The Evolutionary Significance of the Narwhal’s “Tusk”

Contributed by Madeline Haley and Melissa Querrey

First, a short introduction to narwhals by yours truly.

https://www.youtube.com/watch?v=sCf7XQsdNtk&feature=youtu.be

The narwhal, or Monodon monoceros, is a cetacean mammal that inhabits the Arctic waters and is most commonly recognized for its large “tusk”, which closely resembles the horn of the mythical unicorn. Contrary to popular belief, this “tusk” is actually a modified tooth that forms during development from a pair of tooth buds and projects outward from the maxilla, or upper jaw. While both males and females can grow tusks, males tend to have tusks more often than females.

There has been much debate among researchers about the true function of the narwhal’s tusk. It was initially thought that the tusk was only used as an evolutionary means of self-defense and breaking the ice that covers the surface of their aquatic habitats so breaths of air can be taken. However, recent study of the anatomy of the tusk by Nweeia and colleagues revealed nerves that lead directly to the brain, giving evidence of its additional function as a sensory organ.This sensory feature serves several purposes to the narwhal by detecting changes in the external environment, such as salinity and temperature. Because these functions of the narwhal’s tusk increase its chances of survival and are retained in the population, it can be said that they are a result of natural selection.

Additionally, secondary functions of the tusk have developed due to sexual selection, which have facilitated the tusk’s persistence. Based on the discovery of broken tusk fragments and scarring, it can be inferred that male narwhals use their tusk in an aggressive fashion in order to assert sexual dominance and eventually find a mate.

While the narwhal’s tusk may seem like an obnoxious physical display, it is clear that evolutionary forces of both natural and sexual selection have driven the species to utilize its tusk in a way that enables its survival and overall individual and reproductive fitness.

Finally, check out this awesome video about narwhals.
https://www.youtube.com/watch?v=ykwqXuMPsoc

And, for more information:

Palsboll, P.J, Heide-Jorgensen, M.P, & R. Dietz. 1997. Population structure and seasonal movements of narwhals, Monodon monoceros, determined from mtDNA analysis. Heredity 78: 285-292.

Nweeia, M. T., Eichmiller, F. C., Hauschka, P. V., Donahue, G. A., Orr, J. R., Ferguson, S. H., Watt, C. A., Mead, J. G., Potter, C. W., Dietz, R., Giuseppetti, A. A., Black, S. R., Trachtenberg, A. J., & Kuo, W. P. 2014. Sensory ability in the narwhal tooth organ system. The Anatomical Record, 297: 599–617.

Nweeia, M.T., et al. 2009. Considerations of anatomy, morphology, evolution, and function for narwhal dentition. The Anatomical Record 295, 6: 1006-1016.

Silverman, H. B., & M. J. Dunbar. 1980. Aggressive tusk use by the narwhal (Monodon monoceros L.). Nature 284.5751: 57-58.

Brear, K., et al. 1993. The mechanical design of the tusk of the narwhal (Monodon nonoceros: Cetacea). Journal of Zoology 230.3: 411-423.

Mirceta, S., Signore, A.V., Burns, J.M., Cossins, A.R., Campbell, K.L., & Berenbrink, M. 2013. Evolution of Mammalian Diving Capacity Traced by Myoglobin Net Surface Charge. Science 14: 1234192

“Narwhals.” Narwhals. National Geographic, n.d. Web. 18 Apr. 2014.<http://video.nationalgeographic.com/video/narwhals?source=relatedvideo>.

 

 

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