The bull gator lay in the sand under the oak trees. A few days earlier he had been hauled out of a murky lake in central Florida. Researchers instantly named him Mr. Big. He was sofa-size, with fat jowls framing his head like a couple of throw pillows. He would have measured 13.5 feet (4 meters) if a rival hadn't chomped the last foot off his tail.

Four people sat on his back. Excited alligators do more than thrash—they can spin like wound-up rubber bands. Yet Mr. Big, with his mouth taped shut and a towel over his eyes, was completely docile, as inert as luggage. He behaved like a gator basking in the sun rather than one in the middle of a science experiment.

Gregory Erickson, the scientist, stood a few paces away, grimly holding a plastic pole tipped with a little square plate called a force transducer. He intended to put this in the animal's mouth, to measure the force of its bite. Erickson also intended to retain all his body parts, which explained his serious countenance.

A man on the gator's back removed the towel and the tape. The animal opened its eyes and hissed. It was a factory noise, a steam pipe venting. The mouth opened as slowly as a drawbridge. The maw on Mr. Big was spacious enough to house a poodle. Erickson presented the force transducer to the largest tooth at the back of the right upper jaw—and the jaws snapped shut.

"Trouble, we...