Narwhal tusk’s hidden double helix keeps it straight

A narwhal’s tusk seems to draw one clean spiral through the Arctic water. Inside that tooth, scientists have now found another spiral running the other way. The international team, including Henrik Birkedal and researchers from Aarhus University in Denmark, says the hidden structure is what helps the whale grow nature’s only known straight tusk of this kind.

The team used imaging technology at several scales to map the tusk’s construction. Its mineralized collagen fibrils run along the tooth, but not in a single direction: the inner dentine layer twists right, while the outer cementum layer twists left. The visible spiral is therefore the result of two opposing helical arrangements, rather than one simple coil.

That geometry appears to act as a form of biological self-correction. Growth materials are not deposited evenly around the tooth’s root, yet the opposing fibril directions help the tusk remain straight as it lengthens. The researchers say the spiral itself does not appear to add another function, such as increasing surface area.

The scale of the structure matters because the tusk is not a decorative rod attached to the animal. It is a canine tooth in the left upper jaw of male narwhals, emerging through the lip at two or three years of age and growing to 1.5 to 2 meters. Some males develop two tusks, while rare males develop none. Earlier research has also connected the tooth with sensing water temperature and salinity, sexual selection, tapping and stunning prey.

So what changes, concretely? The finding gives scientists a mechanical explanation for how a continuously growing tooth can stay straight despite uneven growth. It could sharpen the study of other specialized biological structures, but it does not settle why narwhals have tusks or how the double helix forms during development. Those questions still require further investigation, and the tusk’s overall role remains debated: one account emphasizes social status and limited survival value, while another calls it critical for survival.