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That purpose does not include the deliberate consumption of humans--another misconception spread by Jaws. Great whites, most experts believe, prefer high-fat prey because fat is packed with calories. People are too scrawny, which is why, after taking a first bite--perhaps because a human, especially one wearing a black wet suit and flippers, looks something like a seal--a great white will usually turn up its nose at whatever remains. Most other shark attacks are probably also cases of mistaken identity: a swimmer's flapping feet and hands may look like the movements of a fish darting through the water.
About the only time sharks attack humans on purpose is when their territory is invaded or their courtship rituals are interrupted. And just about the only time they eat humans is when there's lots of blood in the water, after an airplane or ship accident, for example.
While marine biologists like Klimley and Holland are trying to unravel the mysteries of sharks' behavior and their role in the marine food chain, immunologists and physiologists are attempting to understand the animals' biochemistry. The idea that sharks can actually be beneficial to human health was established decades ago: vitamin A came primarily from shark-liver oil until 1947, when it was first synthesized in the laboratory. The unctuous liquid is also, for reasons still unknown, highly effective in shrinking human hemorrhoids.
Today biomedical scientists are on the trail of deeper mysteries. It has been known for some time that sharks have a low incidence of disease in general and extremely low rates of cancer. Known carcinogens injected into sharks by researchers don't trigger malignancies; they don't even cause the sorts of genetic damage that leads to tumors in other animals.
No one knows why. One clue may be that the chemical squalamine, found in the stomach, liver and gallbladder of the dogfish, can inhibit the growth of human brain tumors. Sharks also have a primitive but highly active immune system, which may play a role. Their resistance to cancer, however, has nothing to do with their cartilage, despite extravagant claims by people who peddle shark-cartilage pills. While the cartilage has proved promising as an ingredient in temporary artificial skin for burn patients, no proof whatever exists that it can prevent tumors in humans.
Assertions that it does are based on a tiny grain of scientific truth. Shark cartilage--and cow cartilage, for that matter--does contain minute quantities of a compound that inhibits blood-vessel growth, and tumors depend on the rapid growth of internal blood vessels that can feed them. But this substance is locked up in the cartilage and doesn't leak out to the rest of the body. To extract it, scientists have to soak huge amounts of cartilage in harsh chemicals for weeks at a time.
Nevertheless, shark cartilage is hot, and sharks are being slaughtered wholesale to produce it; a single processing plant in Costa Rica reportedly turns 235,000 sharks into cartilage pills every month. Sharks are also taken by the millions for their fins--a practice that scientists and conservationists find especially disturbing. Often the fins are hacked off and the sharks are thrown back into the water, alive but mortally wounded, to bleed to death.