The narwhal sports an eight-foot-long spiraled tooth that makes it resemble a unicorn of the sea. Some thought that the whale, typically 13 to 15 feet long, used it to break arctic ice; others theorized that it served as a lance in male jousts. The tooth, in fact, may be a giant sensor for navigating and hunting. Through electron microscopy of two male tusks, researchers from Harvard University, the Smithsonian Institution, and the National Institute of Standards and Technology discovered that a single horn possesses some 10 million nerves running from its surface to its core. Instead of inflicting the narwhals with a massive ice cream headache, this sensitive tooth appears capable of detecting changes in water temperature, pressure and particle gradients linked with salinity and prey. Their findings surfaced last December at the 16th Biennial Conference on the Biology of Marine Mammals in San Diego.