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Animal Behavior
Why Sharks Will Sometimes Attack People
Mistaken identity, curiosity, and flaws in vision.
Updated September 21, 2023 Reviewed by Devon Frye
Summer means beaches, and, at least in the popular imagination, shark attacks. Sharks attack humans extremely rarely, with only about 16 bites in the whole U.S. in a given year, and one fatality every two years.
Indeed, humans and sharks can readily co-exist in shark habitats. A recent study on the California coast using drone footage found that surfers were almost always in close proximity to seasonal gatherings of sharks, but the interactions did not lead to increased bites.
How do sharks know not to attack us? And in the rare cases where sharks do attack humans without provocation, what goes wrong? To answer this question, we need to examine the shark's sensory world, especially its vision.
The Sensory World of the Shark
Living underwater, the sensory environment of the shark is utterly foreign to us land lubbers. Sharks use electrical and mechanical senses to feel the flow of water past their skin and to detect currents and disturbances from other creatures around them.
There is really no comparable sensory experience for us, even when we are in the water. To a shark, you probably have a kind of mechanical and electrical "presence" as an object, which they feel with their whole bodies. These senses are especially important in moment-to-moment adjustments in chases and attacks. Sharks, unlike us, also possess highly sensitive olfactory systems that help them to track prey in shallow water and to navigate among feeding grounds over miles of open ocean.
But to hunt, sharks require sharp vision, first to identify prey, then to make an approach and attack. Interestingly, their eyes are remarkably similar to ours.
A great white shark's eye is about an inch and a half in diameter, while ours is about an inch. Their eyes can see a decent chunk of space in front of them with both eyes, like we can, though their eyes are perched on the side of the body. Like us, sharks are capable of both far and near vision, though they use a curious adaptation to achieve this.
Unlike humans and most vertebrates, which adjust from near to far vision by changing the shape of their eyes' lenses, sharks use a different approach. They use tiny muscles to move their lenses forward and backward within their eyes, much like what happens inside a pair of binoculars when you turn the focus wheel.
Sharks Are Completely Colorblind
Basically, all of the cellular machinery in the light-sensitive retina of the shark also exists in us, and the retina grows and develops in similar ways.
But sharks entirely lack color vision. They are referred to as cone monochromats since they have only one type of cone, while most humans are trichromats with three cone classes. Even compared to "colorblind" human dichromats, who have two cone classes, and can see ranges of hues, sharks have a big disadvantage.
Considering physiological evidence along with behavioral studies, sharks probably see in pure grayscale. Interestingly, this seems to be a useful adaptation: both their close cousins the rays as well as more distant cousins the chimaeras (a weird shark-ish group named for mythical fire-breathing monsters) have two or more cone types. Shark ancestors probably did too, meaning their lineage then lost one or more cone types.
So this is not a case where a species just physically can't escape from some biological constraint. Instead, having just one cone is seemingly "better" than two—if you are a shark. Interestingly, the seals they hunt are also cone monochromats (their lineage also having lost at least one cone class at some point), so this might represent a kind of evolutionary "arms reduction treaty."
Vision in Time and Space
Beyond the eye, ecology has likely shaped sharks' visual abilities. Being apex predators, just as we are, they require great sensitivity for visual motion of prey, Like us, sharks have high "frame rates" of around 30 frames per second—meaning sharks can resolve changes in the visual scene over time rather quickly.
With respect to visual motion, sharks also need to compensate for motion of the prey, the water, and the shark itself. This may help explain sharks' outsize cerebellum, which is thought to coordinate motion and sensation.
But spatial vision may be less important for motion-related skills like hunting. This could explain the lack of color vision in sharks. Sharks also lack a fovea of densely-packed cones like we have, a feature that gives us greatly improved spatial resolution.
Why Sharks Attack
Sharks may be missing some of the fine details that differentiate us as a particular class of objects from something tasty like a seal. This relative deficit could combine with the fact that when humans paddle in the water, they resemble the pattern of visual motion produced by swimming seals, at least according to computer simulations.
Wetsuits, which give us seal-like skin, have been proposed to add to the similarity. In addition, murky water stirred up before an attack may contribute to a mistaken identity.
However, this idea has recently been challenged. Sharks species that do not feed on seals have been found to bite humans. Patterns of attack also seem to differ for humans compared to seals. In addition, shark attacks have been found to occur in perfectly clear water.
Just as we can never really understand what a shark sees, we can't know what they think of us either. Instead of mistaking us for prey, sharks may just be exploring their environments with the best manipulator available: their enormous, toothful mouths. Sharks have been observed biting—but not consuming—a variety of animate and inanimate objects of different sizes and shapes.
Consider an analogy. Around my home in shark-free upstate New York, it is common to see fossils of shellfish in rocks—if an alien scientist observed me picking one up and feeling it with my hands, they might mistakenly conclude that I think the fossil is dinner, since it resembles edible shellfish.
We may be making a similar mistake in trying to understand why sharks attack us. Instead of, or in addition to, looking like a tasty seal to a shark, perhaps you look more like a chew toy.
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References
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Hart, N. S. (2020, October). Vision in sharks and rays: Opsin diversity and colour vision. In Seminars in Cell & Developmental Biology (Vol. 106, pp. 12-19). Academic Press.
Rex, P. T., May III, J. H., Pierce, E. K., & Lowe, C. G. (2023). Patterns of overlapping habitat use of juvenile white shark and human recreational water users along southern California beaches. Plos one, 18(6), e0286575.
Ryan, L.A., Slip, D.J., Chapuis, L., Collin, S.P., Gennari, E., Hemmi, J.M., . . . & Hart, N.S. (2021). A shark’s eye view: testing the ‘mistaken identity theory’ behind shark bites on humans. — J. Roy. Soc. Interf. 18: 20210533
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