A Narwhal’s tusk protrudes out the left side of the upper jaw, which forms of a left-sided corkscrew and can be as long as 3 meters.

Narwhals   © Paul Nicklen

Narwhals (Monodon monoceros) are cetaceans that live year-round in the Arctic circle.
They feed in deep bays and inlets, where they find a good supply of Arctic cod, squid, and other food such as flatfish, pelagic shrimp, and cephalapods.

The function of the narwhal tusk has been a matter of some debate.

Researchers thought it to be related to mating activity. Males have been found with broken tusks which might be a result of a battle. Most females are missing the tooth completely which is reason for the earlier studies supporting the theory that the tusk is used as a weapon by the males in battles over possession of females.
However, there are very few instances where people have seen narwhals using their tusks to engage in combat or to hurt each other. Instead, narwhals have been seen rubbing tusks together gently, like a social gesture.
There is no evidence to suggest that the tusk is used to break through pack ice or kill prey either.

At one time it was thought the tusk might be used to stir up bottom sediments in search of food. Other uses, however, seem to be more probable, since female narwhals are able to feed without a tusk.

More recent studies observing the tooth itself have suggested another theory: Human teeth are protected by a layer of hard enamel with dentin and cementum underneath. The centre of our teeth, the pulp, is where all the nerves are.
In narwhals, it is the exact opposite; the softer part of the tooth is on the outside, surrounding a dense centre that runs the length of the tooth. The soft surface is littered with tens of millions of small pores, which brings the nerves in direct contact with the cold Arctic waters.
Suggestions exist that these tusks act like sensory organs, allowing the animal to sense the changes in the environment surrounding it. They may be able to detect changes in temperature, salinity, pressure, and particle gradients, which would be highly useful when hunting in the deep sea.
Perhaps this gives the narwhal a better sense of touch, which would explain why the narwhals rub tusks together. This may be a heightened form of social contact with one another.

So, it is possible that the tusk is a sexual characteristic, but it is also possible that it has a second purpose.

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