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The Narwhal (Monodon monoceros) is an Arctic species of cetacean with a body similar to that of a Beluga whale and Irrawaddy Dolphin. It is a rare creature found south of latitude 70°N. It is one of two species of whale in the Monodontidae family (the other is the beluga whale). It is possibly also related to the Irrawaddy dolphin.

The name "Narwhal" is derived from the Old Norse word náhvalr, meaning "corpse man". This probably is a reference to the mottled white and grey colouring of the skin of the adult. It may also refer to the way a narwhal can lie belly up, motionless, for a few minutes at a time. The Narwhal is also commonly known as the moon whale.

The most conspicuous characteristic of male narwhals is their single extraordinarily long tusk, which is a tooth that projects from the left side of the upper jaw and forms a left-handed helix. The tusk can be up to 3 m (nearly 10 ft) long (compared with a body length of 4–5 m [13–16 ft]) and weigh up to 10 kg (22 lb). One in 500 males has two tusks, which occurs when the right tooth, normally small, also grows out. Rarely, a female narwhal may also produce a tusk.

The purpose of the tusk has been the subject of much debate. Early scientific theories supposed that the tusk was used to pierce the ice covering of the narwhal's Arctic Sea habitat. Others suggested the tusk was used in echolocation. More recently, scientists believed the tusk is primarily used for showmanship and for dominance: males with larger tusks are more likely to successfully attract a mate. This hypothesis was suggested by the activity of "tusking", in which two males rub their tusks together.

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