An international team of researchers has solved the mystery of how narwhal tusks get their twisted structure. The findings have been published in the journal Nature Communications. Narwhals are often called the “unicorns of the seas”, and not without reason. In the Middle Ages and Renaissance, the narwhal tusk was often sold as a unicorn horn. The mythical unicorn was so revered that the Danish-Norwegian king Frederick III commissioned a coronation chair made entirely of narwhal tusks as a sign of his power and social status. Despite extensive research, we still don’t know exactly what the narwhal uses its tusk for. Scientists have many theories: sexual signals, hunting, internal fighting or games. But for now scientists have solved another mystery: how exactly does the tusk get its characteristic twisted structure? A close-up of the head of a narwhal showing the tusk protruding from the upper jaw. Credit: Mads Peter Heide-Jørgensen The structure Narwhals are not the only animals that sport this long, pointed tooth, but it is still unique: it is the only one that protrudes straight in a spiral and does not follow a curved shape. Narwhals have two upper teeth: the left canine of males grows into a twisted shell, protruding from the jaw bone and passing through the left upper lip. The right tooth in men and both teeth in women are embedded in the skull. There are rare cases of females having one tusk and males without one, and even individuals with double tusks have been observed. The tusk can exceed a whopping 2 m in length and always rotates in a helix to the left. Even in the rare cases where it is the right tooth, it becomes a fang. “Like bones or other teeth in the animal kingdom, the narwhal tusk is a complex composite material whose structure extends from small nanoscale building blocks to the visible form of the entire tooth,” says Marianne Liebi, co-author of the study. The tusk is made of dentin: the same material that forms the core of human teeth. On the outside, it is covered by a thinner layer of cementum, a tissue that in other mammals normally occurs only at the root of the tooth. Both parts contain collagen fibers, which act as internal reinforcement, which in turn are reinforced by small mineral crystals that give hardness to the material. Narwhals on the surface showing their fangs. Credit: Mads Peter Heide-Jørgensen The challenge It is exactly this complicated architecture of the tusk that presented the main challenge to the researchers. “Collagen fibers and mineral crystals are on the nanometer scale, while the spiral of the tooth is only visible in centimeters and meters in length,” explains Liebi. Visualizing the shape of the tusk at both ranges required a specific X-ray technique: tensor tomography. In this method, the tooth is rotated and scanned point by point, and when X-rays find regularly arranged structures within the material, they produce patterns. From these patterns, scientists can calculate how nanometer-sized collagen fibers are oriented inside. Finally, from millions of measurements, a three-dimensional image of the internal architecture of the tusk can be constructed, from the tiny nanoscale fibers to the macroscopic shape of the entire tooth. But, to the research team’s surprise, the data revealed nothing. “That puzzled us quite a bit,” says Liebi. Based on previous mechanical tests, the scientists thought the spiral shape would be reflected in the collagen fibers and mineral crystals. “But instead of a propeller, all we saw was a regular pattern.” The answer to this riddle forced them to take a step back. Instead of focusing on individual pixels, they needed to analyze spatial orientation. “When we compared how these points were oriented relative to each other, a directional trend began to appear,” recalls the study’s first author, Adrián Rodríguez-Palomo. “All we had to do then was connect the dots, like in a children’s paint-by-numbers kit, and suddenly the structure became visible: two interlocking spirals.” The outer layer of the tooth forms a counterclockwise helix, while the mineralized collagen structures within the tooth follow a spiral pattern in the opposite direction. Reinforcement This newly discovered structure is no coincidence: the double helix configuration makes the tooth more stable against bending and twisting. This is reflected throughout the animal kingdom, for example in the helical reinforcement of the deep-sea glass sponge Euplectella aspergillum, which helps it resist strong ocean currents. The study also determined that the double spiral structure is encoded in the animal’s genetics and remains stable throughout its life. The maximum lifespan of a narwhal is about 80 years and the tooth continues to grow with the animal. This discovery provides new insights into how complicated structures form in the natural world, which can be used to inspire materials design in fields such as construction or medicine. It could also offer a glimpse into past environmental and climatic conditions. “Since whales can live up to 80 years, their teeth form a kind of historical record of changing environmental conditions throughout the animal’s life,” says Henrik Birkedal, who led the research project, from Aarhus University in Denmark. “And since 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. That is what we are working on now.” Read the full findings here. Featured image: Tusked narwhals swimming in northwest Greenland. Credit: Carsten Egevang
