Known as "the unicorns of the sea," narwhals are unique for the solitary tusk that protrudes through the tops of their heads. The horn is actually a canine front tooth that can reach as long as nine feet. But until recently scientists weren't sure what, if any, purpose it had.

Now, new research suggests that the tusk is used as a sensory organ, helping the narwhal pick up changes in its environment. Researchers say males of the species may use the horns to look for food or find mates. The results of the study were published in the journal The Anatomical Record.

"People have said it's everything from an ice pick to an acoustic probe, but this is the first time that someone has discovered sensory function and has the science to show it," the study's lead author, Dr. Martin Nweeia from the Harvard School of Dental Medicine, told the BBC.

A team of international investigators worked together to understand the function of the narwhal's unusual protuberance. To do so, they captured several of the elusive animals and anchored them using a net anchored perpendicular to shore.

The researchers found that the outer cementum layer of the tusk is porous, the inner dentin layer has microscopic tubes that channel toward the middle, and the pulp in the center has nerve endings that connect to the animal's brain. The structure makes the tusk sensitive to temperature and chemical differences in the environment.

When the tusk was exposed to different levels of salt in the surrounding water, for example, the researchers noticed a change in the narwhal's heart rate.

The animals can basically "taste" the concentrations of chemicals in the water. Because of that, researchers believe males may use the tusk to find food. They also appear to be able to find females that are ready to mate.

Nweeia told the BBC that he's fascinated that narwhals put all their energy into growing a single tusk rather than having a set of teeth to help them eat their diet of large fish.

"If you were looking for an ideal and fascinating tooth to study, there's no question this would be it."

Mary Jo DiLonardo Mary Jo writes about everything from health to parenting — and anything that helps explain why her dog does what he does.