Voyager: What is a narwhal's tusk used for?

Adult narwhals have two sets of teeth, but their teeth don't develop inside of the mouth as in humans. They remain permanently small and hidden under the skin. However, in male members of this near-threatened species, one of these teeth usually becomes elongated and ultimately sticks right out through the bones and tissue of the face. These tusks can be up to 2.67 meters (9 feet) long and are the main physical feature that sets narwhals apart from other whales. Very rarely, females can also develop a tusk and males can sometimes grow two of them.

Why do they have these tusks? 

There is one new theory that the tusks might play a sensory role but the most likely answer so far is that they play a role for male narwhals in establishing a social order. One common interaction that is observed at breeding grounds is two male narwhals crossing their tusks. Sometimes tusks are broken during these clashes. It’s not known if losing tusks diminishes narwahls’ social stature, but individuals that don't have tusks seem to live otherwise healthy lives, so it doesn't seem that the tusks are used for feeding. 

– Joshua Jones, staff research associate, Marine Physical Laboratory