"Males may rub them together for as-yet unknown sensations"

I'm not into whale-blogging, but with a quote like that, the narwhal has become today's subject.

Here's the story (from LiveScience):

For hundreds of years the purpose of the tusk on the narwhal, or "unicorn" whale, has stumped scientists and Inuit elders alike. It is an evolutionary mystery that defies many of the known principles of mammalian teeth.
A new study suggests the whales use their tusks to determine the salinity of water and search for food.

So it looks like if a mammal wants to develop an antenna, a good bet is to modify a tooth.

After four trips to the Canadian High Arctic to study these whales, Martin Nweeia of the Harvard School of Dental Medicine has discovered that the narwhal's tooth, while seemingly rigid and hard, has remarkable sensor capabilities. With ten million tiny nerve connections tunneling from the central nerve of the tusk to its outer surface, the thing is like a membrane with an extremely sensitive surface and can detect changes in water temperature, pressure, and particle gradients.
Because these whales can detect particle gradients in water, they can discern the salinity of the water, which could help them survive in their Arctic ice environment. They can also detect water particles that hint at the fish that make up their diet.
There is no known comparison in nature and certainly none more unique in tooth form, expression, and functional adaptation.
"Why would a tusk break the rules of normal development by expressing millions of sensory pathways that connect its nervous system to the frigid arctic environment?" Nweeia said. "Such a finding is startling and indeed surprised all of us who discovered it."

Oh, and about that headline:

The findings point to a new direction of scientific investigation. The tusk is also sensitive to touch, and narwhals are known for their "tusking" behavior, when males rub tusks with each other. Because of the tactile sensory ability of the tusk surface, the whales are likely experiencing a unique sensation.

This is exactly why I'm not a marine biologist.