Narwhals usually have one horn apiece, but this fellow has bagged a double. (AFP photo)
People have generally assumed that narwhal horns were used for defense, but according to an article in Le Figaro, scientists still aren't in agreement:
A research group including specialists in cetaceans as well as dentists are exploring other assumptions. One of their favorites would be a sensory probe. In the support of this theory is the presence of 10 million nerve endings in the pulp of the gigantic tooth. The narwhal could thus detect the salinity of water, its temperature or its pressure, and also the presence of its favorite prey animals. [my translation]
See also Alex's 2005 post on the Super Sensing Tusk of the Unicorn Whale.