In the icy waters of the Arctic Ocean, a group of narwhals swims, with the long horns protruding from their foreheads shining in the sunlight. This 3-meter-long “horn” is actually a specialized tooth, not only a symbol of male narwhals but also a precise “ocean sensor” capable of detecting seawater temperature, salinity, and prey location.

The narwhal’s tusk is one of the most peculiar organs in nature. It is formed by the specialization of one tooth in the upper left jaw, growing in a spiral shape and penetrating the entire snout. The surface of the tusk is covered with a layer of porous cementum, and there are countless tiny dentinal tubules inside, extending from the tooth surface to the pulp. When seawater enters the pores of the cementum, it is transmitted to the nerve endings in the pulp through the dentinal tubules, and then to the brain through the maxillary branch of the fifth cranial nerve.

Experiments have proven that the narwhal’s tusk has sensitive perception abilities. When researchers alternately exposed the tusk surface to high-salt solution and fresh water, the narwhal’s heart rate changed significantly, indicating that they can perceive subtle differences in seawater salinity. This ability is particularly important when searching for food—narwhals mainly feed on deep-sea squid and fish, and they can use their tusks to detect seawater disturbances caused by prey activities and temperature changes in different water layers, thereby accurately locating prey.

The narwhal’s tusk also has social functions. Male narwhals engage in “fencing” behavior with their tusks—colliding tusks with each other without causing harm, which may be related to establishing social status. In addition, stable isotope analysis shows that there are differences in the foraging areas of male and female narwhals, indicating that the tusk may also affect their niche differentiation. This multi-functional specialized tooth is a key adaptive feature for narwhals to survive in the extreme environment of the Arctic Ocean.