[Photo] An 1820 illustration titled "Male Narwhal or Unicorn. Greenland Shark." from the publication "An account of the Arctic regions with a history and description of the northern whale-fishery," by W. Scoresby. Credit: NOAA

The narwhal is a toothed whale, but it has no teeth in its mouth. Instead the male sports one long tooth, or tusk, that sticks up to 6 feet out of the upper jaw.

Tusks exported from the Arctic, perhaps by the Vikings, were sold from Europe to the Far East clear back in the Middle Ages. Buyers did not know where the tusks came from, inspiring myth.

The purpose of the tusks is not clear, but only males have them, so likely they're used to determine social rank, some researchers say.

One recent study, however, found that the tusks are used to determine the salinity of water and search for food.

SOURCE: NOAA; Kristin Laidre, University of Washington; Mads Peter Heide-Jorgensen, Greenland Institute of Natural Resources