An international team of researchers has solved the mystery of how the narwhal tusk maintains its straight growth despite a characteristic twisted structure, according to a study published in the journal Nature Communications.
The Secret of the Double Helix
Narwhals possess the only tusks in the animal kingdom known to grow in a completely straight line, reaching lengths of up to 10 feet. While the tusks always appear to spiral in a left-handed helix, researchers discovered that this macroscopic form is the result of two opposing internal spirals.
Using advanced 3D X-ray technology and three European particle accelerators known as synchrotrons, the team analyzed the tusk’s building blocks. These building blocks consist of mineralized collagen, a protein Henrik Birkedal, a chemist at Aarhus University in Denmark and senior author of the study, describes as having a texture similar to half-cooked spaghetti
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The research revealed that the tusk is composed of two types of dental tissue with opposing molecular orientations:
- Cementum: The outer layer, which forms a left-handed spiral.
- Dentine: The inner core, which forms a right-handed spiral.
According to Birkedal, these opposing spirals are what enable the tusk to grow straight. The study further determined that this double-spiral structure is encoded in the animal’s genetics and remains stable throughout its life.
Overcoming Research Challenges
The discovery required a shift in analytical approach. Initially, the team used a method where the tooth was rotated and scanned point by point to calculate the orientation of nanometre-sized collagen fibres. However, the resulting data showed only a regular pattern rather than a helix, which puzzled the researchers.
The breakthrough occurred when the team stopped focusing on individual pixels and instead analyzed spatial orientation. Adrian Rodriguez-Palomo, the study’s first author, noted that a directional trend appeared only when comparing how those points were oriented relative to each other.
Biological and Historical Context
The tusk is actually an overgrown tooth. In male narwhals, the left canine grows through the jaw and upper lip, while the right canine remains buried in the head. While most males have one tusk, some females also grow them, and some individuals have been observed with two.
The tusk continues to grow as the animal ages, with a maximum lifespan of approximately 80 years. Researchers found growth layers inside the tusks similar to tree rings. Birkedal suggests these bands could potentially provide information about the life history of the whales, as each band may have a distinct chemical and mineral makeup.
Historically, these tusks were sold as magical unicorn horns during the Renaissance and Middle Ages. This reverence was such that the Danish-Norwegian king Frederik III had a coronation chair constructed entirely from narwhal tusks to symbolize his social status and power.
Debate Over Tusk Function
While the structural mystery is solved, the purpose of the tusk remains a subject of scientific debate. Some hypotheses suggest the tusk is used for play, infighting, hunting, or sexual signaling. Birkedal noted a dominant hypothesis that the tusk is used for show—to attract mates or establish social dominance—or that its straightness prevents it from interfering when narwhals suction feed on squid and fish.
Other theories suggest the tusk is used to sense water salinity and temperature. Martin Nweeia, a dental anthropologist at Harvard University and a leading expert not involved in the study, disagrees with the “for show” theory, stating that the primary function of this tusk is as a sensory organ system
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To conduct their research, the international team utilized tusks collected by the indigenous Inuit people of Greenland, as well as specimens from museums.
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