A juvenile great white shark. Photo / Getty Images
A juvenile great white shark. Photo / Getty Images
Researchers simulated a shark's visual system to test the "mistaken identity theory" of unprovoked shark bites.
Baby white sharks learn to hunt on the fly. Although months-old pups feast on fish and other small fry, older juveniles are finally big enough to tackle seals and other meatier meals.
It mightseem easy to spot a blubbery seal in the waves. But young white sharks have less-than-stellar eyesight and are also likely colourblind, rendering the ocean in shades of gray. So you can hardly blame a young white shark for seeing an appetisingly shadowy oval above and chomping.
For decades, scientists have floated this theory of "mistaken identity" as an explanation behind unprovoked shark bites on humans, which are rare. A paper published Wednesday in the Journal of the Royal Society Interface puts this theory to the test. Based on their simulations of how a juvenile white shark sees the world, they found no meaningful difference between a plump sea lion, a person paddling on a surfboard or even a person paddling on their own — supporting the theory that sometimes, sharks make mistakes.
Charles Bangley, a marine ecologist at Dalhousie University in Nova Scotia who was not involved with the research, said the paper supports common-sense advice for anyone hoping to avoid being bitten by a shark: Swim in calm, clear water away from seals and other prey.
Catherine Macdonald, a lecturer at the University of Miami and the co-founder of the marine science program Field School, said the study was "well-executed" and opened up new questions about sharks and people.
"Does it make a difference if it's an honest mistake?" Macdonald said. "We can't tolerate those risks to people regardless."
A simulated juvenile shark's view of a swimming surfer. Photo / Laura Ryan et al. via The New York Times
Baby white sharks are blissfully unaware of the bad rap they are born into. "White sharks have been described as these mindless killers," said Laura Ryan, a neurobiologist at Macquarie University in Australia and an author on the paper. But she hopes they may begin to be seen "in another light by understanding their world."
White sharks, along with bull sharks and tiger sharks, are responsible for most injuries and fatal bites to humans — who are most often surfers. And the most frequent biters appear to be juveniles, between 8-10 feet long. But white sharks usually release a person after the first bite, which may suggest they do not actively hunt humans as prey.
"As a general rule for shark bites, the vast majority of them are one and done," Macdonald said.
Although mistaken-identity theory made logical sense, it would seem easily debunked on the basis of our own visual perception: The average sighted person could likely distinguish between the silhouette of a surfer and a seal paddling side by side.
Recent investigations into shark vision have expanded scientists' understanding of how the cartilaginous predators see their environment: probably in grayscale and with a minimal ability to see detail. To try to see the world as a shark, Ryan said to imagine taking your eye examination underwater without goggles; things are a little less sharp. As such, the visual cues a hunting shark most relies on are probably motion and brightness contrast.
Spurred by this knowledge, the researchers did an experiment. From the bottom of aquariums at the Taronga Zoo in Sydney, the researchers attached a GoPro to an underwater scooter traveling at the speed of a cruising shark. They recorded videos of two sea lions, one fur seal, swimming people and people paddling on three different types of surfboards (the boards came from the personal collection of Ryan, who surfs).
Ryan and colleagues edited the GoPro footage in a computer program to translate the lens of a video camera to the retinas of a young white shark. They stripped the video of some color and rotated them all so the overhead objects moved from the bottom to the top of the screen. Then the researchers ran the videos through a series of statistical analyses at a range of resolutions to glean whether a juvenile white shark might be able to discern between the objects.
In the shark's-eye view, the researchers found no significant difference between a swimming person, a paddling surfer or a meandering seal or sea lion. Ryan said she was surprised that sharks might confuse even a swimming person without a surfboard with a seal.
The study suggests young sharks must be using other tactics to distinguish between people and prey, Macdonald said, adding that the number of people in the water has increased significantly in recent years without an associated spike in bites.
"They eat seals every day, and bites on people are incredibly rare." Macdonald said. "So if they're not solving the problem visually, then how do we think they're solving it?" If the answer lies with a shark's other senses, such as scent, this could determine what interventions would make sense to prevent encounters in the wild, she added.
Ryan continues to surf. Whenever thoughts of sharks come up, she used to think about statistics: how a bite is not likely to occur. After this study, she reminds herself of something new: "I find some comfort in knowing that they're not mindless killers."
