A narwhal's tusk is actually a tooth growing inside out and a vital sensory receptor, according to a Harvard researcher who has devoted her studies to revealing the secretes behind the unicorns of the sea.

"A lot of people really don't believe they are still alive," Martin Nweeia, a dentists and narwhal expert from Harvard University, told BBC.  

Narwhals (Monodon monoceros) are some of the rarest whales in the world and can be found living in ocean habitats throughout the Arctic. They are often difficult animals to study since they spend long Arctic winters tucked away under large ice sheets. Generally, only males grow oversized tusks from their upper jaw. However, females do occasionally grow them as well.

One misconception associated with these mysterious animals is that they use their horns to spear their prey, but this not the case, Nweeia told BBC. A narwhal's tusk can grow about two or three meters in length, and since they have no other appendages they couldn't free their prey if they used the spearing technique to hunt. Instead, narwhals prefer to slurp their food, mainly squid, fish and shrimp. 

Although their tusk does not help narwhals hunt, it does perform several vital functions. Nweeia and a team of researchers concluded that the animal's unicorn-like horn is actually a very unique and sensitive tooth – its only tooth – that is softer on the outside and harder and denser on the inside. Its main functions are sensing temperature, pressure, and hydration status. Researcher confirmed this sensory ability by examining significant changes in heart rate when the tusk was either exposed to high-salt or fresh water. This suggests their tusks act like antennas, searching through the water. 

"This tooth is almost like a piece of skin in the sense that it has all these sensory nerve endings," Nweeia told BBC.

Much still remains unknown about these elusive sea animals. Since most of the world's narwhals regularly pass through the Lancaster Sound in the Arctic, conservationists are already working to protect these critical habitats. This will ensure researchers are able to reveal the animal's secrets before it's too late.

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