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| Narwhals – often called the “unicorns of the sea” – have fascinated people for centuries with their long tusks. Now an international research team has used X-ray light to reveal the internal structure of this unique tooth for the first time, from nanometer to centimeter scale. Photo Credit: © Carsten Eqevanq, Greenland Institute of Natural Resources, North West Greenland (2021) |
Scientific Frontline: Extended "At a Glance" Summary: Narwhal Tusk Nanostructure
The Core Concept: The narwhal tusk, an elongated tooth reaching up to two meters, possesses a highly complex internal structure characterized by two opposing microscopic helices that provide exceptional structural integrity.
Key Distinction/Mechanism: Unlike typical curved teeth in other mammals, the narwhal tusk grows in a counterclockwise spiral. Recent tensor tomography reveals that at the nanoscale, the tusk is constructed of two interlocked spirals: a left-handed helix in the outer cementum layer and a right-handed helix within the inner dentine layer.
Origin/History: The nanostructural details of the opposing helices were discovered and published by an international research team in August 2026, utilizing data from three European synchrotron facilities (including the Swiss Light Source).
Major Frameworks/Components:
- Dentine Core: The primary internal structure, featuring mineralized collagen fibers arranged in a right-handed spiral.
- Cementum Layer: The external layer, normally confined to tooth roots, which in the narwhal tusk forms a left-handed spiral.
- Nanoscale Building Blocks: The interplay of collagen fibers (tensile strength) and mineral crystals (hardness), functioning similarly to reinforced concrete.
- Tensor Tomography: The advanced X-ray scattering technique used to map the spatial orientation of these nanoscale components into a macroscopic 3D model.
Branch of Science: Marine Biology, Zoology, Biomechanics, Materials Science, Physics (synchrotron radiation analysis).
Future Application: Understanding this biological architecture could inform the development of novel, highly durable, and stress-resistant synthetic composite materials, mimicking the interlocking helix design. Additionally, the tusk's growth rings provide a historical record of Arctic climate and ecological shifts.
Why It Matters: The discovery resolves a longstanding biomechanical mystery regarding how the lengthy tusk withstands intense hydrodynamic forces without breaking, highlighting a sophisticated evolutionary adaptation in biological composite materials.
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| The individual pixels in the tomogram do not yet reveal any hint of a helix (left image). Only when their spatial orientations are connected do the two spirals become visible (right image). Image Credit: © Adrian Rodriguez-Palomo et al. |
Up to 2 meters long and twisted into a perfect spiral, the narwhal (Monodon monoceros) tusk is one of the most unusual structures in the animal kingdom. An international research team, including scientists from the Paul Scherrer Institute (PSI), has deciphered for the first time how this helix is organized inside the tooth—with a surprising twist. At the nanometer scale, the building blocks of the spiral rotate not just one way, but in two opposite directions.
King Frederik III knew the true origin of unicorn horns, and in the seventeenth century, he sent an expedition to Greenland to probe the reality behind the precious artifact. That is because the mythical creature purportedly lives there, in Arctic waters, and has fins rather than hooves. The expedition was successful and bestowed on the Danish ruler a throne made entirely of "unicorn horns"—and with that, prestige and power in Europe. This was despite the fact that the legendary horn is not a horn at all, but merely a tooth.
This tooth, however, is unique in the animal kingdom. While other long teeth—such as the tusks of elephants and walruses, or the incisors of beavers—simply follow a curved shape, the male narwhal’s tooth grows forward in a screw-shaped form. Usually, it is the left upper canine that twists in an elegant counterclockwise spiral, piercing the upper lip and reaching lengths of up to 2 meters—sometimes up to half the animal’s body length.
It was precisely this unusual growth that aroused the curiosity of an international and interdisciplinary research team. Using state-of-the-art X-ray methods, the researchers sought to determine whether the direction of rotation of the visible spiral is determined by the arrangement of its nanometer-sized building blocks.
