Male narwhals possess a great spiraled tooth that projects from their heads. Researchers are unsure of the exact purpose of the tooth. The narwhal tooth is the only straight tusk in the world -- all other known tusks are curved. It's also one of the only spiral teeth. The left tooth comes up through the jaw and corkscrews through the lip. Occasionally, someone spots a double-tusked narwhal, which happens when the whale's right tooth grows into a tusk too. But disdaining nature's liking for symmetry, this second tusk doesn't mirror the first -- it spirals in the exact same way as the other one, to the left. The tusk is flexible, able to bend about a foot in any direction without breaking. The tusk can grow up to 9 feet (2.7 meters) or more, which is amazing when you consider that the male is only about 15 feet (4.6 m) long at maturity. Researchers are unsure of the exact purpose of the tooth. Some believe it serves as an attractive ornament for mating, while others believe it is used as a weapon to fight rivals.