Narwhal Tusks Grow Straight Thanks to Opposing Spiral Layers, Study Finds

Narwhals, Arctic whales known for a single elongated tooth that can reach three meters, are the only animals with a straight tusk that consistently spirals to the left. A study published August 18 in Nature Communications reveals that this visible left-handed helix is only the outer layer of a more complex structure.

Chemist Henrik Birkedal of Aarhus University and colleagues examined tusks sourced from Greenlandic Inuit subsistence hunters and the Greenland Institute of Natural Resources. Using microscopy, X-ray imaging, and a CT scan of an entire skull, they found that the tusk consists of two main layers: an outer cementum layer and an inner dentine layer.

The cementum forms a left-handed helical arrangement of collagen threads, matching the visible spiral. Surprisingly, the inner dentine layer arranges its collagen threads in a right-handed helix, twisting in the opposite direction.

Mechanical testing indicated that these opposing spirals give the tusk greater resistance to bending and twisting than a single-spiral or non-spiral rod would have. The counter-twisting structure also helps the tusk grow straight rather than curving.

Birkedal said the intricate boundary between cementum and dentine is more complex than previously understood. The researchers also observed that the opposing spiral pattern appears in annual growth layers, similar to tree rings.

The team is now analyzing those growth layers to see whether they can reconstruct a narwhal's life history and environmental conditions from its tusk. They also plan to study the tusk's growth zone in the skull to understand how the structure forms.

Narwhal expert Martin Nweeia of Harvard University, who was not involved in the study, described the tusk as "the most extraordinary tooth on the planet" and welcomed the detailed structural analysis.

While the tusk's biological function — possibly for mating competition or sensory perception — remains under investigation, the new findings explain how its unique geometry is achieved at the material level.