An international team of researchers has discovered that the narwhal’s tusk contains two opposing spirals of mineralized collagen, a structural design that allows the tooth to grow straight while remaining both hard and flexible. The findings, published August 18 in the journal Nature Communications, explain the secret behind the only straight tusk found in nature.
Double-Spiral Structure Found in Narwhal Tusks
The narwhal tusk is actually the left canine tooth of the male whale, which erupts through the upper lip and can reach lengths of more than two meters, or roughly 6.5 feet. While the tusk visibly spirals to the left, researchers found that this macroscopic chiral structure is the result of two internal layers twisting in opposite directions.
The outer layer, known as cementum, consists of mineralized collagen fibrils that form a left-handed helical arrangement. In contrast, the inner layer of dentine twists into a right-handed helical shape. According to the researchers, these opposing helices act as a biological counterbalance, canceling out twisting forces during early growth stages to enable the tusk to grow straight.
Advanced X-Ray Mapping Techniques
To map the tusk’s structure across several scales, the team led by chemist Henrik Birkedal of Aarhus University in Denmark utilized multiple imaging techniques. These included birefringence microscopy, small-angle x-ray scattering, and x-ray diffraction.
The team specifically employed a 3D X-ray technique called tensor tomography. This method sends powerful X-rays through the tooth to analyze how they scatter from nanoscale mineralized collagen fibrils. To achieve the necessary resolution and power to map the entire interior of the tooth, the researchers combined the capabilities of three synchrotrons:
- MAX IV in Sweden
- Swiss Light Source in Switzerland
- European Synchrotron Radiation Facility (ESRF) in France
The analysis revealed that while the building blocks are primarily oriented along the longitudinal axis of the tooth, they systematically deviate at small angles to create the twisted structure.
Mechanical Strength and Biological Function
The “double-spiral” architecture functions similarly to reinforcement embedded in concrete, providing the tusk with the toughness required to withstand strong hydrodynamic forces generated during swimming.
Researchers tested this toughness by bending rod-shaped samples of the tusk in two different directions. They found that rods cut along the length of the tusk resisted bending approximately 63 percent more than those cut across it.
While the structural secrets are becoming clearer, the exact purpose of the tusk remains a subject of debate. Most experts believe it primarily serves as a sexual signal, as it is typically found in males. Other theories suggest it may be a sensory organ or used to compete for mates. Drone footage has also captured narwhals using their tusks to play, such as prodding and flipping fish.
Growth Patterns and Future Research
The study found that the double-spiral structure is preserved across the tooth’s annual growth layers, which are similar to the growth rings found in trees. According to study first author Adrian Rodriguez-Palomo, who worked as a doctoral student at Chalmers University of Technology in Sweden, this suggests the growth pattern is genetically programmed and stable throughout a narwhal’s life, which can exceed 80 years.
Researchers are now exploring how these growth layers can be used to “map” the life history of narwhals and gain insights into the living conditions of the Arctic, particularly as the region undergoes rapid changes. Additionally, the team intends to investigate the area of the skull where the tusk grows to better understand its formation.
The researchers suggest that this biological motif could serve as inspiration for the design of bioinspired composite materials with engineered mechanical properties.
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