mechanisms hypothesized to operate at a
distance are unnecessary for the fly-repulsion effect) but indicate that
interspecific variation in stripe width is associated with selection
pressures other than biting flies.