Over at Querencia I learn from Reid Farmer that the use of the narwhal's horn has finally been discovered.

But a team of scientists from Harvard and the National Institute of Standards and Technology has now made a startling discovery: the tusk, it turns out, forms a sensory organ of exceptional size and sensitivity, making the living appendage one of the planet's most remarkable, and one that in some ways outdoes its own mythology.

The find came when the team turned an electron microscope on the tusk's material and found new subtleties of dental anatomy. The close-ups showed that 10 million nerve endings tunnel from the tusk's core toward its outer surface, communicating with the outside world. The scientists say the nerves can detect subtle changes of temperature, pressure, particle gradients and probably much else, giving the animal unique insights.

No virgins were harmed in the process of this investigation, only some odd theories. My personal favorite was that it produced a piezo-electrical charge (PDF):

I think the tusk is a sensor of some kind. It could be used for detecting sound, temperature or salinity. I also think there may be a voltage potential across the tooth. Most bones and teeth have what is known as a piezo effect: they contain crystals that generate a voltage when a mechanical force is applied to them. So when a twisted crystal like the narwhal's tooth is moving with a tremendous force through water, there's probably some kind of voltage across it.