Zebra Stripes and Horseflies

This is a more critical look at the hypothesis that zebra stripes evolved to decrease biting by flies presumably because of the number of deadly diseases transmitted this way. Maybe you will start seeing striped or checked fly sheets in the pasture.
DrO

Zebra stripes, tabanid biting flies and the aperture effect
Proc Biol Sci. 2020 Aug 26;287(1933):20201521.
Authors
Martin J How 1 , Dunia Gonzales 1 , Alison Irwin 1 , Tim Caro 1 2

Affiliations
1 School of Biological Sciences, University of Bristol, 24 Tyndall Avenue, Bristol BS8 1TQ, UK.
2 Center for Population Biology, University of California, Davis, CA 95616, USA.

Abstract
Of all hypotheses advanced for why zebras have stripes, avoidance of biting fly attack receives by far the most support, yet the mechanisms by which stripes thwart landings are not yet understood. A logical and popular hypothesis is that stripes interfere with optic flow patterns needed by flying insects to execute controlled landings. This could occur through disrupting the radial symmetry of optic flow via the aperture effect (i.e. generation of false motion cues by straight edges), or through spatio-temporal aliasing (i.e. misregistration of repeated features) of evenly spaced stripes. By recording and reconstructing tabanid fly behaviour around horses wearing differently patterned rugs, we could tease out these hypotheses using realistic target stimuli. We found that flies avoided landing on, flew faster near, and did not approach as close to striped and checked rugs compared to grey. Our observations that flies avoided checked patterns in a similar way to stripes refutes the hypothesis that stripes disrupt optic flow via the aperture effect, which critically demands parallel striped patterns. Our data narrow the menu of fly-equid visual interactions that form the basis for the extraordinary colouration of zebras.