Researchers using advanced X-ray scans have discovered that narwhal tusks owe their straight growth and rugged strength to two opposing helical spirals locked together inside the tooth. The finding solves a biological puzzle inspired by unicorn myths.
For centuries, the protruding spiral tooth of the male narwhal fueled European myths about unicorns. Danish King Frederik III even commissioned an entire coronation throne crafted from the twisted white tusks, which was completed in 1671. Long before inspiring royal furniture, however, narwhals used their outsized teeth for survival. As reported by Earth.com, the animals rely on the appendage to stun small fish before eating them, while drone footage shows them swatting and playing with the tusk the way another animal might use a paw.
That iconic tooth is actually the left canine, which erupts through the male narwhal’s upper lip and can grow longer than 6 feet (2 meters). The tusk continuously corkscrews to the left for a lifespan that can reach 80 years or more. While researchers have long studied the macroscopic spiral, splitting a tusk lengthwise revealed that the two halves spring apart on their own, untwisting by more than half a turn. Something locked inside the tooth held considerable strain, though until now, nobody knew what caused it.
Synchrotron X-Ray Scans Reveal Intertwined Spirals Inside Dentine and Cementum
To uncover the source of that tension, an international team led by Aarhus University chemist Henrik Birkedal combined multiple imaging techniques. Working with colleagues in Sweden and France, the researchers aimed powerful X-rays at thin tusk slices at synchrotron facilities in Switzerland and Sweden, alongside the European Synchrotron Radiation Facility in France. The beams mapped internal structures just a few hundred atoms across.
Initial results brought immediate confusion. According to Chemistry World, the international team combined x-ray diffraction, small-angle x-ray scattering, and birefringence microscopy to map the tusk across several scales. When the X-rays hit the slices, the individual points produced an unexpected picture.
“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.”
Marianne Liebi, physicist at the Paul Scherrer Institute
The breakthrough came when the team stopped viewing each measurement point in isolation and instead compared neighboring points. Adrian Rodriguez-Palomo, a doctoral student at Chalmers University of Technology who later continued the work as a postdoctoral researcher at Aarhus University, described the turning point in the analysis.
By connecting those directional points like a children’s paint-by-numbers kit, the researchers uncovered two intertwined spirals running in opposite directions, as detailed by Earth.com. On the surface, a thin outer layer of cementum forms a left-handed spiral. Beneath that cap, the main bulk of the tusk consists of dentine—the same material found beneath human tooth enamel—where mineralized collagen threads spiral in the opposite direction to the right.
Opposing Helical Arrangements Explain Straight Growth and Mechanical Strength
The discovery explains both how the tusk grows straight despite its corkscrew shape and how it survives the physical demands of an Arctic existence. The opposing helices in the outer cementum and inner dentine meet and lock together where the two layers touch. Researchers believe these counter-twisting forces cancel out twisting forces during early growth stages, allowing the tusk to grow straight.
That dual-spiral architecture also provides physical reinforcement. Like bone or other biological composites, the tusk bridges nanoscale building blocks with macroscopic form to handle opposing demands.
“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. The narwhal tusk, too, must be both hard and flexible. Only then can it withstand the strong hydrodynamic forces that swimming generates.”
Marianne Liebi, physicist at the Paul Scherrer Institute
Mechanical testing confirmed that toughness. When researchers bent rod-shaped samples cut from the tusk in multiple directions, rods cut along the tusk’s length resisted bending about 63 percent more than those cut across it. Two male narwhal tusks collected by Greenlandic Inuit subsistence hunters supplied the material for the analysis.
Future Biomimetic Applications and Arctic Pressures
The microscopic architectural motif could find applications beyond marine biology. Researchers suggest the opposing helical structure could serve as inspiration for designing bioinspired composite materials with engineered mechanical properties.
Yet further study is required to understand precisely how the tusk is formed during a narwhal’s development. That research unfolds as the species faces broader environmental pressures in the wild. As documented by Earth.com, narwhals are already forced to shift their migration patterns as Arctic sea ice forms later each year, adding new urgency to understanding the biology of one of the ocean’s most distinctive mammals.
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