The nature of the narwhal: 'The one that is good at curving itself to the sky' | Helen Sullivan

The whole thing that is great about the teeth of the narwhal is that nothing makes sense'

• The Nature of ... is a column dedicated to interesting animals, insects, plants and natural phenomena

There are some animals about which it is easy to forget the fact that they have teeth, so that every time they flash their grins, it is as though you are seeing a new animal. Dogs, cats of all kinds, sharks and crocodiles are not among these. Horses, rabbits, fish and geese live toothless in my mind.

Narwhals are among the actually toothless, if you discount their tusks. Inside their mouths, which are shaped in a permanently sweet smile, there are no teeth as we understand teeth to be.

But the males have a long, unicorn-like projection protruding - just off centre - from what might be described as their upper lip. I find them quite festive, like ornaments that should be hung on a Christmas tree. Maybe it's their wintry, icicle-like tusk. I try to forget that this tusk is a tooth.