UM Researcher Discovers Major Sensory Area in Alligators

Daphne Soares stumbled onto her discovery of a previously unknown major sensory ability in alligators when she was sitting on the back of a huge bull alligator in a pick-up truck, cruising through a Louisiana swamp.

"I was looking at his jaw as we rode along and thought 'I wonder what those little spots are for,'" said the University of Maryland doctoral student, referring to the 'beard' of pinprick-sized holes that line the jaws of alligators and their crocodilian relatives.

Soares' question led to her discovery of what she has named Dome Pressure Receptors, tiny pressure-detecting mounds that give crocodilians the ability to detect the movement of prey in water. Soares estimates the receptors probably evolved about 200 million years ago in Jurassic Age predecessors that may have been the first amphibian crocodilians. Her findings appear in the May 16 issue of the Journal Nature.

Tiny Holes, Big Find

While the existence of the tiny holes had been noted in earlier research and used mainly as a marker to distinguish different types of crocodilians, which include alligators, crocodiles and the slender-snouted gavial, their function and natural history had not been determined. Soares¿ research shows that Dome Pressure Receptors, or DPRs, make up a unique sensory organ that give crocodilians the ability to detect small disturbances on the water surface. "Crocodilians hunt at night, half-submerged in water, waiting for prey to disrupt the water surface. Their jaw rests right at the interface of air and water," Soares said. "When they're hungry, they quickly attack anything that disturbs that interface."

Sensing the Change

Using juvenile alligators, Soares determined the function of the receptors by blocking the gators' hearing and placing them in shallow water in total darkness, leaving them with no way to sense activity through their eyes, ears or noses. When the gators submerged their jaws, Soares dropped a small bit of water onto the water's surface. The alligators reacted instantly, lunging and biting in the direction of the disturbance. (Video can be viewed at http://www.wam.umd.edu/~daph/DPR.html)

She then covered the DPR area of the jaw, and when she dropped the water, the animals did not react. "The DPRs detected the pressure difference waves in the water when the surface of the water was broken," Soares said. "Without the DPRs, that orienting behavior is blocked. "The primary function of the DPRs seems to be prey detection. The disturbance caused by the water droplets is similar to, a baby bird dropping into the water from a nest, for example," Soares said. "DPRs might have other roles. Crocodilians also use the water-air interface for communication, such as head slaps and bellowing.

"Our findings show that the alligator is a very touch sensitive creature. Its brain is enormously devoted to DPR information," said Soares.

Fossil Jaws

Soares' hypothesis about the DPR function as a sensory organ for an amphibious lifestyle was strengthened when she examined the jaws of fossil crocodiles. She found that the way the nerves reach the DPRs from the brain gives rise to markings on the jaw bones. These markings, called foramina, are holes the nerves go through to innervate the DPRs.

In examining the fossils, Soares discovered that only animals known to have had a semi-aquatic lifestyle had the same pattern of foramina as today's crocodilians, all of which are amphibious. Conversely, fossil jaws of extinct crocodiles known to have been totally aquatic or totally land creatures did not have the foramina pattern.

"Evidence shows that this sensory organ may have appeared around 200 million years ago, in the Jurassic period" Soares said. "It's fun to imagine these enormous extinct crocodiles sitting half way 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 where to get its next meal."

Alligator Bites:

Accidental Research: Soares¿ discovery was kind of an accidental piece of research. She was using alligators to study how birds hear, the subject of her doctoral dissertation. "Crocodilians are the closest living relative of birds," Soares says. "Their ears and the way they process sound are virtually identical."

Gator numbers: Soares has about 30 or so young gators swimming in tanks the size of small hot tubs in her labs. They range from about two months to two years in age. The oldest measure about two feet.

Gatorspeak: Says Soares, "Alligators are very vocal. They growl, grunt, call out in distress. They use sound to communicate threats, mating and dominance calls. The babies call out to the mother to help them out of the nest after they hatch."

Gator Bites: Soares has been bitten dozens of time, especially by the new hatchlings. "They come out of the egg biting,¿ she says. "The bites of the bigger ones can get infected really easily." By the time the gators are two years old, they are becoming aggressive and chomp with ferocity. "Alligators are made for closing their jaws," Soares explains. "An adult can close its mouth with thousands pounds of pressure per square inch."

Life In The Swamp: Soares studies adults and gets her alligators and eggs at the Rockefeller Wildlife Refuge along Louisiana's Gulf coast. To harvest the eggs, Soares wades up to the nest with a bucket, while another scientist keeps any nearby mother gators at bay with a long stick. "We can keep them away by tapping them on the nose most of the time, but there was one mother that bit the stick and broke it," Soares recalls. "I was a little nervous."

Air Gator: Because the eggs have to be kept perfectly still to hatch, Soares flies home on the airplane with the eggs on her lap. She hasn't sent any eggs through the airport x-ray machine since September 11, so she's not sure what delays she might encounter on her next trip. "I've been pulled aside before," she says. "You can see the little skeletons in the eggs on the x-rays."

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