Loading today’s stories
Science3 distinct publishers3 min readUpdated
Narwhal tusks hide two spirals twisting against each other, and the mismatch is the point
3D X-ray work at three synchrotrons shows the tusk's outer layer coils left while its core coils right, the two meeting at the dentin-cementum boundary. The stiffness gain is still an inference.
The Scientist · Science desk
What happened
- An international team of researchers led by Aarhus University used advanced 3D X-ray techniques to determine how the narwhal tusk obtains its twisted structure; the discovery was published in Nature Communications.
- 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.
- In the outer cementum the mineralized collagen fibrils form a left-handed spiral, while in the inner dentin they form a right-handed spiral, and the two opposing structures meet at the transition between dentin and cementum.
- The double-spiral structure is far more stable against bending and twisting than either a single spiral or a straight rod would be.
Compiled by The ScientistSomething wrong?How this is made
Why it matters
An international team led by Aarhus University has reported in Nature Communications that a narwhal tusk is not one spiral but two, wound in opposite directions [1][2]. The outer layer coils left, the inner core coils right, and the two meet at the boundary between cementum and dentin in what the authors call a biological counterbalance, where opposing forces cancel at an internal interface [3][4].
That geometry, not the animal, is the part with consequences. According to the study, the double spiral is considerably more stable against bending and twisting than either a single spiral or a straight rod, and the same counter-twisted architecture appears in other biological materials that have to carry large loads [5][6].
Mapping it took the largest available instruments. The team combined several X-ray imaging methods, principally tensor tomography, which fires high-intensity X-rays through the tooth and reads how they scatter off nanoscale mineralized collagen fibrils [7]. Beam time came from three synchrotrons: MAX IV in Sweden, the Swiss Light Source, and the ESRF in France, together giving enough resolution to map the whole interior at atomic, nano and micro scale [8]. Senior author Henrik Birkedal, a chemistry professor at Aarhus, describes the machines as turbocharged CT scanners and the collagen itself as half-cooked spaghetti, firm but flexible [9][10].
The tusk is the animal's left canine, growing out through jaw and lip [11], and unlike a human tooth it has no enamel, just dentin inside and cementum outside [12]. Most fibrils run along the long axis but deviate systematically by small angles, and it is that consistent tilt, opposite in the two tissues, that produces the helices [13]. The interface where they meet is more intricate than previously assumed [14]. Every narwhal tusk turns left, and according to Scientific American these are the only tusks known to grow perfectly straight; Birkedal says the opposing spirals are what hold that line [15][16][17]. The pattern also persists across the annual growth bands, which the authors read as evidence of a genetically programmed handedness stable over a life that can run to about 80 years [18].
Two limits on the engineering read. The mechanical benefit is presented as an inference from measured architecture rather than from a bend test: the publisher's own framing is that the double spiral "may explain" the resistance to bending, and Discover reports it as a likely source of stability [19][20]. And the transfer path is stated as a possibility, with Discover suggesting such structures could one day be adopted in construction or medical materials [21]. The accounts available here give neither the fibril tilt angles nor the thickness ratio between the two layers, which is exactly what a composites designer would need to copy the arrangement rather than admire it.
What the tusk is for remains unsettled [22]. Birkedal's stated dominant hypothesis is display, for social dominance or mate attraction, rather than hunting directly, with a secondary idea that a straight tusk stays clear when narwhals suction feed [23]. The sources also disagree on maximum length, from more than 2 meters to about 10 feet [24][25][26].
Next: Birkedal's group says it is testing whether the chemistry of individual growth bands records the rapidly changing North Atlantic [27], and the team plans to image the skull chamber where the tusk originates [28].
Loading topic controls
Loading related stories