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Dolphins and killer whales are descended from four-footed land-living air-breathing carnivorous mammals. In the sea they lost their limbs but not their taste for meat, or their teeth. Indeed, living in the ice-strewn waters of the Arctic is the narwhal, a member of the family that has grown one of its teeth to an extraordinary length. However, only the male narwhal grows the great tusk that can reach nine feet long. The ones with the smaller tusks are females. No one knows exactly what purpose the tusk serves but it may be used in courtship since males have, though very rarely, been filmed fencing with each other, their tusks poking out of the water.