Narwhal tusks stay straight by twisting both ways

Narwhal tusks grow straight because different layers twist in opposite directions, an international team of researchers said in a study published in Nature Communications.

The researchers used advanced 3D X-ray technology, including European particle accelerators called synchrotrons, to study the structure of the tusks. They found that the inner dentin forms a right-handed spiral, while the outer cementum forms a left-handed spiral.

The team said those counteracting forces help explain how the tusk keeps its arrow-straight shape, unlike the curved tusks of animals such as elephants.

Narwhal tusks are usually the left canine tooth of male narwhals, growing through the jaw and upper lip. They can reach up to three metres long.

The researchers also found growth layers inside the tusks similar to tree rings, which they said could offer clues about narwhals’ lives.

The purpose of the tusk remains disputed. Study co-author Mads-Peter Heide Jorgensen of the Greenland Institute of Natural Resources said its most likely role is to establish dominance and impress a potential mate. Harvard University dental anthropologist Martin Nweeia, who was not involved in the study, said he strongly disagreed and considers the tusk primarily a sensory organ system.