Narwhals have essentially no functional teeth in the mouth. Males grow one tooth from the upper left jaw straight through the upper lip, reaching two to three metres. The right canine usually stays embedded, though roughly one male in five hundred develops both into a double tusk. About fifteen per cent of females grow a tusk, generally shorter.
The spiral grooves run anticlockwise viewed from the base, and they do so in every animal, including in the rare double-tusked males where both teeth turn the same way rather than mirroring. The tooth is unusual in construction: the cementum is porous and dentinal tubules connect the pulp to the outer surface, which is why the tusk is thought to register salinity and other water properties.
Function remains contested. Tusks are used in surface rubbing between males and have been filmed tapping fish, but the sensory, social and possible weapon roles have not been ranked by evidence.