Narwhal Tusk

Over winter break, I was exploring the curiosities on Wikipedia, which is where I came across this picture of a narwhal skull with two tusks (http://upload.wikimedia.org/wikipedia/commons/thumb/a/a7/Narwalschaedel.jpg/140px-Narwalschaedel.jpg). Since then, I have done some investigating on narwhals (Monodon monoceros). Most narwhals, both male and female, have only one tusk, which is actually a tooth that extends from the left side of the narwhal's skull. The right side is usually bare of teeth. However, some narwhals develop differently and grow only a tusk that protrudes from the right side. Even stranger, some narwhals develop both tusks, one on the right and one on the left. This leads to a specimen like that in the image below. Male narwhals will rub tusks with other male narwhals during mating season, possibly as a display of dominance. Some researchers claim that the tusk acts as a sort of sensory organ, which I presume is much like a cat's whiskers only much less delicate.

I have tried to find research that may explain why some narwhals develop one tusk while others develop two. As of yet, I have found nothing. However, I came across research performed by Nweeia et al. (2012) that examines the tusks in-depth. There has been some debate as to whether the narwhal's teeth are best described as incisors or canines. I admit that they appear to be both. Nweeia et al. conclude that the narwhal's tusks are "surrounded by maxillary bone over the entire length of their bone socket insertion, and are thus more accurately termed caniniform or canine teeth." Furthermore, Nweeia et al claim that the wide range of phenotypes among narwhal tusks suggest that the development of the narwhal's tusks are "no longer guided by function but rather by random germ cell differentiation."

This still leaves me wondering exactly what determines whether a narwhal will have a left or a right tusk (or both). Could it be genetic? Must something happen to the narwhal during its development either as a young cetacean or as an embryo? Is this at all similar to people who are left-handed or right-handed? I guess I still don't have a clue, and I will probably just keep wondering.

Also, just for fun, I have included an image of "The Avenging Narwhal Play Set (with 4 Magical Tusks and 3 Adorable Animals to Impale)" (http://tommcmahon.typepad.com/photos/uncategorized/2008/05/05/narwhal2.jpg).

External links: http://onlinelibrary.wiley.com.ezproxy.morris.umn.edu/journal/10.1002/(ISSN)1932-8494 http://wwf.panda.org/what_we_do/where_we_work/arctic/wildlife/whales/narwhal/

The Avenging Narwhal Play Set

Two-tusked narwhal skull.