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Mystery Bumps

Scientists knew that alligators' jaws are covered in bumps but it took biologist Daphne Soares to figure out why

She hooked up the electrode to a bank of equipment capable of amplifying and recording brain activity in response to nerve stimulation. When nerve cells fired, loudspeakers sounded a series of rhythmic pops.

It took a month of experimenting before Soares could even begin to investigate what kinds of stimulation would make the nerve fire. She tested whether the bumps might be sensitive to light, or to electrical fields, like the skins of some eels and fish. She blew tiny particles of ground-up fish over the bumps to see if they were sensitive to taste or odor. But the loudspeakers stayed silent.

Then, one day, Soares reached into the tank and the speakers went brrraap.

After conducting further experiments, Soares concluded that the ripples in the water had caused the bumps to vibrate, which in turn caused the nerve to fire and send a message to the brain. This made sense for an animal that spends much of its life lying quietly in the shallows waiting for its prey to swim by or come to the water’s edge for a drink. And it also offered an explanation for why crocodilians that lived on dry land did not have bumps.

Still, Soares wasn’t ready to break out the champagne. "If someone pokes you in the eye, your optic nerve fires, but that doesn’t mean your eye is a poke-detector," she says. Was detecting ripples the bumps’ primary purpose? What if they served some other function that Soares didn’t know about? What if alligators found their prey, as do most animals, by sight and hearing?

Soares covered several alligators’ ears with fine plastic and placed them in a tank of water. In complete darkness, using an infrared camera to record the animals’ reactions, she let a single drop of water fall onto the surface. Though deprived of sight and hearing, all of the animals swam directly toward the spot where the drop hit the water.

Soares next covered several alligators’ snouts with the same plastic wrap, so that the bumps no longer came into contact with water or air, thus ensuring they had no way of detecting surface waves. She again dropped water into the tank. The alligators did not respond. Now she was ready to celebrate.

In a paper she published last year in the journal Nature, Soares wrote that the alligators’ bumps are "pressure receptors" that evolved millions of years ago and solved the problem of how a creature with armorlike skin could have tactile sensitivity. This sensory system is "a specialization of crocodilians that other animals don’t have," says Clark. It’s a "major breakthrough in crocodilian sensory physiology," says Valentine Lance of the Center for the Reproduction of Endangered Species in San Diego.

Soares still has questions she wants to answer. Alligators have bumps on their jaws and mouths, she points out, while other crocodilians have them all over their bodies. How come? "It’s a wonderful mystery. Their world is so different from ours."

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