Researchers have deciphered the internal structure of narwhal tusks, revealing a hidden double spiral that provides the elongated canine tooth with strength and stability. The findings were published in the journal Nature Communications.
Using advanced three-dimensional X-ray imaging, researchers found that the exterior of the tusk—made of cementum—displays a left-handed spiral. In contrast, the interior, composed of dentine, exhibits a right-handed spiral. This structural arrangement allows the tusk to grow straight, differentiating it from the curving tusks of other animals such as elephants and walruses.
The study was led by materials chemist Henrik Birkedal of Aarhus University in Denmark. According to Birkedal, the vast majority of male narwhals and a small percentage of females possess one tusk, while a few males carry two. The tusks serve multiple functions, including establishing social hierarchies among males during contests, playing, and acting as sensory organs.
Narwhals inhabit the frigid waters of the Arctic Ocean near Canada, Greenland, Norway, and Russia, feeding on fish, squid, and shrimp. The researchers determined that the large-scale helical structure originates from arrangements of mineralized collagen at a microscopic level in fibers called fibrils.
Birkedal explained that the collagen fibrils are reinforced by nanoparticles of a calcium-mineral to create a strong composite material. While these fibrils align almost parallel to the long axis of the tusk, they systematically deviate to form two helical patterns with opposite handedness in the cementum and dentine.
This double-spiral structure produces a biological counterbalance where opposing forces meet at the contact point between the interior and exterior regions. This configuration delivers mechanical properties superior to a single spiral or a straight rod, offering high stability against bending and twisting. Researchers suggest this mechanism could serve as an inspiration for the design of bioinspired materials.
Historically, narwhal tusks were objects of mystery and high economic value. In the Middle Ages, they were sold as unicorn horns believed to neutralize poisons and cure diseases. Historical documentation from the 1500s and 1600s, including writings by Swedish cartographers and Danish natural historian Ole Worm, tracked the identification of the narwhal and its tusks. The artifacts also carried significant royal prestige, notably used to construct the Coronation Chair of Denmark between 1671 and 1840.
"The discovery of natural composite structures in narwhal tusks offers compelling insights for material science and engineering. Understanding how biological systems achieve structural stability and strength through opposing helical designs can inspire advanced manufacturing and product development across various industries looking for durable, bioinspired materials." — Dr. Shishir Gupta, Founder & CEO, StartupLanes
