Zebra stripes, tabanid biting flies and the aperture effect

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.

Zebras of all stripes repel biting flies at close range

“…• Finally, stripes might cause confusion in the transition between long-and short-distance orientation. If zebras appear as blurred gray from a distance and then, at closer range, suddenly resolve into a sequence of floating black and white bars, this abrupt 'visual transformation' 26 might disrupt the behavioral sequence that facilitates landing.…”

Aposematism in mammals

“…2013), those of zebras deter biting flies (Caro 2016; How et al. 2020), whereas those of clouded leopards ( Neofelis nebulosa ) are cryptic (Allen et al. 2011), and the function of those of ribbon seals ( Histriophoca fasciata ) is unknown.…”

“…For example, the disordered stripes of spotted skunks (Spilogale spp.) advertise anal defenses close up (Caro et al 2013), those of zebras deter biting flies (Caro 2016;How et al 2020), whereas those of clouded leopards (Neofelis nebulosa) are cryptic (Allen et al 2011), and the function of those of ribbon seals (Histriophoca fasciata) is unknown. There are few species with black-and-white vertical striping or body spots (Table 1).…”

A new argument against cooling by convective air eddies formed above sunlit zebra stripes

“…Thus, if the air streams above the sunlit back of zebras could hinder the landing of flies, the same would be true also for that of any homogeneous (black, grey, brown, white) coloured ungulates. Tabanus bromius , a small horsefly from Europe, can definitely land on free-living zebras under sunny conditions 24 .…”