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. 2021 Oct;18(183):20210533.
doi: 10.1098/rsif.2021.0533. Epub 2021 Oct 27.

A shark's eye view: testing the 'mistaken identity theory' behind shark bites on humans

Laura A Ryan  1 , David J Slip  2 , Lucille Chapuis  3 , Shaun P Collin  4 , Enrico Gennari  5   6   7 , Jan M Hemmi  8 , Martin J How  9 , Charlie Huveneers  10 , Victor M Peddemors  11 , Louise Tosetto  1 , Nathan S Hart  1
Affiliations

A shark's eye view: testing the 'mistaken identity theory' behind shark bites on humans

Laura A Ryan et al. J R Soc Interface. 2021 Oct.

Abstract

Shark bites on humans are rare but are sufficiently frequent to generate substantial public concern, which typically leads to measures to reduce their frequency. Unfortunately, we understand little about why sharks bite humans. One theory for bites occurring at the surface, e.g. on surfers, is that of mistaken identity, whereby sharks mistake humans for their typical prey (pinnipeds in the case of white sharks). This study tests the mistaken identity theory by comparing video footage of pinnipeds, humans swimming and humans paddling surfboards, from the perspective of a white shark viewing these objects from below. Videos were processed to reflect how a shark's retina would detect the visual motion and shape cues. Motion cues of humans swimming, humans paddling surfboards and pinnipeds swimming did not differ significantly. The shape of paddled surfboards and human swimmers was also similar to that of pinnipeds with their flippers abducted. The difference in shape between pinnipeds with abducted versus adducted flippers was bigger than between pinnipeds with flippers abducted and surfboards or human swimmers. From the perspective of a white shark, therefore, neither visual motion nor shape cues allow an unequivocal visual distinction between pinnipeds and humans, supporting the mistaken identity theory behind some bites.

Keywords: object motion; prey detection; shape discrimination; shark attack; shark vision.

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Figures

Figure 1.
Figure 1.
Direction and strength of motion cues from the 2DMD model of (a–c) a pinniped swimming, (d–f) a human paddling a surfboard, (g–i) a human swimming and (j–l) a rectangular float towed through the water. Modelling was performed assuming a spatial resolving power of either 5 cpd (b,e,h,k) or 2.5 cpd (c,f,i,l). Red open dots indicate angle vectors that were significantly different from the pinniped and black dots were not significantly different. Units are an arbitrary scaling value. Dashed lines on (a) show the division of the object for analysis of the different ‘faces’. Only the rectangular float differed from the pinniped at 2.5 cpd.
Figure 2.
Figure 2.
Mean direction and strength of motion cues for each object from the 2DMD model of (a–c) a pinniped swimming, (d–f) a human paddling a surfboard, (g–i) a human swimming and (j–l) a rectangular float towed through the water, calculated for the four different faces (leading edge, left side, trailing edge, right side) at 5 cpd (b,e,h,k) and 2.5 cpd (c,f,i,l). Red open dots represent motion directions that were significantly different from that of the pinniped and black dots were not significantly different. Units are an arbitrary scaling value. Panels (a,d,g,j) show example frames of motion direction over pixel location, where pixel colour corresponds to the motion direction in the colour wheel (inlay a).
Figure 3.
Figure 3.
Five examples of the roundness shape analysis for the different object categories. (a–c) A pinniped with its flippers abducted; (d–f) a pinniped in a streamlined position; (g–i) a human swimming; (j–l) a human paddling a surfboard; and (m–o) a rectangular float. The mean distance from the centroid to the edge of the shape measured in 2° increments (b,e,h,k,n) and the normalized FFT amplitudes (c,f,i,l,o) are also shown. Red asterisks represent FFT frequencies that were significantly different from the pinniped in a streamlined pose, and blue asterisks represent FFT frequencies that were significantly different from the pinniped with its flippers abducted.
Figure 4.
Figure 4.
Five examples of the edge projection shape analysis for the different object categories. (a–c) A pinniped with its flippers abducted; (d–f) a pinniped in a streamlined position; (g–i) a human swimming; (j–l) a human paddling a surfboard; and (m–o) a rectangular float. The mean distance from the centroid to the edge, divided into 180 equal length curves (b,e,h,k,n), and the normalized FFT amplitudes (c,f,i,l,o) are also shown. Red asterisks represent FFT frequencies significantly different from the pinniped in a streamline position and blue asterisks represent FFT frequencies significantly different from the pinniped with its flippers abducted.
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