Although great white sharks are often portrayed as ruthless human killers, a new study suggests that shark attack victims might simply be the subject of mistaken identity. Because they are color blind or at best have only very limited color perception, sharks can confuse humans for seals, their natural prey, because they look the same when viewed from below.
Fatal shark attacks are very rare (with approximately ten such events per year), but can still result in severe shark mitigation measures that significantly reduce population numbers and endanger many shark species.
“Shark bites on humans are rare but are sufficiently frequent to generate substantial public concern, which typically leads to measures to reduce their frequency,” wrote the study authors. “Bites also have negative consequences for sharks as they often result in the implementation or continued use of lethal shark mitigation measures, including the deployment of gill nets and drum lines to reduce shark populations.”
Until recently, the reasons why sharks attacked humans were poorly understood. By analyzing the visual perspectives of young white sharks, a research team led by Macquarie University in Sidney, Australia, has found that, due to poor vision, sharks might confuse humans with seals, an animal that they frequently prey upon.
“We looked specifically at juvenile sharks and that’s because they are responsible for the majority of fatal bites on humans,” explained study lead author Laura Ryan, a postdoctoral researcher in Biology at Macquarie University.
“Until now, the potential similarity between humans and seals has been assessed based on human vision. However, white sharks have much lower visual acuity than us, meaning they cannot see fine details, and lack color vision.”
In order to test this hypothesis, the researchers used footage of two sea lions and one fur seal swimming in a pool at the Taronga Zoo in Sidney, as well as of humans paddling on surfboards. By filtering this footage through a virtual visual system mimicking shark vision, they discovered that motion cues of humans swimming or paddling on surfboards and of swimming seals did not differ significantly. Thus, when viewed from below, humans and seals may appear indistinguishable to a shark that is preparing to attack.
“Our results indicate that the poor spatial resolving power of the shark retina may result in bites on humans as a result of mistaken identity or ambiguous visual cues,” the study authors concluded.
The research is published in the Journal of the Royal Society Interface.