A narwhal tale with a twist

Up to two metres long and twisted into a perfect spiral: The narwhal’s tusk is one of the most unusual structures in the animal kingdom. An international research team, including scientists from PSI, has now for the first time deciphered how this helix is organised inside the tooth – with a surprising twist. At the nanometre scale, the building blocks of the spiral rotate not just one way, but in two opposite directions.

King Frederik III knew the true origin of unicorn horns, and in the 17th century he sent an expedition to Greenland to probe the reality behind the precious horn. That’s because the mythical creature lives there, in Arctic waters, and has fins rather than hooves. The expedition was successful and bestowed on the Danish ruler a throne made entirely of “unicorn horns” – and with that, prestige and power in Europe. Even though the legendary horn is not a horn at all, but “merely” a tooth.

A tooth, however, that is quite unique in the animal kingdom. While other long teeth – such as the tusks of elephants and walruses, or the incisors of beavers – simply follow a curved shape, the male narwhal’s tooth grows forwards in a screw-shaped form. Usually it is the left upper canine that twists in an elegant counterclockwise spiral, piercing the upper lip and reaching lengths of up to two metres – sometimes up to half the animal’s body length.

It was precisely this unusual growth that aroused the curiosity of an international and interdisciplinary research team. Using state-of-the-art X-ray methods, the researchers wanted to find out whether the direction of rotation of the visible spiral was already determined by the arrangement of its nanometre-sized building blocks.

The results led to an unexpected twist: The building blocks form two opposing helices – one clockwise and one anticlockwise – which interlock to form an extremely stable structure.

The investigations were carried out at three synchrotron facilities in Europe: the Swiss Light Source SLS at the Paul Scherrer Institute in Villigen, the MAX IV Laboratory in Lund (Sweden), and the European Synchrotron Radiation Facility ESRF in Grenoble (France). The researchers have now reported their findings in the journal Nature Communications.

A tooth like reinforced concrete

The tooth can be as old as the animal itself: Narwhal tusks grow throughout the animal’s life and thus can easily reach 70 years of age. What exactly the animals use these striking tusks for, however, remains a matter of speculation. Much suggests that they mainly play a role in mate selection. Occasionally, narwhals can also be observed using their tusks in playful “duels” with one another, suggesting that the most likely explanations have to do with reproduction and social interaction.

Even though relatively little is known about the function of these tusks, their internal structure is now far better understood. “Like bones or other teeth in the animal kingdom, the narwhal tusk is a complex composite material whose structure extends from tiny nanoscale building blocks to the visible form of the entire tooth,” says Marianne Liebi, a PSI researcher and co-author of the study.

It's not only in nature that composite materials play an important role. A classic example from everyday life is reinforced concrete, in which the concrete component – a mixture of cement and sand – withstands strong compressive forces while a steel framework provides tensile strength and stability. “The narwhal tusk, too, must be both hard and flexible,” Liebi explains. “Only then can it withstand the strong hydrodynamic forces that swimming generates.”

Inside the narwhal tusk, a familiar material dominates: dentine – the same substance that forms the core of our own teeth. On the outside, it is covered by a thinner layer of so-called cementum, a tissue that in other mammals normally occurs only at the root of the tooth. Both tissues contain fine collagen fibres reinforced by tiny mineral crystals. In fact, the collagen fibres act as an internal reinforcement, while the tiny mineral crystals give the material its hardness – quite similar to the way reinforced concrete gets its useful properties.

This complex architecture was in fact the main challenge of the study. “The collagen fibres and mineral crystals are on the nanometre scale, while the spiral of the tooth only becomes visible at centimetre and metre lengths,” Marianne Liebi explains. To connect the two and visualise the helix from the nanometre range to the macroscopic form of the tooth, the research team used a special X-ray technique: tensor tomography.

Connecting the dots to reveal the spirals

Everyone’s familiar with X-rays from visits to the dentist: They make teeth visible because teeth absorb the radiation more strongly than the surrounding tissue. What one sees in the image, essentially, is the shadow of the tooth.

Using synchrotron radiation from three large European research facilities, researchers are able to discern much more. When these special X-rays encounter regularly arranged structures within a material, they are scattered and produce characteristic patterns. “From these patterns, we can calculate how the nanometre-sized mineralised collagen fibres inside the material are oriented,” explains first author Adrian Rodriguez-Palomo. He joined the project as a PhD student at Chalmers University of Technology in Sweden and later continued the work as a postdoctoral researcher at Aarhus University in Denmark.

In tensor tomography, the tooth is rotated in the synchrotron beam and scanned point by point. From millions of measurements, a three-dimensional image of its internal architecture is reconstructed – from the tiny building blocks at the nanometre scale to the macroscopic form of the entire tooth.

But when the team began analysing the vast amounts of data, they got a surprise: In the images they saw . . . nothing. “That puzzled us quite a bit,” recalls Marianne Liebi. Mechanical tests had suggested that the spiral shape should already be reflected in the collagen fibres and the mineral crystals. “But instead of a helix, all we saw was a regular pattern.”

The solution only emerged when the team took a step back. Instead of focusing on individual tiny pixels in the three-dimensional dataset, they began analysing their spatial orientation. “When we compared how these points were oriented relative to each other, a directional trend began to appear,” recalls Palomo. “All we had to do then was connect the dots – as in a children’s paint-by-numbers kit – and suddenly the structure became visible: two intertwined spirals.”

From the nanoscale to the macroscopic helix

A closer look at the data also revealed where these two spirals are hidden within the tooth. The outer layer of the tooth, known as cementum, forms a left-handed helix, while inside the tooth the mineralised collagen structures in the dentine follow a spiral pattern in the opposite direction.

This counter-rotating architecture is no accident, but a remarkably refined construct of nature. Two interlocking helices give the tusk exceptional stability, much like the twisted fibres in a rope. They make it particularly resistant to torsion and mechanical stress. And they are also what give the tooth its distinctive spiral form – ultimately lending an unexpected twist to an ancient myth.

The study was supported by the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie programme. It is part of the NordForsk-funded research project “Narwhal tusks – a tale with a twist.” Led by Henrik Birkedal from Aarhus University in Denmark, an international and interdisciplinary team is investigating not only the structure of the narwhal tusk, but also how its growth relates to environmental and climate conditions. The tusk’s growth structures – similar to the annual rings of a tree – allow researchers to draw conclusions about past environmental conditions in the Arctic, such as climate and food availability. Over its nearly hundred-year lifespan, each male narwhal thus records its own environmental history in its tusk.