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Narwhals have extra long tusk-like teeth that grow out so long that they pierce through their lips and come out looking like a horn. The male narwhals have tusks that grow to be almost as long as the rest of the animal! In some rare cases, both the left and right front teeth grow out, so you end up with a narwhal with two horns.
"Narwhals seen along the floe edge in Arctic Bay, Canada. The whales are pushing under the ice to feed on cod. They come up in seal holes and rotten ice in order to catch a breath. This undated photo is part of an exhibit titled "Irreplaceable: Wildlife in a Warming World..." (Paul Nicklen/National Geographic Image Collection)