By Will Dunham
Aug 20 (Reuters) - In the Middle Ages, narwhal tusks were sold as unicorn horns blessed with magical powers such as neutralizing poisons and curing diseases — prized possessions of monarchs and nobles worth more than their weight in gold. They provided no such powers, of course, but even today remain objects of mystery.
Researchers have now deciphered the internal structure of these spiral-shaped tusks — actually a straight and elongated canine tooth that protrudes from a narwhal's mouth,
sometimes reaching up to 10 feet (3 meters) long.
Using advanced three-dimensional X-ray imaging, they found that while the exterior of the tusk, made of dental tissue called cementum, displays a left-handed spiral, its interior, made of dental tissue called dentine, exhibits a right-handed spiral.
"We propose that this arrangement helps the tusk grow straight, unlike other tusks — elephant or walrus, for example — that curve," said materials chemist Henrik Birkedal of Aarhus University in Denmark, who helped lead the study published this week in the journal Nature Communications.
The vast majority of male narwhals and a small percentage of female narwhals have one of these tusks — a few males even have two of them — though their purpose remains unclear.
"The tusk may have several functions. The tusks seem to function to establish social hierarchy amongst males, and is thus a sexually selected trait used during male-male contests. It has also been proposed that the tusk may serve in play and as a sensory organ," Birkedal said.
TUSK'S STRUCTURE GIVES IT STABILITY
Narwhals, which can reach around 17 feet (5 meters) long, inhabit the frigid waters of the Arctic Ocean, mainly near Canada, Greenland, Norway and Russia, and feed on fish, squid and shrimp. Their color changes with age; they are born with a blue-gray hue that turns blue-black in their juvenile years and mottled gray in adulthood, only to become nearly all white in old age.
The researchers found that the large-scale helical structure of the tusk arises from arrangements of mineralized collagen — an important protein in bones, teeth and skin — at a microscopic level in fibers called fibrils.
"The fibrils are strong and flexible. In the narwhal tusk, and in our teeth and bones, the collagen fibrils are reinforced by nanoparticles of a calcium-mineral to make a strong so-called composite material," Birkedal said.
"The collagen fibrils are aligned almost parallel to the long axis of the tusk. However, they deviate systematically to form two helical patterns, one in cementum and another in dentine, with opposite handedness," Birkedal added.
This double-spiral structure produces a biological counterbalance in which two opposing forces meet at the point of contact between interior and exterior parts, giving the tusk beneficial mechanical properties for stability against bending and twisting — superior to a single spiral or a straight rod.
"We think this may act as a source of inspiration for design of bioinspired materials," Birkedal said.
While unicorns and the magical properties of narwhal tusks were mere legend, they were common beliefs centuries ago.
"The first scientific depictions are from the mid-1500s where a Swedish cartographer drew the narwhal like a fish-like creature with a horn," Birkedal said.
In the 1600s, Birkedal added, "The Danish natural historian Ole Worm concluded that narwhal and unicorn tusk powders were one and the same, and described skulls of marine unicorns that resemble narwhals."
The Coronation Chair of Denmark that was used in the coronations of Danish monarchs from 1671 to 1840, commissioned by King Frederik III and now displayed at Rosenborg Castle in Copenhagen, was made of narwhal tusks.
"The tusks thus carried great prestige," Birkedal said.
(Reporting by Will Dunham in Washington, Editing by Rosalba O'Brien)
