Male narwhals have the strangest canines in the animal kingdom. Their left upper canine grows when they’re young, becoming a tusk, as the right canine stays hidden in their skull. During their 80-year lifespan, the tusk continues to grow longer and thicker while keeping its spiral architecture. As such, humans have long been fascinated by this rare structure that inspired the mythical unicorns.
In a study published today in Nature Communications, we’ve come a step closer to understanding the narwhal’s twisted tusk. Researchers at Aarhus University in Denmark convened an international team to analyze the internal architecture of tusks from two male narwhal skulls collected by Greenlandic Inuit subsistence hunters and exported under CITES permits. Six tusks housed at the Greenland Institute of Natural Resources were also used for external measurements.
Read more: “Why Are Narwhals Messing with Hydrophones?”
The team employed three massive synchrotrons in Sweden, Switzerland, and France, respectively, to map the interior of tooth sections using ultrabright X-ray beams. That might sound like overkill, but exceptional resolution and power were needed to capture the structural complexity of a narwhal tooth at atomic, nano, and micro scales. The mechanical properties of intact tusk material were also examined in three-point bending tests.
The analysis revealed that mineralized collagen fibers comprise both the internal and external building blocks of the tusk. On the outside, they form the material “cementum” and spiral to the left. On the inside, as “dentine,” the fibers spiral to the right. The result is an exceptionally stable helix structure, thanks to the opposing forces where the dentine meets the cementum.
The helical forces are so great that if a tusk is sectioned lengthwise, the pieces torque an additional 180 degrees as force gets released. During growth, as the tusk lays on dentine rings to reach up to 6 feet long, it keeps the left-handed twist, suggesting an innate growth pattern, according to the study authors.
As for why males possess such a tusk, the prevailing theory is that it was honed by sexual selection to serve in courtship. In that way, it’s a lot like a peacock’s tail—stunning, audacious, and totally impractical. 
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Lead image: dottedyeti / Adobe Stock
