In narwhals, which tooth usually grows into the long tusk?

In narwhals, the left upper canine usually grows into the long tusk.

The tusk is a spiraled tooth that projects through the upper lip, most often on males. It can reach about 3 metres in length. Narwhals are unusual because the tooth contains sensory nerves and may detect changes in water conditions, although the tusk also appears to have social and display functions.

Most males have a tusk, while some females do as well. A small minority of narwhals develop two tusks when the right canine also erupts. This makes “one tusk per narwhal” an oversimplification.

The tusk is not a horn and is not made of keratin like a rhinoceros horn. It is dentine and enamel, like a tooth. Narwhals live in Arctic waters and are closely related to belugas; together they form the whale family Monodontidae.