Researchers using high-tech X-rays have discovered that the narwhal’s iconic tusk features a rare double-spiral internal structure.
For centuries, the elongated, corkscrew-shaped tooth protruding from the front of an Arctic whale fueled maritime folklore across Europe. Sold by traders as unicorn horns during the Middle Ages, these artifacts fetched prices far exceeding their weight in gold as monarchs and nobles prized them for supposed magical properties like neutralizing poisons. Science eventually corrected the myth, identifying the horn as a specialized canine tooth erupting through the jaw and lip, yet the physical mechanics behind its signature twist remained an enduring puzzle.
An international research team has finally unlocked that secret. By deploying advanced imaging tools, scientists mapped the internal architecture of narwhal tusks down to the atomic scale, uncovering an intricate internal design that nature uses to engineer extreme durability.
Mapping the Microstructure With Particle Accelerators
Examining specimens collected by Greenlandic Inuit subsistence hunters and preserved by the Greenland Institute of Natural Resources, the researchers faced a formidable imaging challenge. The tusks are dense, structurally complex, and can reach up to ten feet long. To see inside them without destroying the historical material, the team reserved time on three massive synchrotron particle accelerator X-ray sources: the MAX IV facility in Sweden, the Swiss Light Source in Switzerland, and the European Synchrotron Radiation Facility in France.
The scientists combined traditional computed tomography scans with tensor tomography and small-angle X-ray scattering. These high-resolution methods tracked how powerful X-rays scattered off nanoscale building blocks inside the tooth. The results mapped the orientation of mineralized collagen fibrils—the protein threads reinforced by calcium-mineral nanoparticles that give bones, teeth, and skin their structural integrity.
While human teeth contain a hard exterior layer of enamel, narwhal tusks lack enamel entirely. Instead, they feature an inner core of dentine and an outer layer of cementum. The advanced scan revealed that the microscopic collagen building blocks do not just form a single, uniform corkscrew. They form two distinct helices moving in opposite directions.
How the Double Spiral Forces the Tusk to Grow Straight
The analysis showed that the outer cementum layer forms a left-handed spiral, while the inner dentine twists into a right-handed spiral. These opposing structural patterns meet at the intricate boundary layer between the two tissues, creating what lead investigators termed a biological counterbalance.
“We propose that this arrangement helps the tusk grow straight, unlike other tusks — elephant or walrus, for example — that curve.”
Henrik Birkedal, materials chemist at Aarhus University
In nature, spiral components often twist in a single direction, which typically causes growing structures to drift or curl over time. By growing from the interface between two opposing force vectors, the narwhal tusk remains remarkably straight.
Mechanical testing confirmed that this dual-helix architecture produces superior stability against bending and twisting compared to a standard straight rod or a single-spiral structure.
Unresolved Mysteries of the Arctic Unicorn’s Tooth
While the physical mechanics of the tusk are now clear, its exact biological purpose remains a subject of ongoing debate among marine biologists. Typically found almost exclusively in males—though rare females grow them and some males do not—the tusk is widely viewed as a sexual signal used in male-male contests to establish social hierarchy.
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Other theories suggest the appendage may serve as a sensory organ or play tool. However, field observations by marine biologists in Greenland monitoring narwhal behavior have found no evidence supporting water-property sensing theories regarding temperature or salinity.