The results revealed an unexpected twist: the building blocks form two opposing helices—one clockwise and one counterclockwise—which interlock to form an extremely stable structure.
The investigations were carried out at three synchrotron facilities in Europe: the Swiss Light Source (SLS) at the Paul Scherrer Institute in Villigen, Switzerland; the MAX IV Laboratory in Lund, Sweden; and the European Synchrotron Radiation Facility (ESRF) in Grenoble, France.
A tooth tells a story: the growth structures of narwhal tusks not only reveal how they are constructed but also provide insights into the environmental and climate history of the Arctic.
Video Credit: © Lars Øivind Knutsen et al. / NordForsk project "Narwhal tusks—a tale with a twist."
Connecting the Dots to Reveal the Spirals
Most people are familiar with X-rays from visits to the dentist: they make teeth visible because teeth absorb the radiation more strongly than the surrounding tissue. What one sees in the image is essentially the shadow of the tooth.
Using synchrotron radiation from three large European research facilities, researchers are able to discern much more. When these special X-rays encounter regularly arranged structures within a material, they scatter and produce characteristic patterns. "From these patterns, we can calculate how the nanometer-sized, mineralized collagen fibers inside the material are oriented," explains first author Adrian Rodriguez-Palomo. He joined the project as a doctoral student at Chalmers University of Technology in Sweden and later continued the work as a postdoctoral researcher at Aarhus University in Denmark.
In tensor tomography, the tooth is rotated in the synchrotron beam and scanned point by point. From millions of measurements, a three-dimensional image of its internal architecture is reconstructed—from the tiny building blocks at the nanometer scale to the macroscopic form of the entire tooth.
However, when the team began analyzing the vast amounts of data, they received a surprise: in the images, they saw nothing. "That puzzled us quite a bit," recalls researcher Marianne Liebi. Mechanical tests had suggested that the spiral shape should already be reflected in the collagen fibers and the mineral crystals. "But instead of a helix, all we saw was a regular pattern."
The solution emerged only when the team took a step back. Instead of focusing on individual tiny pixels in the three-dimensional dataset, they began analyzing their spatial orientation. "When we compared how these points were oriented relative to each other, a directional trend began to appear," recalls Rodriguez-Palomo. "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."
From the Nanoscale to the Macroscopic Helix
A closer look at the data also revealed where these two spirals are hidden within the tooth. The outer layer of the tooth, known as cementum, forms a left-handed helix, while inside the tooth, the mineralized collagen structures in the dentin follow a spiral pattern in the opposite direction.
This counter-rotating architecture is no accident, but rather a remarkably refined construct of nature. The two interlocking helices give the tusk exceptional stability, much like the twisted fibers in a rope. They make it particularly resistant to torsion and mechanical stress. They also give the tooth its distinctive spiral form—ultimately lending an unexpected twist to an ancient myth.
The tusk’s growth structures—similar to the annual rings of a tree—allow researchers to draw conclusions about past environmental conditions in the Arctic, such as climate and food availability. Over its nearly 100-year lifespan, each male narwhal thus records its own environmental history in its tusk.
Funding: European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie program.
Published in journal: Nature Communications
Title: The narwhal tusk assembles its macroscopic helix from building blocks with opposing twists
Authors: Adrian Rodriguez-Palomo, Peter Alling Strange Vibe, Dimitra Athanasiadou, Jonas Palle, Leonard C. Nielsen, Jonathan T. Avaro, Thorbjørn Erik Køppen Christensen, Nina Kølln Wittig, Mads Ry Vogel Jørgensen, Innokenty Kantor, Manfred Burghammer, Jiliang Liu, Stefano Checchia, Christian Appel, Karl Anker Jørgensen, Eva Garde, Mads Peter Heide-Jørgensen, Marianne Liebi, and Henrik Birkedal
Source/Credit: Paul Scherrer Institute | Benjamin A. Senn
Edited by: Scientific Frontline
Reference Number: mb081926_01
