The narwhal is a small, rare arctic whale with a very long (~7-10 ft, or 2-3 meters) tooth that resembles the horn of the fabled unicorn. It has been used to adorn palaces and royal scepters around the world, from England to Japan. The ground tooth of a narwhal has (falsely) been said to cure various diseases. But until recently, the function of this tooth has been a mystery -- the means of its evolution defies normal models of the way mammalian teeth develop.
In 2005, Harvard School of Dental Medicine researcher Martin Nweeia determined the function of the tooth. It acts as a sophisticated hydrodynamic sensing device, capable of measuring temperature, water particle density, salinity, and other information. Although one would imagine a tooth that looks like a horn to be rigid, it has a delicate, membrane-covered surface saturated with millions of sensory nerves. These neural networks feed directly into the central nervous system of the narwhal, giving it a uniquely powerful sensory apparatus for survival in the arctic environment.
The tooth of the narwhal is unique among mammals, which partially explains why it has taken science so long to figure it out. The spiral tusk morphology is unique to narwhals, for example. Also, the tusk is common among males but rare among females, an unusual asymmetry for mammalian teeth.
Before the purpose of the tusk was decisively determined, a number of theories were advanced to explain its purpose. These included a tube for breathing, a heat sink, a swimming rudder, a display for mating, and even a tool to break ice. Because the animal is so rare and lives in such cold areas, there has been a lack of samples with which to make theories. A collaborative effort between Nweeia and the Inuit people of Canada led to a large catalog of behavior of the animals, helping the final determination of the tusk's function.