Lying half-submerged in a murky swamp, alligators and crocodiles resemble nothing so much as floating logs. Yet should a splashing bird or sipping deer break the surface of the water, the fearsome predators will instantly lunge and bite. Their hair-trigger response has been chalked up to a sharp sense of smell and keen hearing. But new research suggests that crocodilians employ an entirely separate sensory system that first evolved during the age of the dinosaurs.
The secret lies in the pigmented bumps known as dome pressure receptors distributed like beard stubble around the jaws of alligators and the faces and bodies of crocodiles. Curious about the organs' function, Daphne Soares of the University of Maryland, College Park, studied the anatomy of American alligators. She found evidence of an extremely important sensory system; the nerve linking the receptors to the brain was the largest of all the animal's cranial nerves, and the corresponding area of the brain was also particularly well developed, she reports in the 16 May issue of Nature.
Soares set out to determine the function of the receptors by recording electrical activity in the nerves of anesthetized alligators. She tried light, electric current, smell--all to no avail. But when she happened to dip her hand in the water, the nerves fired strongly. When Soares placed alligators in darkened tanks half filled with water, plugging their ears to eliminate sound cues, she found that the alligators reliably turned and snapped at disturbances caused by a single drop of water. But they didn't react when she masked the receptors with latex.
By studying fossils, Soares discovered that the receptors first appeared in semiaquatic crocodilians dating back about 200 million years to the Early Jurassic. The most ancient, terrestrial crocodilian fossils lacked them, as did their fully aquatic relatives and all modern lizards.
"This explains why when there's wind you never see alligators around," says Valentine Lance of the Center for Reproduction of Endangered Species at the San Diego Zoo. "The choppy water would interrupt this ability to detect prey." Tracing the emergence of the system adds a new dimension to crocodilian evolution, says Adam Britton, a crocodile specialist with Wildlife Management International in Darwin, Australia. "It must have been really instrumental to the success of the group as a whole."