Narwhals Sport Iconic Straight, Spiraled Tusks. Scientists Just Uncovered Some of the Eye-Catching Feature's Structural Secrets

Narwhals are nicknamed the unicorns of the sea, and for good reason: The Arctic whales sport a spectacular “horn” that can reach up to ten feet long. While they can’t perform magic with this eye-catching feature—technically a tooth—researchers suspect the animals might wield their tusks to compete for mates, since mostly males have them, or use them as a sensory organ. Their purpose is far from the only mystery, though.

Narwhals are the only known creatures with a straight tusk, and one that visibly spirals to the left. In the new work, the researchers examined several narwhal tusks, some of which were collected by Greenlandic Inuit subsistence hunters and others from the Greenland Institute of Natural Resources. A few got sliced up and examined under powerful microscopes and using intense X-rays.

The team even put an entire narwhal skull, including the elongated tooth, in a C.T. scanner to create digital slices. Analyses revealed that the tusk’s building blocks—firm but flexible threads of the protein collagen—had different arrangements. The one on the tooth’s exterior, cementum, formed a left-handed helical layer.

But the one on the interior, dentine, twisted into a right-handed helical shape. These two “opposing spirals” help the tusk grow straight, Birkedal tells Scientific American’s Jackie Flynn Mogensen. What’s more, mechanical testing hinted that the different helices give the tusks strength, allowing them to withstand bending and twisting better than a single spiraled or nonspiraled rod.

Overall, the work suggests that the intricate boundary between the cementum and dentine seems even more complex than researchers previously thought. Fun fact: Narwhals might use their tusks to play Aerial video footage included in a study published last year shows narwhals prodding and flipping a fish in what looks like a play session. Martin Nweeia, a narwhal expert and dentist at Harvard University, was excited to see the new in-depth look at what he calls “the most extraordinary tooth on the planet.” “Naturally, when a curious dentist meets the most extraordinary tooth, there’s an irresistible urge to figure out what’s going on here,” Nweeia tells the Canadian Broadcasting Corporation News’ Bridget Stringer-Holden.

Still, there’s more research to be done. For instance, Birkedal and his colleagues also found that the opposing spiral structure can be seen in annual growth layers in the tusk, similar to growth rings in a tree. Her work has appeared in publications including BBC Travel, Discover magazine, Live Science, Atlas Obscura and Hidden Compass.