Mysteriet med världens längsta tand på väg mot sin lösning

Dental24: En forskargrupp ledd av tandläkaren Martin Nweeia har under ett tiotal expeditioner till Arktis arbetat med att försöka lösa gåtan med narvalens uppseendeväckande långa tand. Frågan är vilken funktion och nytta som narvalen har av sin 2 ½ – 3 m långa tand? Denna gåta som under de senaste 500 åren har förbryllat vetenskapen är nu på väg att få sin lösning tack vare Martin Nweeia och hans forskarteam.

Dr Martin Nweeia är principal investigator vid Narwhal Tusk Research, www.narwhal.org. Han har under 11 arktiska expeditioner lett och samarbetat med ett team för att upptäcka syftet och funktionen med narvalarnas betar.  Dr Nweeia undervisar vid Harvard School of Dental Medicine och är forskningsassistent vid institutionen för Vertebrate zoologi vid Smithsonian Institution och Institutionen för Mammalogy vid Harvard Museum of Comparative Zoology. Han driver också en privat tandläkarpraktik på heltid i Sharon, CT, USA. Martin Nweeia är okså en Sverigevän som besökt landet många gånger och har många vänner här.

Läs Dr Martin Nweeias berättelse om forskningen kring narvalens mytiska tand:

“The erupted tusk of the male narwhal is without doubt the most extraordinary tooth in nature. Among its distinguishing attributes are the long straight axis, the left-handed spiral, and variation in its expression that defies evolution. Though most mammals have a mirrored expression in both size and shape down the midline axis, the narwhal couldn’t be more different. Take, for example, the most common expression of the adult male narwhal. On the left side is a two-and-a-half to three-meter tusk and on the right side an embedded tusk in the skull that is less than a half-meter. It gets stranger. In rare expressions of two tusks, not only is the left tusk usually longer but the morphology of the right tusk, instead of being mirrored, is also left-handed. These are the only examples of such difference in size and shape of teeth in mammals in all of nature. But the story gets better.

Dr. Nweeia, här i svart våtdräkt, håller i en hjärtövervakare med fem elektroder fästa under huden på en infångad narval, samtidigt som olika lösningar med hög salthalt och färskvatten sprutas in i beten. Valen släpptes fri efter ca 20 minuter.

When we compare male and female narwhal, they are unusually different. In most mammals male and female teeth do not significantly differ in size and shape within a species. Yet narwhal couldn’t be more different. Females typically have two embedded tusks very close in size and shape. Males, on the other hand, have one giant tooth on the left hand side and one disproportionately small-sized tooth on the right side. There is no description of asymmetry for mammals in the literature that explains this difference. As if the story couldn’t get any better, now let’s look at just plain old odd expression of teeth. Okay. First we have the only spiraled tusk in nature. Second, we have a horizontally impacted tooth that for this animal is normal. Third, in addition to the horizontal impaction, the tooth pierces through the upper lip. Now that we’ve given you some background on the tooth, let’s try to understand its functional significance. This has been the focus of research for my work with the Narwhal Tusk Research and a question that has perplexed scientific inquiry for the last 500 years. It’s no wonder why. Here’s an animal that eats large fish, most of which can swim faster than it can. It is also an animal that has eight pairs of teeth, six of which are genetically silenced at birth. Makes sense, right? After all, if you’re eating large fish what better choice could you make than to silence all of the teeth that might help you bit and chew your food. Much better to have an eight-foot tusk sticking out in the middle of the freezing ocean. Now let’s look inside the narwhal’s mouth. Hmm. No teeth.

Many biologists would have us believe that narwhals take this big giant tooth and go fencing with their buddies to win over the hearts of their women. Though there are precedents set for other forms of tissue, most notably horns or other single-layered tissue forms, used in aggressive male-male rivalry, there are not an abundance of similar examples for the use of teeth as a primary function in aggressive intraspecific fighting. The narwhal tusk is likely up to something different. When our team examined erupted tusks by scanning electron microscopy, we observed millions of orifices on the surface. So many, in fact, that over a two-and-a-half meter tusk there were an estimated six million of these openings. Stranger still was the fact that these openings were patent and in direct communication to the pulp of this tusk. Add to this confusion that unlike many tusks that have reparative ability, the narwhal tusk has none. So indeed, if these animals are fighting it out to break each other’s tusks, the loser faces dire consequences.

From our preliminary work it is evident that the narwhal tusk possesses sensory capability, and that its association primarily among males makes us believe that it plays a role in sexual selection, though it may have either a newly developing or phasing out of such function for the species as a whole. Approximately 15 percent of females also possess a tusk, and there are some males that have no tusk. Understanding all the variants in tooth expression and the unusual microanatomic structures, we have come to understand that all of these possibilities may be true. Behavioral observations for this species are difficult at best, given the harsh arctic environment and the limited time for observation.

Each year I am part of a large collaborative team of usually 10-15 support and research field workers interested in this extraordinary animal. We catch and release narwhals to help establish migration patterns through satellite tagging and my work is completed during the application of the satellite tag. All the animals are carefully monitored by an on-site veterinarian and any animals stressed or compromised are immediately set free. Over the 12 years I have been part of these teams, we have examined approximately 70 whales. It is a privilege to witness their behavior and understand them in their natural surroundings. One would wonder why the need to create a mythical creature like the unicorn when there is one living that is more extraordinary. Our studies of this whale will continue for many years to come, and the unusual stories unfolded, or better yet unwound.”

By Dr. Martin Nweeia

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Dr. Martin Nweeia is principal investigator of Narwhal Tusk Research, www.narwhal.org.  He has led and collaborated on 11 High Arctic Expeditions to discover the purpose and function of the narwhal tusk. Dr. Nweeia teaches at the Harvard School of Dental Medicine and is a research associate in the Department of Vertebrate Zoology at the Smithsonian Institution and Department of Mammalogy at the Harvard Museum of Comparative Zoology. He has a full time private practice in Sharon, CT, USA.