Alligators may not be the usual exemplar of delicate skin, but some of it is exquisitely sensitive. Gator faces carry pressure receptors so responsive that they can detect ripples on the water's surface from a single falling drop.
That's more responsive than human skin, even lips. "Crocodilians have taken skin sensitivity to the next level," says Daphne Soares of the University of Maryland in College Park.
Alligators and crocodiles have dots on their skin that scientists have long suspected to be sense organs. In the May 16 Nature, Soares describes experiments that provide the first information about what the spots do.
At first, Soares studied alligator tissue and noted that the dots connect to the trigeminal nerve, which is the thickest of the cranial nerves in an alligator. "It's about a quarter of the thickness of your pinky," says Soares. "If something is big, it's probably important to an animal."
To see what information facial dots pick up, she monitored impulses from nerves attached to the dots of young alligators as she offered them various stimuli in a laboratory tank. "I tried electrical currents, I tried lights, I tried stinky things," she recalls. The nerves didn't respond. Then one day, when reaching into the water, she noticed the nerve firing. When she set a rod vibrating in the water, the nerve fired dramatically.
In an experiment to test how the facial dots affect behavior, Soares placed young gators in a tank of shallow water so that they were half-submerged. To reduce input to other senses, she temporarily blocked the animals' ears with Vaseline and turned out the lights. Even so, whenever she dropped a milliliter of water into the tank, the gators turned toward the disturbance. "They'd usually go and bite it," says Soares.
When completely underwater, however, the animals gave no response.
In a further test, she blocked the facial receptors with a spreadable plastic used as the basis for some cosmetic beauty masks. The partially submerged gators no longer tracked the droplets.
Seeking ancient signs of the sensors, Soares checked crocodilian fossils for holes in the jawbones where nerves passed through. She found such holes in semiaquatic crocodilian fossils from as long ago as 200 million years.
However, fossils of a terrestrial crocodilian and a fully aquatic one didn't have the holes.
Because alligators are champs at hunting in the dark, James Perran Ross of the Florida Museum of Natural History in Gainesville says that the sensitive receptors don't surprise him. He welcomes the work because "sensory capacities of crocodilians are not well understood."
Valentine Lance of the Center for Reproduction of Endangered Species in San Diego calls Soares' work "a major breakthrough in crocodilian sensory physiology," adding that the "paper may open up a whole new area of research."
P.O. Box 120551
San Diego, CA 92112
James P. Ross
Crocodile Specialist Group
Florida Museum of Natural History
Dickinson Hall, Box 117800
University of Florida
Gainesville, FL 32611
Department of Biology
University of Maryland, College Park
College Park, MD 20742