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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.
The narwhal's tooth has hydrodynamic sensor capabilities. Ten million tiny nerve connections tunnel their way from the central nerve of the narwhal tusk to its outer surface. Though seemingly rigid and hard, the tusk is like a membrane with an extremely sensitive surface, capable of detecting changes in water temperature, pressure, and particle gradients.