Nweeia, whose latest research was sponsored in part by the National Geographic Society's Expeditions Council, has studied narwhals on four trips to the Canadian high Arctic.
He founded the Narwhal Tooth Expeditions and Research Investigation in 2000, bringing together both scientific experts from a variety of disciplines and members of the region's indigenous Inuit population.
The new findings, which have not been published, are based on analyses of at least six Canadian male narwhal tusks, Nweeia said.
One of a Kind
The narwhal, whose Latin name Monodon Monoceros translates as "one tooth, one horn," typically grows 13 to 15 feet (4 to 4.5 meters) long, not counting its tusk, and weighs about 2,200 to 3,500 pounds (1,000 to 1,600 kilograms).
A protected Arctic species, narwhals are social animals that mostly live in the Atlantic portion of the Arctic Ocean and are found in fewer numbers in the Greenland Sea.
They are hunted by the Inuit for their tusks, meat, and skin. The whales have been known to dive nearly vertically as deep as 3,000 feet (about 900 meters) multiple times per day. (They dive when scared and presumably to feed.)
The mammal's tusk has baffled scientists because it defies known principles and properties of teeth. It is slightly longer than half the animal's length and typically protrudes through the left side of a male's upper jaw plate and lip.
(For comparison, consider a six-foot-tall [two-meter-tall] person with a three-foot-tall [meter-long] incisor jutting straight up into the air.)
Furthermore, unlike the curved teeth of elephants and warthogs, the narwhal tooth is nature's only straight tusk. It consistently spirals on a left-handed, single axis. Scientists speculate the spiral may minimize tusk fractures, and prior research suggests it may aid the tusk's relatively straight growth during development.
Adding to the tusk's uniqueness is its odd gender distribution. The teeth are common in males but not females. Female tusks, when they do appear, tend to be shorter and cleaner with more tightly wound spiral patterns.
The latest findings, researchers say, only add to the narwhal's singularity.
Generally, mammalian teeth are softer on the inside and tougher on the outside to resist wear and abrasion.
But when Naomi Eidelman, an infrared microscopy expert at the Paffenbarger Research Center at the National Institute of Standards and Technology in Gaithersburg, Maryland, used a special technique to map cross-sections of a tusk cell by cell, she reported something dramatically different.
While the tusk contains some materials similar to other mammalian teethdentin, pulp, and cementumit is constructed "inside out," said Frederick Eichmiller, who directs the research center.
A highly mineralized layer surrounds the pulp on the inside, like a steel rod. The outside of the tooth, which resembles enamel at the tip, is actually less mineral and more protein.
Cementum is usually the layer that bonds the tooth to the bone in other mammalian teeth. But in this case, the cementum is "just sticking out into the ocean," Eichmiller said. "This tooth was different from what we've seen before," he said. "Ever."
Scientists theorize the reverse architecture endows the tusk with flexibility, possibly helping it to absorb shock and resist extreme water pressures during deep dives.
The tusk does not appear able to lay down another form of dentin to heal cracks, and perhaps it does not need to, Nweeia says. The researcher adds that the tusk's unusual qualities could have profound implications for modern dentistry and biomaterials science.
"Everything about this tusk is built not to break," he said. "To find a material that is flexible and strongthat is kind of the grail for restorative materials. This guy's got it."
Sensing the Environment
The new findings also provide significant clues to tusk function, a puzzle that has generated conflicting theories, from displaying aggression to breaking ice.
Using scanning electron microscopy, researchers uncovered evidence of dentinal tubules, basic structures that exist in almost all teeth, including humans. The tubules are remnants of a cell process in which millions of tiny nerve connections tunnel their way from the central nerve of a tooth to its outer surface.
Tubules in human mouths are sensitive to cold and are normally covered by enamel. We experience discomfort and pain only when they are inadvertently exposed, through cavities, for example.
Narwhal tubules, however, penetrate the outermost layer of the tooth, directly exposing sensory connections to the Arctic environment. The result is that the tuskdespite its inanimate appearanceactually serves as a kind of membrane with an extremely sensitive surface, researchers say.
Tubules are known to allow for specific sensory functions in mammals, such as gauging air temperature and barometric pressure. But it remains to be seen what they are used for by the narwhal, whose tubules contain a solution similar to blood plasma, Nweeia says.
Sensing salinity is one possible answer; Nweeia and his colleagues have developed customized equipment to test this theory that measures narwhal brain activity when saline solution is introduced to the tusk.
Narwhal migration is tied to ice formation, which affects saline concentrations, and narwhals may be able to detect subtle changes in the environment from miles away, Nweeia said.
Human teeth have evolved so that "we go out of our way not to have cold things against a tubule," he said. "Why does an Arctic whale, who is in frigid waters and incredible pressures all of his life, go out of his way to have open tubules?"
"If you are going to develop something like this, from an evolutionary standpoint it has to be about survival. There are a lot better ways to get a female than growing one of these," he added.
Solving the Puzzle
Future field expeditions are expected to focus on anatomical studies and sensory research.
Scientists at Johns Hopkins University in Baltimore, Maryland, are conducting CT and MRI scans on two narwhal heads, one male and one female. And a dissection team, led by James Mead of the Smithsonian Institution, will convene in January.
Still unanswered, researchers say, are fundamental questions about how and why the tusk evolved.
"There's a difference, but why is there a difference?" Eichmiller said. "That's the part that is the most intriguing."
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