3D X-ray imaging unlocks the secrets of the narwhal's spiral tusk

In the Middle Ages, narwhal tusks were highly valued as magical unicorn horns, treasured by monarchs and nobles. Researchers have now revealed the internal structure of these spiral-shaped tusks using advanced three-dimensional X-ray imaging. Contrary to popular belief, the tusk's exterior, made of cementum, has a left-handed spiral while its interior, composed of dentine, exhibits a right-handed spiral.

This arrangement may help the tusk grow straight, unlike other elongated canine tusks of animals like elephants and walruses. Materials chemist Henrik Birkedal of Aarhus University proposed that this double-spiral structure offers superior stability, mimicking a biological counterbalance for bending and twisting resistance. While the tusks' exact function remains unclear, they are believed to play a role in social hierarchy among males and may have sensory uses.

Narwhals inhabit the frigid Arctic waters, primarily near Canada, Greenland, Norway, and Russia, feeding on fish, squid, and shrimp. Their age is determined by their coloration, which changes from blue-gray to blue-black in juveniles, mottled gray in adults, and nearly all-white in old age. The tusk's unique helical structure arises from mineralized collagen fibers reinforced with calcium-mineral nanoparticles, providing exceptional strength and flexibility.