Crocodilians' Hunting Secret—Blue Freckles?

Ben Harder
National Geographic News
May 16, 2002
An alligator hunts by lurking motionlessly in shallow water, in wait of unwary prey. When another animal unknowingly blunders too close, the predator reacts, lunging with lightning speed while opening—and then slamming shut—its powerful jaws.

Now, a student scientist's accidental observation has led to new insights on how these hunters react with such instant reflexes. These ancient reptiles detect movement using not only hearing and sight, but also a specialized set of nerves that gives them extreme sensitivity to touch.

Alligators and their relatives—collectively called crocodilians—have small, raised, bluish freckles on their faces, scattered around the mouth like the stubble of a beard. These bumps lack pores and hairs, and have thinner skin than the surrounding areas of the face. In most crocodilians, the bumps are distributed in a dense, scattered pattern across the skin.

Although scientists noticed the freckles long ago, they couldn't discern the structures' purpose. Daphne Soares, a doctoral student in neuroscience at the University of Maryland, has now determined that the bumps, which she calls "dome pressure points," are packed with nerves that make them extraordinarily sensitive.

It's that unique sensory ability—which can detect waves produced by a single droplet hitting the water's surface several feet away—that give crocodilians their hunting proficiency.

The new insight stems from a serendipitous observation Soares recently made. On a ride with colleagues through a Louisiana swamp, she was sharing the back of a pick-up truck with a fearsome-looking—though restrained—alligator. "I was looking at his jaw as we rode along and thought, I wonder what those little spots are for,'" she recalls.

Soares checked into the matter and found that no one really had a good idea of what purpose the bluish bumps served. So she decided to investigate the question herself.

A Turn Toward Trouble

"Crocodilians hunt at night, half-submerged in water, waiting for prey to disrupt the surface," says Soares. "Their jaws rest right at the interface of air and water. When they're hungry, they quickly attack anything that disturbs that interface."

Since the domes lie along the surface of the water when crocs and gators are in their hunting posture, Soares wondered whether they might play some role in the animals' snappy reflexes.

To test that hypothesis, she placed an alligator in a completely dark tank of water—thus robbing them of sight—and soundlessly dripped a single droplet to one side of the gator.

When its head was lying along the surface of the water, the animal turned and lunged in the direction of the ripples produced by the droplet. When its head was either fully submerged or completely out of the water, however, the animal ignored the droplet.

In repeated tests, other gators also responded the same way, lunging or at least turning their heads toward disturbances in the water—but only when their stubble of domes was lying along the plane of the water's surface.

Intrigued, Soares then covered gators' domes with a plastic material that diminished their sense of touch. Thus "blinded," the gators ignored water droplets even when their heads did lie along the water's surface.

Hoping to figure out how the domes work, Soares examined alligator skulls to determine what lay beneath the surface of the stubble. The jaws are riddled with small holes through which nerve bundles can relay electrical messages from the domes to the brain.

Ancient Adaptation

Since the nerve-loaded domes represent such an appropriate adaptation to the alligators' stalking habits and their shallow, watery hunting grounds, Soares wondered how long they took to evolve. She discovered that dome pressure points are nothing new—crocodilians appear to have been using them for the past 200 million years.

Soares determined their evolutionary age by examining skulls of various living and extinct crocodilians for the telltale nerve holes. Of 13 different groups of crocodilians she considered, 11 had nerve holes arranged in the same, scattered pattern as the Alligator mississippiensis that first led to Soares' investigation.

Only two crocodilian groups had sparser, and therefore less sensitive, strings of the holes running along their jaws. These animals would have been less effective at hunting half-submerged, Soares thinks.

It's no surprise, then, that one is the oldest known branch of the crocodilians—long since extinct—and the other is an alligator relative that doesn't hunt in the water.

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