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• Science & Nature

Sense and Sensitivity

Great whites have tiny brains but powerful sensory organs

• By Amanda Bensen
• Smithsonian magazine, June 2008

The scientific name given to great white sharks in the 19th century—Carcharodon carcharias, from the Greek for "jagged tooth"—reminds us that these creatures were defined by their most fearsome feature long before the movie Jaws. But great white sharks are more than just masterful masticators; they are equipped with powerful sensory systems that equal or surpass our own in many respects.

Researchers rarely get a good look at a great white's brain; specimens are hard to come by, and a dissection must be done during the hours between death and decomposition. In 1992, biologists Leo Demski of the University of South Florida and R. Glenn Northcutt of the University of California at San Diego obtained a fresh great white's head from a Florida fisherman. They were surprised at the "relative smallness" of the brain, which weighed less than an ounce and a half. But they found that 18 percent of it was devoted to smell, the highest percentage among sharks. That explains why dropping pieces of not-so-fresh fish into the water, as researchers and tour guides often do, is a good way to attract great whites.

Great whites also have excellent vision. Inside their retinas, they have a pattern of cone-shaped cells (which detect color) and rod-shaped cells (which detect contrast in low light conditions) comparable to that in humans. They also have a reflective layer behind their retina—the same thing that makes cats' eyes appear to glow in the dark—which bounces extra light to the retinal cells. This allows the sharks to see in the dimness of the deep ocean.

Great whites and other sharks lack external ears, but that doesn't mean their world is silent. They have inner ears on either side of their head, and a system of hair-lined tubes called a "lateral line" runs beneath the skin of their flanks, making their body sensitive to vibrations caused either by sound or movement in the water.

A. Peter Klimley of the University of California at Davis, who has studied sharks for 36 years, says a great white operates from a "hierarchy of senses" depending on its distance from potential prey. "At the greatest distance, it can only smell something, and as it draws closer it can hear, and then see it," Klimley explains. "When the shark gets really close, it can't actually see the prey right under its snout because of its eye positioning, so it uses electroreception."

Electroreception is a kind of "sixth sense" found in some aquatic animals. Sharks and rays use an organ called the ampullae of Lorenzini, named after the Italian scientist who discovered it in 1678. Tiny holes in the shark's snout lead to a network of narrow canals beneath the skin that are filled with a jelly that conducts weak electrical currents—the sort generated by the heartbeat or other muscle movements of a seal or swimmer.

Despite all these sensory tools, many scientists say that great whites don't always know what they're biting into. They often release an animal after biting it once, particularly if it is a relatively low-fat creature like a sea otter or a human, instead of a high-fat seal or sea lion.

"It may be a textural discrimination [of fat], more than what we would call taste... We once took a seal and stripped the fat off it and put it all in the water. The shark ate the fat, but not the rest of the body," Klimley says. "They are actually very discriminating predators."

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