The tusk is a tooth, and it can feel

The narwhal's tusk is not a horn. It is a canine tooth, grown through the upper lip, spiralling always to the left, up to three metres long.

For centuries it was sold in Europe as unicorn horn, at many times the price of gold. Elizabeth I is said to have kept one valued at the price of a castle. Nobody in that trade had ever seen the animal.

We now think the tusk is a sensory organ — millions of nerve endings reading salinity and temperature through the water. The thing people mistook for a weapon turned out to be an instrument for paying attention.