Narwhals are the Unicorns of the sea - and now, researchers know why the whales sport that distinctive horn.

Narwhals are toothed whales, and the horn of the male can grow to be as long as eight-and-a-half feet long. The unusual feature is the left canine tooth of the males, which breaks through the upper lip of the male. It is sometimes used in contests between males to win mates, but it does not primarily serve a defensive purpose.

Although biologists knew this much about the horn of narwhals, theories of how the animals used the body part differed greatly. Some biologists believed the horns are used for ice picks, while others said the horns picked up noises deep under water. Researchers discovered the horn is, in fact, used as a sensory organ.

Unlike most mammal teeth, the horn of a narwhal is not protected by enamel. Sea water enters the horn through cementum channels, which are also present in human teeth. The liquid then travels through a network of tubules to the center of the tooth, heading base of the tusk. There, the water excites nerve endings in the pulp of the tooth, sending signals to the brain of the animal.

This makes the horn sensitive to temperatures and the animal can "taste" the concentrations of different chemicals in the water. Biologists believe the males may use the horn to find food, as well as females who may be ready to mate.

"The combined evidence suggests multiple tusk functions may have driven the tooth organ system's evolutionary development and persistence," researchers wrote in the study.

Martin Nweeia from the Harvard School of Dental Medicine led the study.

To complete this study, biologists captured wild narwhals, and brought the elusive animals near shore, where they could be studied. They found the Narwhal tooth is extremely sensitive to changes in the surrounding environment.

When salt levels around the animals were changed, it affected the heart rate of the marine animals.

"This is the first time that someone has discovered sensory function and has the science to show it," Nweeia told the BBC.

As part of a follow-up investigation, Nweeia plans on asking fishermen around the Arctic for stories and information about the mysterious creatures.

Details of the new study were published in the journal The Anatomical Record.

The next time you have a toothache, just be thankful you are not a narwhal.

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