However, a research team has now used advanced 3D X-ray techniques to determine how the tusk obtains its characteristic structure.

It turns out that the tooth does not contain just one spiral, but two. 

On the outside, the structure twists to the left, while the interior structure twists in the opposite direction, forming a kind of biological counterbalance, where two opposing forces meet at the interface between interior and exterior parts. The discovery has just been published in Nature Communications.

Researchers have long known that the left-handed shape must reflect a particular organization of the mineralized collagen fibrils the microscopic building blocks that give the tooth its strength but exactly how this organization works has been impossible to map in three dimensions.

Until now.

The Really Big Machines

To see how the building blocks are oriented inside the tooth, the researchers combined several X-ray imaging techniques, particularly a special 3D X-ray technique called tensor tomography. The technique works by sending powerful X-rays through the tooth and analysing how they scatter from the nanoscale mineralized collagen fibrils.

The narwhals tusk is in fact the left canine tooth, which grows through the jaw and lip and can reach more than two meters in length. Unlike human teeth, it has no enamel but consists of dentin on the inside and cementum on the outside.

Because the narwhal tusk is both large and structurally extremely complex, the researchers had to deploy the biggest technological tools available. They combined the capabilities of three enormous synchrotrons particle accelerator X-ray sources MAX IV in Sweden, Swiss Light Source in Switzerland and European Synchrotron Radiation Facility (ESRF) in France to obtain enough resolution and power to map the entire interior of the tooth in three dimensions at the atomic, nano and micro scale.

The analyses revealed a fascinating pattern: while the building blocks are primarily oriented along the tooths longitudinal axis, they systematically deviate at small angles, creating a twisted structure. In the outer cementum, the fibrils form a left-handed spiral, while in the inner dentin they form a right-handed spiral.

More Complex Than Previously Believed

The two opposing structures meet at the transition between dentin and cementum a complex biological boundary that now appears to be even more intricate than previously believed.

This double-spiral structure gives the tusk favourable mechanical properties. The structure is far more stable against bending and twisting than either a single spiral or a straight rod would be. It is an architecture also found in other biological materials that must withstand large forces.

No one has previously carried out such an advanced experiment of this type. We have only been able to do it by collaborating across several disciplines namely chemistry, physics, materials science and biology. Without collaboration with the biologists at the Greenland Institute of Natural Resources, we would not have been able to interpret the results, says Adrian Rodriguez-Palomo, the studys lead author. He was a PhD student at Chalmers University of Technology in Sweden, when he joined the project, and continued the study as a postdoctoral researcher at the Department of Chemistry at Aarhus University. 

To enable this interdisciplinary endeavour, the project reunited groups from Aarhus University, Chalmers University of Technology, and the Greenland Institute of Natural Resources in collaboration with the synchrotron facilities. 

Growth Layers Reveal Stability

Remarkably, the study shows that the double-spiral structure is preserved across the tooths annual growth layers like tree rings, but with a constant twist. This suggests that the left-handed growth pattern is genetically programmed and remains stable throughout the animals life, which can extend to approx. 80 years.

The discovery not only solves a centuries-old natural science mystery about one of the oceans most iconic animals. It also provides new insight into how nature constructs advanced materials with extreme mechanical properties knowledge that could eventually inspire the design of new composite materials for fields such as construction and medicine.

And the research teams investigations do not stop there:

Since whales can live for up to 80 years, their teeth form a kind of historical record of changing environmental conditions throughout the animals lifetime. And because the North Atlantic is currently undergoing very rapid changes, it is obvious to investigate whether we can trace these changes in the hard tissue of the narwhal tusk. That is what we are now working on, says the leader of the research project, Professor Henrik Birkedal from the Department of Chemistry at Aarhus University.

What Does the Narwhal Use Its Tusk For?

Most researchers today believe that the narwhals tusk primarily functions as a sexual signal, since it is typically males that possess one. 

But the role of the tusk has been debated, in part because a very small proportion of females also develop tusks and some males do not. 

Some researchers have suggested that the tusk may be able to detect temperature, salinity, and chemical changes in the water. However, marine biologists in Greenland have found no evidence for such a thing in the narwhals behaviour. 

Others propose that it may be a hunting tool but if that were the case, it would be logical for females to have tusks as well, and there is no indication that females are starving.

Finally, it is possible that male narwhals use their tusks in fights with one another. There are some observations of narwhals with tusk sections embedded in their bodies, but no real evidence of tusk fighting as a normal behaviour. 

Incidentally, the narwhal is the only animal with tusks that normally has just one. Normally, because some male narwhals develop two tusks. 

It is also the only tusk in nature that is straight. 

And a Bit of Mathematics

Strictly speaking, the narwhal tusk is not a spiral but a helical curve on a tapered shape a kind of conical helix. Unlike a classic spiral in a single plane, the structure twists three-dimensionally around the tooths longitudinal axis. However, because the tusk visually resembles a spiral, this article uses the word spiral fully aware that some mathematicians may raise an eyebrow.