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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

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On a hot July day in a Louisiana marsh in 1999, Daphne Soares was riding in the back of a pickup truck next to a thrashing six-foot-long alligator. Though the reptile was tied up and its mouth cinched shut, Soares couldn’t take her eyes off its jaws—but not for the obvious reason.

The University of Maryland neuroscientist had noticed hundreds of dark bumps all over the animal’s upper and lower jaw. "They were each about the size of a little blackhead, and raised like a pimple," she says. But this was no case of alligator acne. She knew that other alligators sported these bumps, yet she’d never seen them on any other creature. "I began wondering what those little spots were for," Soares says.

Those musings had little to do with her reason for visiting the 76,000-acre Rockefeller Wildlife Refuge, which stretches along the Louisiana coast in the Cameron and Vermilion parishes. The 28-year-old graduate student was there to compare the brains of birds and large reptiles for her PhD dissertation on the mechanisms of avian hearing. But "a great thing about science is that you can follow your curiosity," she says.

Back in her lab she found that the bumps were characteristic not only of the American alligator (Alligator mississipiensis) but of the 22 other species in the 220-million-year-old crocodilian family, which includes alligators, caimans, crocodiles and gharials. For years, crocodilian biologists have used the number, placement and patterns of these bumps as a means of distinguishing between different species. But no one had been able to explain their function. So Soares placed some alligator skin under the microscope and began her own investigation.

Her first reaction was disappointment. "I was hoping to see a pore or a hair or an eyeball looking back at me," Soares jokes. "But all I saw was a bump."

Dissecting an alligator’s head, she found that the bumps were connected to nerves, which led into the skull through holes in the bones of the jaws. These holes, known as foramina, are found in all animals and serve as conduits for blood vessels and different facial nerves, such as those that transmit optical and olfactory signals.

To identify which kind of nerve was connected to the bumps, she injected a dye into the nerve itself. As the dye was absorbed slowly by the nerve cells, it left a telltale trail that enabled Soares to determine that each bump was connected to the large trigeminal nerve, which stimulates the skin and muscles of the face. Whatever their purpose, these bumps appeared to be important.

It occurred to Soares that the fossil record might be helpful. So Soares approached James M. Clark, a paleontologist at George Washington University and an expert on crocodilians. She learned that not all ancient skulls had these foramina: those with similar holes in the jaw were species known to have lived where land meets water, as alligators and crocodiles do today; crocodilians without bumps lived entirely on dry land.

"But I still didn’t know what these things did!" says Soares. Clearly the bumps were some kind of sensors that passed along information about the animal’s environment directly to the brain. And they appeared to be useful primarily to animals that spent their lives between land and water.

In 2001, Soares won a three-month Grass Foundation Fellowship to the Marine Biological Laboratory in Woods Hole, Massachusetts, a major center for neuroscience research. Until Soares arrived with 20 baby alligators in June, however, no one had used its sophisticated equipment on a crocodilian. At the lab, she taught herself to open the skull of a sedated but living animal and to insert an electrode into the area of the brain that receives electrical signals from the trigeminal nerve.

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