The narwhal's single, spiral tusk has always been a mystery. Now a Connecticut dentist has discovered that the eight-foot-long modified tooth has as many as 10 million tiny nerves reaching from its surface to the central core and, ultimately, the whale's brain.

Although various scientific theories of what purpose the tusk serves have been put forth--from piercing through ice to jousting for dominance--none have proposed it as a sensory organ. But Martin Nweeia, who is also a Harvard University professor and marine mammal researcher at the Smithsonian Institution, believes on the basis of his finding that the tusk is capable of detecting changes in pressure, temperature and, most significantly, the relative concentration of various particles at varying depths. "For understanding particle gradients, it would be great," he notes. "The whale has a capacity to dive 400 meters and still understand the particle gradients for salinity in water or prey."

Nweeia and his colleagues discovered the nerves when they scanned two narwhal tusks with an electron microscope, revealing tiny tubes extending from the pulp at the center of the tooth to the outside environment. "This animal has gone out of its way to be exposed to the cold Arctic environment," says Nweeia, who presented the findings yesterday at a conference on marine mammals in San Diego. "Why? We don't see that in any other animal."

The narwhal tusk is the only one in nature that does not curve and forms in a spiral. This may help explain its unusual strength and flexibility--it can yield at least a foot in any direction without breaking. And the tusk is definitely not used for male jousting, according to field observations as well as interviews with Inuit hunters who have tracked the whale for millennia. Rather male narwhals more typically rub their tusks together, perhaps a form of grooming or other sensory pleasure, and they frequently raise their heads out of the water with the tusk held high.

When the nerves in human teeth are exposed to cold the result can be excruciating. Yet, the "unicorn" of the Arctic has exposed itself to potentially the worst case of that on record. "It's very compelling evidence that there are very important pieces of info that it is processing," Nweeia concludes.