This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: The narwhal's impressive tusk has long been the subject of myths. In the Middle Ages, they were sold as unicorn horns believed to possess magical powers.
Now, an international team of researchers led by Aarhus University has solved the mystery of how the tusk acquires its twisted structure. The narwhal's long, white, spiraled tusk is not magical—but it is nevertheless somewhat mysterious. For example, scientists are still not certain what the narwhal uses it for.
However, a research team has now used advanced 3D X-ray techniques to determine how the tusk obtains its characteristic structure. It turns out that the tooth does not contain just one spiral, but two. On the outside, the structure twists to the left, while the interior structure twists in the opposite direction, forming a kind of biological counterbalance, where two opposing forces meet at the interface between interior and exterior parts.
The discovery has been published in Nature Communications. Researchers have long known that the left-handed shape must reflect a particular organization of the mineralized collagen fibrils—the microscopic building blocks that give the tooth its strength—but exactly how this organization works has been impossible to map in three dimensions. To see how the building blocks are oriented inside the tooth, the researchers combined several X-ray imaging techniques, particularly a special 3D X-ray technique called tensor tomography.
The technique works by sending powerful X-rays through the tooth and analyzing how they scatter from the nanoscale mineralized collagen fibrils. The narwhal's tusk is in fact the left canine tooth, which grows through the jaw and lip and can reach more than 2 meters (6.6 feet) in length. Unlike human teeth, it has no enamel but consists of dentin on the inside and cementum on the outside.
Because the narwhal tusk is both large and structurally extremely complex, the researchers had to deploy the biggest technological tools available. They combined the capabilities of three enormous synchrotrons—particle accelerator X-ray sources—MAX IV in Sweden, Swiss Light Source in Switzerland and European Synchrotron Radiation Facility (ESRF) in France—to obtain enough resolution and power to map the entire interior of the tooth in three dimensions at the atomic, nano and micro scale. The analyses revealed a fascinating pattern: While the building blocks are primarily oriented along the tooth's longitudinal axis, they systematically deviate at small angles, creating a twisted structure.
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