When a surfer swims out to catch a wave, his silhouette creates the image of a shark swimming below. Against the background of the bright sky in the water, a person can resemble the body of a seal or the favorite prey of a shark.
We’ve often heard that this is why sharks attack people. However, a new study recently challenged this theory.
Eric Clois, a marine biologist and veterinarian based in French Polynesia, and Carl Meier, a biologist at the University of Hawaii, published paper in Behavior which analyzes why sharks mistake humans for seals and provides an alternative response to shark attacks: personality and nurture.
How often do sharks attack people?
The most important thing to know about shark attacks is that they almost never happen. Less than 100 unprovoked bites are reported every year and six people die from them worldwide on average per year. Contrast this with the metric for unlikely events such as a lightning strike that kills approx 28 people every year only in the US.
“They get a lot of attention from journalists,” says Clois, referring to shark bites and the media frenzy they can sometimes create. “But if you look at the data, there are far fewer shark bites than crocodiles, dogs, etc.”
Why do sharks attack people?
H. David Baldridge first proposed the scientific theory of sharks mistaking humans for seals, also known as the mistaken identity theory, but he used cautious language to describe the idea.
Baldrige wrote in a 1973 technical report that “it is certainly not beyond reason that a man dressed as [in a wetsuit] it may appear to a shark as a seal or other marine animals on which the shark may feed.”
Debunking the theory of mistaken identity
The mistaken identity theory received relatively little public attention until another paper on white shark hunting behavior 10 years later brought the idea into the scientific mainstream.
It has since been repeated in both scholarly and popular sources, sometimes without the nuances and caveats of Baldrige’s original writing. But Clois and Meyer say the science of mistaken identity is anything but settled.
How sharks hunt
Their case against Baldrige’s theory rests on several different lines of evidence. One is the array of senses the shark uses against its potential prey. Clois says most research into shark-human interactions has focused primarily on sight and smell.
“Those are just two senses, and there are two others that are critical from my point of view: sound and mechanoreception,” he says, referring to sharks’ ability to sense movement in the water. “I strongly believe that these two senses are actually more important than sight and smell.”
What do sharks eat?
Another piece of evidence: Many of the shark species that most commonly bite humans don’t even eat seals or other marine mammals. Blacktip sharks, for example, account for over 15 percent of all shark bites worldwide and eat mostly small or medium-sized fish.
Even if a shark in murky water can mistake a human surfer for a seal, it seems less realistic to mistake us for a mackerel or a flounder. On the subject of visibility, Baldrige’s theory seems to suggest that there will be more bites in murky water than in clear conditions. In fact, most shark bites occur in clear water.
But is there a more likely explanation?
Read more: What is unique about the Blue Shark?
Characteristics of a shark
Clois and Meyer support an alternative theory based on the characteristics of sharks called the prey exploration hypothesis. Previous studies have shown that sharks have individual personality traits, including varying degrees of boldness or curiosity.
Lacking parental care, sharks must learn about their environment and potential prey on their own, and they do this by exploring, often with their mouths. Simply put, they might not think of us as seals. They don’t know what we are, and the bravest, most curious ones bite us from time to time to find out.
Looking into the eyes of a shark
When researchers simulated the way a great white shark’s retina would interpret the silhouette of a surfer in paper published in Journal of the Royal Society Interface in 2021 they found images visually similar to a seal or sea lion.
However, in Clois and Mayer’s paper, they argue that the results of this study, while technically impressive, can be interpreted to support both theories. The fact that younger white sharks are more likely to bite humans than older ones, the authors of the 2021 paper argue, also supports their prey-study hypothesis. To them, the younger shark is likely to have less experience with humans, leading to the rare, exploratory bite.
Clua has simple advice for anyone worried about shark encounters, although bites in the wild are incredibly rare. “If you’re diving, swimming, surfing, keep doing what you’re doing. But never do it alone – for two reasons. First of all, being two of you can change the shark’s decisions because it’s a different choice to bite one prey versus bitten prey that are together. And if the shark makes that decision, the second person is there to help.