Narwhals are sometimes referred to as the unicorns of the sea because of the long, pointed horns that extend from their heads. Male narwhals' tusks can grow up to nine feet long, and as Nadia Drake writes at Wired they are actually modified teeth that protrude out from the corner of their mouth, rather than forehead-centered horns.
Scientists do not know what purpose the narwhal's tusk serves, exactly. They've speculated that it might be used for skewering enemy animals or for breaking through the icy Arctic waters where the animals live. One team hypothesized that the tusk serves as a sort of sensory organ, Wired describes, and recently decided to investigate that idea.*
To put their hunch to the test, the researchers devised a "tusk jacket," Drake writes—a sort of plastic hoodie that fit comfortbly over the narwhals' tusks but excluded the outside environment. The team changed the concentration of salt in the water that filled the tusk jacket, which acts as a proxy for temperature (more ice equals colder water with more salt, while less ice means warmer water with less salt). Wired:
He found that narwhal heart rates rose in response to high salt concentrations, presumably because these concentrations normally suggest that the sea is freezing and entrapment is possible. The animals’ heart rates dropped when the tusks were washed with fresh water, suggesting they could detect this change.
The team only tested the tusks for a response to salt but thinsk the whales might also use their tusks for seeking out prey or finding mates. Why, what would you do with an extra long, sensitive tooth?
*Update: This research was funded partially by Smithsonian Institution and was led by Martin Nweeia, a member of the Smithsonian's Department of Vertebrate Zoology.