The narwhal tusk is the left upper canine, erupting forward through the lip and reaching two to three metres in males. It always spirals in the same direction, counterclockwise viewed from the base, and where a second tusk develops it spirals the same way rather than mirroring the first.
In structure the tooth is inverted relative to normal mammalian teeth. The hard enamel-like exterior is thin or absent along much of the length, the cementum is porous, and dentinal tubules run from the surface to a nerve-rich pulp. This has led to the proposal that the tusk functions as a sensory organ, potentially detecting salinity or other water properties, and narwhals have been observed reacting to changes in water chemistry applied to the tusk.
The sensory function is not the whole answer, because only males typically bear tusks and narwhals of both sexes feed and navigate successfully without one. Drone footage has shown narwhals striking fish with the tusk. Tusk-crossing behaviour at the surface, once read as duelling, may be display or social assessment. No single explanation has settled the question.