Narwhal (Monodon monoceros)
family Monodontidae) of the Arctic, found in groups of 15–20 along coasts and sometimes in rivers. Narwhals are mottled gray, attain a length of 11.5–16 ft (3.5–5 m), and have no dorsal fin. They have only two teeth, at the upper jaw tip. The male's left tooth is a straight, protruding tusk, up to 8.9 ft (2.7 m) long, that is grooved on the surface in a left-handed spiral. It was prized in medieval times as the unicorn horn. The tusk is thought to have evolved as a sexual display. Narwhals eat fishes, cephalopods, and crustaceans. They are hunted by humans for their tusks and meat.
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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.