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Jaws—or lips?

Natural History, Dec, 2006 by A. Peter Klimley, Matt Hinton, R. Aidan Martin, Anne Martin

R. Aidan Martin and Anne Martin ("Sociable Killers," 10/06) claim to differ with my theory that white sharks avoid feeding on people because they prefer fattier prey. But the Martins offer no reason a shark would not eat a human it has bitten--out of "curiosity"--except to say the shark may be fought off.

In fact, there are many cases in which white sharks have not eaten incapacitated humans. An abalone diver off northern California was seized by the head and carried in the jaws of a white shark before being released; the diver swam ashore. In two other cases a shark carried a suffer underwater by the ankle or leg, then let the victim go. Such observations led me to suggest that white sharks be called Lips, not Jaws. Like primates, they handle large food items gently in their lips, tasting them before deciding to eat them.

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White sharks also reject other lean but vulnerable species. Sea otters commonly float up on the shores of Monterey with bite wounds and embedded fragments of shark teeth, yet a sea otter has yet to be found in a white shark's stomach.

Crude experiments supplement such anecdotes. The rib cage of a northern elephant seal, with the fatty layer removed, was presented to a white shark off San Francisco. The shark bumped the carcass once, but after several minutes left it uneaten. Yet when chunks of seal fat were put in the water at the same place and time, the sharks readily ate them. More carefully controlled experiments are needed; they will, I believe, support my hypothesis.

A. Peter Klimley

University of California

Davis, California

In the twenty-five years I've lived in far northern California there have been at least a dozen nonfatal white-shark attacks on local suffers. One man was sitting on his board in five or six feet of water, when a white shark in the ten-foot range swam under him, circled back, and grabbed him by the thigh. He was dragged under, but the shark let go immediately.

About a week later I launched my kayak through the surf a few yards from where the attack had occurred. "Time to go trolling!" I joked to myself. Not ten minutes later there was a tremendous bang at the back of the boat, and I was dumped overboard. Underwater, I saw a white shark about ten feet long turning back toward me. I was still holding my kayak paddle, and aimed a blow at the shark's head. I don't know if I actually hit it, but the next thing I knew I was on the surface. I swam to shore, remembering something John E. McCosker, a biologist at the California Academy of Sciences in San Francisco, had said in a television program--that white sharks attack to disable their prey, then circle until it stops struggling.

Matt Hinton

Trinidad, California

R. AIDAN MARTIN AND ANNE MARTIN REPLY: A. Peter Klimley suggests that white sharks reject humans as food because they are too lean, but most shark attacks on humans are very different from predatory attacks. The 85 percent of victims who survive often report that the animal moved slowly and methodically, and that the wounds from the initial (and often only) strike were minor compared with the damage typically inflicted on prey. White sharks are highly curious, and they gently "feel" objects with their teeth and gums. We believe they rarely consume humans because their encounters are usually motivated by curiosity rather than hunger.

Mr. Klimley has suggested that fat helps maintain the white shark's body warmth. But though a pound of dietary fat carries twice the calories of a pound of protein, the useful energy from protein, or heat liberated by its digestion inside the animal, is superior. Pound for pound, protein not only generates the same amount of metabolic heat as fat, but protein also "burns" more slowly, spreading the heat out longer and so maintaining body temperature more effectively. In addition, the importance of marine mammals in the white shark's diet has been exaggerated, partly because of observer bias (it is relatively easy to observe attacks on seals) and partly because gut-content analysis is limited (fish remains are relatively delicate and inconspicuous). Fourteen quantitative studies show that almost 75 percent of the white shark's diet is low-fat food (fishes, squids), and slightly more than 20 percent comes from marine mammals.

Matt Hinton recounts a neat story! White sharks may employ the "bite, spit, and wait" tactic when tackling large and powerful prey, such as northern elephant seals, but with smaller and more agile prey they typically do not delay between attacking and feeding.

Jaws—or lips?

Natural History, Dec, 2006 by A. Peter Klimley, Matt Hinton, R. Aidan Martin, Anne Martin

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