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. 2020 Aug 26;287(1933):20201521.
doi: 10.1098/rspb.2020.1521. Epub 2020 Aug 19.

Zebra stripes, tabanid biting flies and the aperture effect

Martin J How  1 , Dunia Gonzales  1 , Alison Irwin  1 , Tim Caro  1   2
Affiliations

Zebra stripes, tabanid biting flies and the aperture effect

Martin J How et al. Proc Biol Sci. .

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.

Keywords: insect flight; optic flow; tabanid; vision; zebra stripes.

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Conflict of interest statement

We declare we have no competing interests.

Figures

Figure 1.
Figure 1.
Illustration of the main test hypotheses. (a) Zebra scene (Wikimedia Commons). Dashed squares illustrate horsefly views chosen at 4.0 m and 0.5 m distances. Yellow star illustrates a fictive landing site towards which a virtual fly is heading. (b) Motion maps generated from the same down-sampled images expanding from the central landing point. Colour encodes motion direction (according to the surrounding border scale) and saturation encodes motion strength at (i) 4.0 m illustrating typical optic flow, (ii) 0.5 m with high spatial sample frequency (sf) illustrating the aperture effect, and (iii) at 0.5 m with low spatial sample frequency illustrating spatio-temporal aliasing. The distribution of motion vectors across each of these scenes is represented in the polar histograms below. (c) Scene at the two viewing distances down-sampled to approximate tabanid visual resolution (0.5 cycles/°). Horizontal red line indicates an image transect, the intensity values of which are plotted below.
Figure 2.
Figure 2.
(a) Custom-made fabric pinned over commercial horse rugs. Fabric was printed with uniform black, uniform grey, vertical stripe, horizontal stripe, and checked patterns. (b) Horsefly landing rate on each of the five patterned rugs. Black lines = median, dotted lines = 25th and 75th percentile, shaded areas = violin plot of the data. Letters above each column indicate which are statistically different (Friedman's test with multi-compare and Bonferroni correction, p < 0.05).
Figure 3.
Figure 3.
Fly distance from horses wearing patterned rugs. (a) Dorsoventral (top) and anteroposterior (bottom) views of 107 fly trajectories plotted around the horse location (see also electronic supplementary material, figure S2 and movie S3). Each red line shows a single horsefly trajectory around horses wearing (in this case) the vertically striped rug. (b) Closest recorded distance of each fly to the horse (figure 2 for conventions). (c) Proportion of digitized fly trajectories at different distances from the host target, colour coded according to (b) (n = 101–107 fly trajectories per rug type).
Figure 4.
Figure 4.
Approach speed of flies to each rug type. (a) Speed versus distance during fly approach, colour coded according to (b). Black dotted line is the median approach speed for the whole dataset. Shaded areas are the 25th and 75th percentile ranges. Star indicates the range over which grey significantly differs from the patterned rugs (Kruskal–Wallis multi-compare post hoc with Bonferroni correction, p < 0.05). See electronic supplementary material, table S1 for more statistics. (b) Approach speed relative to overall median. See figure 2 for graph conventions.
Figure 5.
Figure 5.
Fly turning behaviour around horses wearing patterned rugs. (a) Histograms of speed, pitch, and yaw of pooled data demonstrating the cut-off points used to define slow/fast flight (red and blue, respectively) and turning/straight flight (pink and grey, respectively). (b) Example flight segmented into fast (blue) and slow (red) turns viewed from above. Black arrow indicates direction of flight. (c) Rate of slow (top) and fast (bottom) turns around the four rugs. (d) Distance of slow and fast turns from horses. See figure 2 for graph conventions for c and d.
Figure 6.
Figure 6.
Horizontal and vertical flight characteristics around patterned rugs. Ratio between horizontal and vertical flight speeds (a,b) and absolute elevation angle (c) relative to distance and (d) overall median, figure 2 for graph conventions for graphs right.
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