A great white shark from below. A new study found that shark attacks are indeed likely cases of mistaken identity. Photo: Francesco Califano on Unsplash
The idea of a shark attack is terrifying. It’s one of the worst things a person can imagine — limbs previously attached to trunk, floating past in great clouds of blood, a mindless grey monster ripping flesh from bone, starving for human meat as we flail helplessly on our tiny little fiberglass raft. Scary, right? It’s long been posited that the vast majority of shark attacks, however, are simply cases of mistaken identity, and a recent study has confirmed that sharks can mistake humans for their natural prey based on visual similarities.
“Until now, the potential similarity between humans and seals has been assessed based on human vision,” lead author, Dr. Laura Ryan, said in a statement. “However, white sharks have much lower visual acuity than us, meaning they cannot see fine details, and lack color vision. In these experiments, we were able to view the world through the eyes of a white shark.”
Although shark bites are indeed extraordinarily rare, their effects on victims can be catastrophic. They’re bad all around — death or dismemberment, of course, but a shark attack can also have far wider consequences. Communities reliant on tourism can see economic impacts, first responders can be mentally wrecked, and last but not least, shark attacks often lead to shark deaths with shark mitigation devices like gill nets and drum lines.
Those devices don’t just kill sharks, either. Non-target species like dolphins and turtles are often found dead in a net or hung up on a drum line. And since many species of sharks are declining in numbers and many of those species of sharks are apex predators that are incredibly important to the entire food chain… well, killing them off at a rate of some 100-million-per-year is… bad.
There are many species of shark that have bitten people in the past, but three are the culprits in the majority of incidents: great whites, bull sharks, and tiger sharks. Shark bites are generally put into two categories: provoked or unprovoked.
“A provoked shark bite may be an aggressive/defensive behavior as a result of a direct disturbance by a human, such as a diver touching a shark, a fisherman catching or spearing a fish or shark, or intrusion into a shark’s territory,” the authors of the study explained. “Unprovoked bites are the most puzzling and arguably generate the most fear.”
Great whites in particular are thought to rely relatively heavily on sight to find their prey, especially when it’s within a few feet. “White sharks are more successful when hunting prey located at the surface, where the silhouette probably aids the identification of prey against the background skylight,” the authors continued. “In addition, white sharks have visual adaptations that enhance prey detection at the surface, such as cone photoreceptors and a retinal region for acute vision (area centralis) that samples the dorso-lateral region of the visual field, a zone above and to the sides of the head.”
Surfers are at a higher risk than most other groups, especially from young white sharks. As the theory goes, that’s because, when viewed from below, we look a lot like a seal when we’re on a surfboard. Sharks have a particular way of attacking a seal or other pinniped, and it meshes well with bites on surfers.
“Following an initial strike on a pinniped, white sharks typically retreat, allowing the animal to weaken and bleed extensively before returning to feed,” the study said. “Humans are also usually released after the initial strike, although the shark rarely returns to consume the victim. This behavioral difference may be partly attributed to the removal of a shark bite victim from the water before the shark can consume them, intervention from other people or the victim fighting back. However, it may also suggest that white sharks do not actively seek out humans as prey and that bites may be a case of mistaken identity.”
There is indirect evidence, however, that could suggest that juvenile sharks might know that surfers aren’t seals. Comparisons of a shark’s bite force on pinnipeds and humans shows that a bite to a seal has more force than a bite to a human. This could indicate that it’s more of an exploratory bite and not just the result of mistaken identity. Unfortunately for us, even those exploratory bites can be very, very bad.
Up until now, however, everyone just kind of assumed that sharks make mistakes sometimes. No real study into it had been done. We wear bright colors and striped wetsuits and put eyeballs on our surfboards and electric devices on our ankles to ward off attacks, all of which — despite the marketing teams’ best efforts — appear to be pretty useless. Recent progress in our understanding of how sharks see is helping with understanding why they might attack us. Warning: the next bit from the study is a little heavy on the science side.
“Sharks are completely color blind or at best have only limited color perception,” the study reads. “Sharks also have poor spatial resolving power, with the highest estimates based on retinal anatomy at approximately 10 cycles per degree (cpd; range 2–10 cpd), which is considerably worse than humans (30 cpd). Benthopelagic and pelagic species that feed on more mobile prey have higher spatial resolving power. Temporal resolution and contrast sensitivity have been measured in a few elasmobranchs. Temporal resolution is higher in species from brighter light environments (range 12–44 Hz) and contrast sensitivity does not vary significantly between the benthic species it has been measured in, all detecting contrasts below 2.5 percent.”
In essence, that all means that motion and brightness contrast are probably what most sharks use to find out what they’re going to eat. What we didn’t know, though, is how much of a role visual cues play in becoming prey. Everyone always tells you not to splash around if you see a shark, but is that really good advice? Turns out, yes. Yes it is.
The study took video footage of pinnipeds swimming, humans swimming, humans paddling a surfboard, and a plain old rectangular float. Researchers filmed them from below, from a shark’s vantage point, and then digitally filtered the footage using, “spatial and temporal parameters derived from or estimated for the visual system of juvenile white sharks.”
The recordings were shot at the Taronga Zoo Aquarium in Sydney Australia. They filmed two Australian sea lions, a New Zealand fur seal, two humans swimming in a variety of strokes, and a standard short, longboard, and hybrid (board).
The results of the study found that it is indeed more than likely that when sharks bite humans, it’s because they probably think they’re biting something they think is going to be delicious. “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’s conclusion reads. “Modeling here was done under ideal viewing conditions, so this scenario is likely to be of greater significance under more realistic conditions of dim light, surface chop, or turbid water.”
