The tooth grows outward, spiraling counterclockwise, and it can reach 9 feet long. It is also flexible, as an 8-foot long tooth can bend 1 foot. A narwhal might grow two such teeth, although this is not common.
Male narwhals grow this tusk most often. Female narwhals rarely grow the tooth.
Though on average it only has the one tooth, the narwhal can still ingest food, and is it carnivorous. The main things it eats are polar cod, arctic cod, Greenland halibut, Gonatus squid and shrimp.
The narwhal eats a lot in winter. This separates it from subarctic whales, which eat a lot in the summer.
Historically, people hunted the narwhal for its oil as well as for its tusk, which is ivory. As far back as the Middle Ages, tusks were sold to the Mediterranean and other parts of Europe as well as eastern Asia.
It's not entirely clear what the narwhal uses this specialized tooth for. However, in the summer males make a whistling noise while crossing their tusks carefully. This whistling and tooth crossing could be about reproduction and social order. An alternative theory is that it is a sensory organ for such things as salinity, pressure and temperature.
Narwhal teeth are an integral part of the legend of the unicorn. The story began as far back as the eighth century B.C., with a description by a man named Ctesias of a magical horned creature. This description made people associate rhino horns with the unicorn horns.
A man named Aelian, who lived from around 170 to 235 A.D., described the unicorn horn as a twisted spiraling shape. This made people connect the narwhal tooth with his description.