Scientists Finally Explain the Physics Behind the Original Unicorn Myth
A male narwhal grows one tooth into a corkscrew that pushes straight through its upper lip and keeps twisting in the same direction for the rest of its life, sometimes 80 years or more. Split that tooth down the middle and the two halves spring apart on their own, unwinding by more than half a turn. Something inside the tusk has been holding real structural tension the entire time, and until now, nobody knew exactly what.
A team led by Henrik Birkedal, a chemist at Aarhus University in Denmark, set out to find it, working with colleagues in Sweden and France. Using powerful X-rays capable of mapping structures only a few hundred atoms across, the researchers scanned thin slices of narwhal tusk at synchrotron facilities in Switzerland, Sweden and France, building a three-dimensional picture of exactly how the tooth’s internal fibers were arranged.
The tusk itself is the source of centuries of folklore. It is not a horn at all but a left canine tooth, erupting through a male narwhal’s upper lip and growing past six feet in length while corkscrewing steadily to the left. That slow, continuous spiral is unlike anything else in nature, and in the 1600s, twisted white tusks arriving at European trading ports convinced buyers they had acquired genuine unicorn horns. Denmark’s King Frederik III believed the story firmly enough to commission an entire coronation throne built from them, completed in 1671. Narwhals themselves use the tusks for more practical purposes, stunning small fish before feeding on them, and drone footage has captured narwhals swatting and handling their tusks the way another animal might use a paw.
The first scans produced results the team had not anticipated. Researchers expected to find a single helix running through the tooth’s collagen and mineral fibers, the same basic building blocks that make up bone. Instead, the initial data looked strangely uniform. “That puzzled us quite a bit,” said Marianne Liebi, a physicist at the Paul Scherrer Institute in Switzerland and a co-author of the study. “But instead of a helix, all we saw was a regular pattern.”
The breakthrough came from comparing each measurement point against its neighbors rather than examining them individually. “When we compared how these points were oriented relative to each other, a directional trend began to appear,” said Adrian Rodriguez-Palomo, the study’s first author, who began the work as a doctoral student at Chalmers University of Technology before continuing it at Aarhus. “All we had to do then was connect the dots, as in a children’s paint-by-numbers kit, and suddenly the structure became visible: two intertwined spirals.”
Those two spirals run in opposite directions. An outer layer called cementum, the material that typically caps a tooth’s root, forms a left-handed spiral visible on the tusk’s surface. Beneath it, within the tooth’s dentine, the same collagen-and-mineral fibers spiral the opposite way, to the right. The two layers meet and interlock exactly where cementum and dentine touch, a structure researchers say has never previously been documented in collagen, despite the tissue’s ubiquity across the animal kingdom.
The design is not just unusual. It appears to be functional. “Like bones or other teeth in the animal kingdom, the narwhal tusk is a complex composite material whose structure extends from tiny nanoscale building blocks to the visible form of the entire tooth,” Liebi said. “The narwhal tusk, too, must be both hard and flexible. Only then can it withstand the strong hydrodynamic forces that swimming generates.” When researchers bent rod-shaped samples cut from the tusk in different directions, pieces cut along its length resisted bending about 63% more than pieces cut across it, and failed far more gradually. Under a microscope, the crosswise samples snapped cleanly, similar to a shattering ceramic plate. The lengthwise samples instead tore apart slowly, their fibers pulling loose and redirecting the crack sideways as it spread, a failure pattern consistent with years of repeated drag stress from diving.
Open questions remain. The study examined tusks from only two adult male narwhals, both from a Greenland population with unusually low genetic diversity for its size, leaving open whether every narwhal tusk forms its double spiral the same way, and whether rare double-tusked individuals follow the same pattern. Researchers suspect the answer to how a single growing tooth ends up spiraling in opposite directions on its inside and outside lies at the boundary where the two tissue types meet, though confirming that would require observing tusk growth from the very beginning in a wild whale that can live eight decades, something no one has yet managed.
In the meantime, the design itself has practical value well beyond narwhal biology. A natural structure that resists twisting while remaining both strong and flexible is, as the researchers note, a tested blueprint, refined over millions of years, for anyone designing drill shafts or suspension cables meant to withstand the same kind of stress. The study was published in Nature Communications.
