Narwhal Horn Mystery Finally Solved After Centuries

Predominantly inhabiting the frigid waters of the Arctic, narwhals are marine mammals distinguished by their remarkable tusk, which is technically a tooth. Researchers have recently shed light on the mystery of its spiral structure.

A Very High-Resolution 3D Model of the Narwhal’s Tusk

This singular feature, typically the left canine in males, can reach up to two meters in length. Like human teeth, it is composed of collagen fibrils, but unlike ours, it lacks enamel. The interior is made of dentin, and the outer layer of cementum, which helps anchor the tooth in the maxillary bone in humans, explains Henrik Birkedal of Aarhus University.

In the animal kingdom, spiral structures “wrap” generally in both directions, as illustrated by snail shells. Intriguingly, in narwhals, the twist is always oriented to the left.

To learn more, Birkedal and his colleagues used a new form of imaging based on X-rays, which enabled them to create a very high-resolution 3D model of the tusk, revealing the peculiarities of its structure at atomic, nanometric, as well as microscopic scales.

It turned out that the layers of dentin and cementum oriented in opposite directions, diverging at remarkably regular angles from the longitudinal axis of the tusk of this cetacean to create its characteristic twist.

Torsional Tension

According to the team, these opposing constraints at the interface of the two tissues would allow the narwhal’s tusk to maintain a straight shape.

« When we cut a tusk along its length, it began to coil because the inner layer of dentin is thicker than the outer layer of cementum », explain the authors of the new study, published in the journal Nature Communications. « It twists by nearly 180 degrees when this torsional stress is released. »

Another significant finding: like tree rings, it seems that narwhal tusks preserve a record of the environmental conditions in which they form. Growing throughout the animal’s life, they could thus offer a unique glimpse.

In case you were wondering, here’s why you don’t see narwhals in aquariums.