An international research team has uncovered the internal architecture of the narwhal’s iconic tusk, revealing that the tooth is built from a double-helix structure.
X-rays Reveal Double-Helix Structure in Narwhal Tusks
Male narwhals possess what are considered some of the strangest canines in the animal kingdom. Their left upper canine grows during youth to become a tusk that can reach up to six feet long, while the right canine remains hidden in the skull. Unlike an elephant’s curved tusk, the narwhal tusk grows straight, maintaining its spiral architecture throughout an 80-year lifespan. To analyze how this structure forms, researchers at Aarhus University in Denmark convened an international team to examine specimens collected by Greenlandic Inuit subsistence hunters and exported under CITES permits, alongside six tusks housed at the Greenland Institute of Natural Resources.
Advanced Imaging and Synchrotron Technology
Mapping the structural complexity of a narwhal tooth at atomic, nano, and micro scales required exceptional resolution and power. To achieve this, the team combined multiple imaging techniques, including X-ray computed tomography, scanning X-ray diffraction, scanning small-angle X-ray scattering, tensor tomography, and birefringence microscopy.
According to Popsci, the researchers reserved time on three massive synchrotron particle accelerator X-ray sources: the MAX IV in Sweden, the Swiss Light Source in Switzerland, and the European Synchrotron Radiation Facility (ESRF) in France. The team also conducted standard morphological measurements and mechanical three-point bending tests on intact tusk material.
How Opposing Spirals Create Strength
The analysis showed that both the internal and external building blocks of the tusk comprise mineralized collagen fibers, which consist of collagen fibrils mineralized with nanoparticles of hydroxyapatite. These fibrils and nanoparticles orient themselves along the tusk’s longitudinal axis, creating a high degree of anisotropy at all scales alongside tiny systemic deviations at small angles that form the twisted structure.
On the outside, a thin layer of cementum forms a left-handed helix. On the inside, the dentine forms a right-handed helix. The flexible fibers and stiff mineral matrix enable the tusk to withstand strong forces—such as bending and twisting—without cracking. The opposing forces where the dentine meets the cementum create such great helical forces that if a tusk is sectioned lengthwise, the pieces torque an additional 180 degrees as the force is released.
The really special part is that these helices always have the same handedness—similar to if all [people] only had left hands,
Henrik Birkedal, a chemist and co-author of the study at Aarhus University in Denmark, told Popsci. This is, to the best of our knowledge, unique.
Context, Purpose, and Future Research
Despite centuries of fascination that helped inspire the mythical unicorn, debate continues regarding the exact purpose of the tusk. The prevailing theory is that the organ was honed by sexual selection to serve in courtship—functioning much like a peacock’s tail—because most, but not all, males possess one. While some scientists have suggested the tusk might help whales detect temperature, salinity, and chemical changes, or serve as a tool for fighting, foraging, or play, definitive evidence remains lacking.
Furthermore, researchers are looking at the tusks as chronological records. Because whales can live for up to 80 years, their teeth record changing environmental conditions throughout their lifetimes. And because the North Atlantic is currently undergoing very rapid changes, it is obvious to investigate whether we can trace these changes in the hard tissue of the narwhal tusk.
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