Zebra stripes, tabanid biting flies and the aperture effect.

Martin J How,Tim Caro,Alison Irwin,Dunia Gonzales

doi:10.1098/rspb.2020.1521

Martin J How, Tim Caro + Show 2 more

Open Access

https://doi.org/10.1098/rspb.2020.1521

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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.

Highlights

The unusual and striking colouration pattern of the three species of extant zebra (Equus sp.) has generated many intriguing functional explanations over the last 150 years [1]
Lions kill zebras more than expected from their abundance across 40 study sites in Africa [4] suggesting that confusion is an unlikely functional explanation for stripes. (iii) A third theme, that black and white stripes have different heat signatures [5] that set up convection currents that cool zebras, could only operate under very restricted circumstances: over the animal’s dorsum, not over its flank or legs, and, problematically, not under breezy conditions or when the animal moves, as these would negate convection currents acting anywhere on the body [3]
We used rugs placed on domestic horses to investigate the mechanisms underlying the influence of black and white stripes on tabanid fly behaviour

Summary

Introduction

The unusual and striking colouration pattern of the three species of extant zebra (Equus sp.) has generated many intriguing functional explanations over the last 150 years [1]. (ii) Conjectures about stripes confusing predators are poorly supported by observations of plains zebras (Equus quagga) fleeing in ways that do not enhance protean behaviour, nor obscure the outline of individual animals, and because they do not promote motion dazzle or cause lions to misdirect their attack [3]. (iv) In contrast with these ideas, stripes are an established potent force in reducing landings of biting flies based on experimental studies with striped artificial targets [7,8,9], horse models [10], human models [11], painted cows [12], and comparisons of live plains zebras with domestic horses [13].

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