Scientists Uncover the Molecular Engineering Behind the Narwhal's Straight Tusk

An international team including researchers from Denmark's Aarhus University has worked out how narwhals grow the only straight tusk structure known in nature — a helical, continuously growing tooth that can reach more than two meters over the whale's 80-plus-year lifespan. The study, led by Henrik Birkedal, was published in the journal Nature Communications.

The tusk is actually one of a pair of canine teeth that grows through the narwhal's upper lip, while the second tooth stays embedded in the jaw. Using a suite of imaging technology, the researchers mapped the tusk's structure across multiple scales and found that its helical shape comes from two layers of mineralized collagen fibrils twisting in opposite directions: the inner dentine layer twists to the right, while the outer cementum layer twists to the left. Every individual narwhal's tusk spirals counterclockwise.

The researchers found that the spiral itself does not add any extra function, such as increased surface area — its role is to keep the tusk growing straight, a form of self-correction that compensates for growth material not being deposited evenly around the tooth's root. "The structural arrangement of tusk building blocks has remained elusive," the team wrote, adding that the discovery advances understanding of how biological tissues combine structural patterns to achieve specific mechanical functions.

Earlier research by Harvard University scientists had already shown the tusk plays a role in sensing water temperature and salinity, in addition to its uses in sexual selection and in tapping and stunning prey, and that its structure is the inverse of human teeth, with a hard center and a flexible, porous outer layer. The researchers say further study is needed to understand exactly how the tusk forms from the whale's set of teeth during development.