An international research team has solved a long-standing biological mystery by discovering that the iconic tusk of the narwhal features a complex double helix structure.
X-Rays Reveal a Complex Double Helix Inside Narwhal Tusks
The narwhal tusk is actually the animal’s left canine tooth, which grows straight through the jaw and lip to reach lengths exceeding two meters, or 6.5 feet. Unlike human teeth that feature a tough outer enamel, the narwhal tusk consists of an interior core of dentin covered by a thin exterior layer of cementum. While human cementum serves to anchor teeth in the jaw, the narwhal’s outer layer forms part of a sophisticated architectural system.
Advanced Particle Accelerators Unlock Atomic and Nanoscale Secrets
To map the interior of the tooth across atomic, nano, and macroscales, researchers had to utilize some of the biggest technological tools available. The investigation combined multiple imaging techniques, including X-ray computed tomography, scanning X-ray diffraction, scanning small-angle X-ray scattering, birefringence microscopy, and a special 3D method called tensor tomography.
Because of the tusk’s massive size and structural complexity, the team secured time on three massive particle accelerator X-ray sources: the MAX IV facility in Sweden, the Swiss Light Source in Switzerland, and the European Synchrotron Radiation Facility (ESRF) in France. These enormous tools provided the necessary resolution and power to reveal how the tooth’s microscopic building blocks are organized.
The analysis showed that both dentin and cementum are made of microscopic collagen fibrils mineralized with nanoparticles of hydroxyapatite. While these building blocks generally orient themselves along the longitudinal axis of the tusk to maintain a high degree of anisotropy, tiny systemic deviations at small angles create the twisted structure. Specifically, the outer cementum layer forms a left-handed helix, while the inner dentin forms a right-handed helix. Together, they create a biological counterbalance where opposing forces meet at the transition interface.
Unique Handedness and Superior Mechanical Strength
This dual-spiral arrangement is responsible for the tusk’s remarkable material properties, providing the extraordinary stiffness and strength needed to withstand powerful bending and twisting forces without cracking. The flexible fibers and stiff mineral matrix also allow the tusk to grow straight rather than curved like an elephant’s tusk.
Henrik Birkedal, a chemist at Aarhus University in Denmark and co-author of the study, noted the exceptional nature of the configuration. The really special part is that these helices always have the same handedness—similar to if all [people] only had left hands,
Birkedal told Popular Science, adding that the setup is unique to the best of their knowledge.
Researchers also highlighted the crucial role of collaboration. As noted in the analysis, input from biologists at the Greenland Institute of Natural Resources was essential for interpreting the results.
Ongoing Debates and Future Research Goals
Despite uncovering the structural secrets behind the tusk’s twist, scientists are still debating its primary purpose in nature. Most experts believe the tusk functions primarily as a sexual signal because it is predominantly males that possess them, though narwhals have also been observed using them while playing, foraging, and engaging in non-sexual activities.
While some researchers have proposed that the tusk might act as a sensor for temperature, salinity, or water chemistry, or serve as a tool for fighting, concrete evidence remains lacking. Furthermore, because females typically do not have tusks, scientists agree it is unlikely to be a primary hunting tool.
Looking ahead, the research team plans to conduct further investigations. They have already noted a finer underlying microstructure involving collagen fiber bundles extending radially outward in the cementum, which they intend to study using nano-beam experiments. Additionally, because whales can live up to 80 years, researchers hope to analyze the hard tissue to reconstruct a historical record of changing environmental conditions in the rapidly altering North Atlantic.
