The narwhal is the unicorn of the sea, a pale-colored porpoise found in Arctic coastal waters and rivers.
These legendary animals have two teeth. In males, the more prominent tooth grows into a swordlike, spiral tusk up to 8.8 feet long. The ivory tusk tooth grows right through the narwhal's upper lip. Scientists are not certain of the tusk's purpose, but some believe it is prominent in mating rituals, perhaps used to impress females or to battle rival suitors. Females sometimes grow a small tusk of their own, but it does not become as prominent as the male's.
Reference http://www.nationalgeographic.com/animals/mammals/n/narwhal/
