The shark has sharp turns

In his "Biomechanics" column on the hammerhead shark ["Head Turner" November 2002], Adam Summers reports that the shark does not bank its winglike head as it turns. He thus dismisses the idea that the head provides lift and maneuverability, as does the wing of an aircraft. But sharks are different from aircraft in two important respects (apart from the obvious ones).

First, water provides sharks with substantial buoyancy, whereas air does not confer the same benefit to aircraft. Second, a shark can make a yawing (sideways) turn without banking, simply by bending its body and adjusting vertically oriented fins to keep from sinking. Airplanes turn while remaining aloft primarily by controlling the positions of ailerons mounted on horizontally oriented wings.

Nevertheless, the shark's head may help the animal maneuver--if not in making sideways turns, then at least in pitching it up or down. In that respect, the forward position of the large head is optimal, though further analysis is needed to confirm any such hydrodynamic function.

Frank E. Fish West Chester University West Chester, Pennsylvania

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