Dr. Martin Nweeia Studies an Enigma

Martin Nweeia, a clinical instructor in the Department of Restorative Dentistry and Biomaterials Sciences, has been closer to narwhals than most of us. So close, in fact, that he has placed electrodes connected to electrocardiograms and electroencephalograms on these whales as part of his research into the purpose and function of the narwhal’s single tusk—a mystery that has eluded scientists for more than 500 years.

Harvard School of Dental Medicine

Spring 2013

Volume 73, Number 1

Narwhals, elusive creatures whose closest relative is the beluga whale, live in the Arctic. Each year, Nweeia travels to the northern tip of Baffin Island in Canada, donning a dry suit in 36-degree waters to conduct novel experiments to learn about the narwhal tusk. He has led four expeditions to the Canadian High Arctic and collaborated on eight additional expeditions with the Arctic Research Division of Canadian Fisheries and Oceans, braving polar bear encounters, 120-mile-per-hour winds in an Arctic hurricane, and a near-fatal aircraft landing to study this unusual whale.

The narwhal’s tusk is a hollow spiral canine tooth that projects from the left side of the male animal’s upper jaw. Tusks can extend up to 10 feet long in an adult. “If you had to choose the most extraordinary tooth in nature to study, this would be it,” says Nweeia. “There is so much about its expression, anatomy, and evolution that just doesn’t make sense.”

Research Implications

Nweeia and his team conduct experiments on the narwhals (which are captured and then released) that show a connection between an introduced stimulus and a physiological response, including brain activity and heart rate. “We have been successful in attaining positive heart-rate responses, as narwhal heart rate increases significantly when high salt solutions are introduced and likewise relaxes when fresh water is introduced,” says Nweeia. “This makes sense from an oceanographic perspective, as high salt concentrations would normally be associated with rapid ice formation—the only natural way high salt areas would exist. In such cases, this condition is an immediate threat to the narwhal and naturally would be associated with an increased heart rate.”

A positive correlation between increased heart rate and introduced sensory stimulus demonstrates that the tusk is capable of sensing its environment. It is an excellent in vivo model for the Brannstrom hydrodynamic theory of mammalian teeth—dentinal tubules conduct sensory information through interstitial fluid conduction to odontoblasts at the dentin- pulpal interface and to sensory nerve–associated tissue in the pulp, and then conduct information to the brain via the maxillary branch of the fifth cranial nerve.

If you had to choose the most extraordinary 

tooth in nature to study, this would be it.”

—Martin Nweeia

The research is first focused on the function of the tusk. “We are perplexed at a dental organ system that in its embryological development has eight pairs of teeth and then genetically silences them at birth and uses its tooth-forming ability to generate this large single tusk. Why?” asks Nweeia.

“There must be some compelling reason, especially since the narwhal eats some pretty large fish. Why not have  a set in the mouth used for biting and chewing?” Of the eight pairs, six are silenced and one of the two remaining pairs forms the tusks; in females, the tusks are embedded horizontally in the upper jaw, and in males, the right side is embedded and the left erupts to the full tusk. The final remaining pair form vestigial teeth, which have been shown by Nweeia’s work to have no functional significance.

The narwhal dentin is another area of interest, particularly the tissue’s uncharacteristic combined strength and flexibility. “We are looking at this tissue for possible future restorative uses,” explains Nweeia, “as these are qualities one seeks in developing good restorative materials.”

The researchers are also studying the adaptation of this organ system in relation to all modern odontocetes (toothed whales), as whales in general have adapted unusual mechanisms to develop and express teeth. The strap-toothed whale, for example, has two teeth in its lower jaw that wrap around the upper jaw, preventing it from opening.

In addition, notes Nweeia, “we have few links in evolution to inform us how such structure develops, so the more we can discern about the narwhal tusk’s expression, the more we can piece together parts of this missing evolution.”

Finally, the narwhal may provide some information on genetic silencing in teeth that can be actively studied and thus help unlock some of the mysteries of uncontrolled growth, such as cancer cells. “There is no known mechanism for turning such tooth buds off,” says Nweeia, “and studies would assist such work. Our research is not involved with this latter aspect, but is certainly one that deserves attention.”

Nweeia’s research is funded by two divisions of the National Science Foundation (NSF), one in neuroscience and the other in polar research. His work has attracted international attention, as he combines Inuit traditional knowledge with the basic sciences. Nweeia recently led a symposium at the Inuit Studies Conference, hosted by the Smithsonian Institution, that focused on methods of combining traditional knowledge with science, which brought leading researchers in arctic science together. His study is one of the models cited by NSF for inclusion of traditional knowledge and science. In a recent article in Scientific American, Nweeia describes how one Inuit hunter’s observation of narwhal molting in fresh water inlets can change the scientific understanding of this unusual marine mammal and overshadow, change, and add to scientists’ perceptions.

Nweeia and his team have received significant media coverage of their work investigating the function of narwhal tusks. A new BBC series, David Attenborough’s Natural Curiosities, featured Nweeia’s findings on the tusk in a show that first aired in February on the Eden channel. Also recently released was a new book, Narwhals: Arctic Whales in a Melting World, by Todd McLeish. The book features Nweeia’s research, assisted by collaborating teams of Harvard researchers, including Peter Hauschka, an associate professor of developmental biology at HSDM and Boston Children’s Hospital, and Winston Kuo, an assistant professor of developmental biology at HSDM and director of the Harvard Catalyst Laboratory for Innovative Translational Technologies. Nweeia was named a National Geographic Explorer of the Week in September 2012.

In addition to his work at HSDM, Nweeia is a research associate in the Department of Vertebrate Zoology at the Smithsonian Institution and the Museum of Comparative Zoology at Harvard University. “I’m just a curious kid who found a fascinating dental organ system to study,” he says. “With a great team of researchers, many of them at Harvard, we have persisted over 12 years of hard work to unlock some of the mysteries of this wonderful cetacean and its extraordinary tusk.”