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Dental scientist probes mysteries of narwhal's 'unicorn' tooth

April 07, 2014

By Craig Palmer, ADA News staff

Navigating the icebergs: Narwhals surface during spring migration outside Pond Inlet, Nunavut, Canada. Photo by Glenn Williams
Boston—Dr. Martin Nweeia and research colleagues cite new evidence of how "the most extraordinary tooth in nature" interprets its icy Arctic environment for the narwhal, the near mythological whale with the spiraling tusk that is a tooth 6 to 9 feet long.

But what use, that tooth of unicorn measure?

Dr. Nweeia views his explorations on the nature of teeth as relevant to the practice of dentistry. "The unusual properties of the tusk's microanatomy and expression give us a new perspective of teeth," he said of his narwhal tooth anatomy study published by the journal The Anatomical Record and first posted online March 18.

"In my own dental practice, I am always communicating how unusual and sensory our teeth are in function. We all tend to get this passive sense of teeth as instruments used in biting and chewing and often forget their tissue origins and abilities as sensory organs."

Speaking of sensory organs, Dr. Nweeia's narwhal research (visit narwhal.org for more information) reveals the sensory pathway between tooth and brain of the Arctic whale. The narwhal tooth system is a hydrodynamic sensor capable of detecting particle gradients, temperature and pressure and is able to detect high salt and fresh water gradients, the dental scientists reported.

Field work: Dr. Nweeia, left, and an Inuit hunter finish an experiment in Admiralty Inlet, Artic Bay, Canada. Photo by Gretchen Freund
Dr. Nweeia practices general dentistry in Sharon, Conn., teaches part time at the Harvard School of Dental Medicine and tracks narwhals in their native habitat between the Arctic Circle and the North Pole in search of understanding the function of that distinctive tusk/tooth. Research takes him to the northern tip of Baffin Island in the Canadian territory of Nunavut, where he perches on ice floes or does field sampling from shore-based camps, donning a dry suit to wade in 36-degree water, braving 120-mile winds and watching warily for polar bears.

"Animal health and safety is always the primary concern, and all experiments cease and the animal is released at the first sign of overt stress when indicated by the monitoring veterinarian," the scientists reported. "Fortunately, none of the animals exhibited problems requiring early release, and most experiments, tagging and specimen collections were completed within 30 minutes."

Dr. Nweeia's team found nerves, tissues and genes in the narwhal tusk pulp that are known for sensory function and that help connect the tusk to the brain, said a Harvard news release. Armed with this new model, Dr. Nweeia needed to confirm that sensory information is actually transmitted along this pathway to the brain from the tusk in living narwhals.

The research team tested the hypothesis by slipping a "tusk jacket"–a clear tube sealed with foam at either end—onto several narwhal off Baffin Island, sloshing high- or low-salt water through the tube and over the tusk in separate tests. The scientists measured changes in heart rate and found significant changes depending on water salinity. Why salinity matters is another question for further research, though Dr. Nweeia advances several hypotheses, sexual attraction and food location among them.

"Imagine: Exploration, wonder and mystery are all wound up in this magnificent spiraled tusk and sensory organ," he said. "This is the first tooth that has been shown by in vivo testing to have sensory function to a normal variable in its environment that is not necessarily associated with a flight or fright reaction."

"Traditional knowledge has unlocked many of the mysteries our team has searched for," Dr. Nweeia told the ADA News. "In the 10-year parallel study of Inuit traditional knowledge, collected from communities in the High Arctic of Western Greenland and Eastern Nunavut in Canada, there have been keen insights on behavior, migration, anatomy and morphology. The observations of hunters and elders in these Arctic areas have been invaluable in our work.

"My favorite illustration of the view from science and traditional knowledge lies in the definition of 'environment,'" he said. "For the Inuit, the definition is 'everything beyond your heart."

The 2014 paper, "Sensory ability in the narwhal tooth organ system," uses anatomy, histology, genetics and neurophysiology to "add to the discussion of the functional significance of the narwhal tusk," the research abstract said. "The combined evidence suggests multiple tusk functions may have driven the tooth organ system's evolutionary development and persistence."

This study was funded by National Science Foundation grants with additional support from the Harvard School of Dental Medicine, the Museum of Comparative Zoology at Harvard, the Smithsonian Institution, the Explorers Club, the National Geographic Society, Castle & Harlan Inc., Natural Sciences and Engineering Research Council of Canada, Department of Fisheries and Oceans Canada and the Nunavut Wildlife Management Board.

See related story here.