Study shows how alligators seek prey with sonar


Mar 11, 2001
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How gators can tell when we enter the water

By LEE BOWMAN, Knoxville News

May 15, 2002

A scientist has discovered how alligators and crocodiles are able to seek out and attack prey at night, in murky water.

It turns out they've got a sort of sonar - pressure sensors along the jaw that can even pick up a single drop of water disturbing the surface of a lake or other body of water.

Daphne Soares, a doctoral student in neuroscience at the University of Maryland, says she got the idea for her study while sitting on a huge, though restrained, bull alligator in the back of a pick-up truck rolling through a Louisiana swamp.

Soares does alligator field work in the Rockefeller Wildlife Refuge along the Gulf Coast, using the animals mainly as part of her dissertation study on how birds hear. The ears of the two groups are virtually identical.

She recalls studying the gator's jaw and thinking, "I wonder what those little spots are for,'' referring to the "beard'' of pinprick-sized holes that line the jaws of gators and their crocodilian relatives.

The tiny holes have been noted before, and mainly used by biologists to distinguish different members of the crocodile family. But until Soares's research, published Thursday in the journal Nature, no one had figured out the function of the holes.

Soares has termed the holes "dome pressure receptors'' - an apparently unique sensory organ developed millions of years ago by reptiles that hunt at night, half-submerged in the water.

"They wait for prey to disrupt the water surface, their jaw rests right at the interface of air and water. And when they're hungry, they quickly attack anything that disturbs that surface,'' Soares said.

In Maryland, Soares set up an experiment using juvenile alligators in small tanks to see what the organ did. First, she blocked the gators' hearing and nostrils and left them in total darkness, leaving them no way to sense activity through eyes, ears or noses.

When the gators submerged their jaws, Soares dropped a bit of water onto the surface, and the animals reacted instantly, lunging and snapping in the direction of the disturbance.

Then, she covered the receptor areas along the jaw, and repeated the experiment. This time, the animals didn't move. "The receptors detected the pressure-difference waves in the water when the surface of the water was broken. Without the receptors, that orienting behavior is blocked,'' she explained.

Soares also speculates that the croc clan uses the organ for communication, sensing some of the grunts, head slaps and tail movements of other animals partially submerged nearby.

She solidified her theories by looking at the jaws of fossil crocodiles, and found that nerve connections between the holes and the brain leave distinctive markings on jawbones.

Yet she found that only those ancient crocs with a semi-aquatic lifestyle had the markings; those that appear to have lived entirely in water or entirely on land did not.

"The evidence shows that this sensory organ may have appeared around 200 million years ago, during the Jurassic period,'' Soares said. "It's fun to imagine these enormous extinct crocodiles sitting half-submerged in the water at night, waiting for dinosaurs to come and drink.

"Just at the moment the dinosaur broke the water surface with its mouth, it would have sent pressure waves in the water, telling the crocodile just where to get its next meal.''

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(Lee Bowman covers health and science for Scripps Howard News Service. E-mail BowmanL(at)

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