Why Can't We Be Friends?

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The bull shark, Carcharhinus leucas, usually grows no longer than 10 ft. and weighs up to 500 lbs., but what it lacks in size it makes up for in aggressiveness. Experts regard it as the most pugnacious of sharks. It has, according to Robert Hueter, director of the Center of Shark Research at the Mote Marine Laboratory in Sarasota, Fla., the highest level of testosterone in any animal, including lions and elephants. Its lower spiked teeth are designed to hold prey while the upper triangular serrated teeth gouge out flesh. "The bull is an ambush type of predator, it makes this big mortal wound," says Hueter. It is fearless, taking on prey as large as it is.

A unique feature of bull sharks is their ability to live in both salt- and fresh water; they have attacked people in Lake Nicaragua in Central America and have been seen above St. Louis, Mo., in the Mississippi River. Those born in the Mississippi delta usually spend about six months in the brackish water before migrating along the coast to Florida to winter in the Keys.

The bull is the only shark that prowls regularly in water shallow enough for humans to walk in--and it may be territorial. Australian shark biologist Ian Gordon has been getting into the water off Florida beaches and deliberately agitating bull sharks to observe their reaction. He says his research so far suggests that underwater geography and a sense of territory can provoke an attack. "Even if you don't know it's there, the shark will feel like it is being cornered."

Human shark victims almost always seem to be inadvertent intruders rather than targeted prey. Scientists who work with sharks know how dangerous they can be, and many are critical of the guided shark-feeding tours that are proliferating in Florida and the Bahamas. Sharks there have begun to associate the sound of an outboard motor with food, and there have been attacks by sharks apparently impatient to be fed, according to George Burgess, head of the International Shark Attack File. Shark feeding is illegal in two Florida cities, and a campaign to ban it statewide is under way. "When you are training animals, you are changing their basic behavior and their respect for human beings," says Burgess.

That would be a strange development: the ocean's fearsome hunters lured unnaturally into the company of humans--then learning to bite the hands that feed them. Nature has its bounds.

--Reported by Alice Jackson Baughn/Ocean Springs, Paul Cuadros/Gainesville, Lisa Clausen/Melbourne, Jeanne DeQuine/Bahamas and Jeannie McCabe/Honolulu

TIME.com For pictures and profiles of the world's most dangerous sharks, go to time.com