A narwhal's tusk is what anatomical structure?

A narwhal’s tusk is an elongated tooth, usually the left upper canine.

The tusk grows in a spiral pattern and can reach about 3 metres in length, especially in males. Most males have one tusk, while some grow two; a smaller proportion of females also develop a tusk. The tooth has millions of nerve endings and an unusual outward-facing sensory structure.

Narwhals live in Arctic waters and use sound for orientation and communication beneath sea ice. Researchers have proposed several functions for the tusk, including display, social competition, and sensing changes in the surrounding water. It is not a detachable horn or a weapon made from bone.

Narwhals are toothed whales and are closely related to belugas. Their long tusks gave rise to historical “unicorn” legends, but the structure is a real tooth emerging through the upper lip.