Friday, November 25, 2011

Picture of the Indiscriminate Interval #000007 - Narwhal

Monodon monoceros at the American Museum of Natural History.
Narwhals are a bit strange even by cetacean standards. I'll let the title of this Tet Zoo article speak for itself: "A 3-m tooth that can bend 30 cm in any direction and is hypersentitive to salinity, temperature and pressure... and the sonic lance hypothesis".


They get weirder. Without the tooth (or sometimes, teeth) it is difficult to picture how this flat-skulled cetacean could be the same as a bulbous-headed Narwhal. As brought up in my Dorudon post, there are colossal amounts of soft tissue involved.


Another soft-tissue feature not hinted at by the skeleton are unusually shaped flukes... in males. Fontanella et al. (2010) suggest that the concave leading edge and lack of sweepback of the flukes increases lift and thrust to compensate for the drag caused by the tusk in males. The implications of the occasional tusked female narwhal were not discussed by the authors.


One female Narwhal was estimated to be 114.8 (± 10.2) years old (Garde et al. 2007) which, if correct, would make Narwhals the third oldest known mammals after humans (122 years) and Bowhead Whales (211 years?). Garde et al. (2007) used a sample of 75 individuals (15 juvenile) from a heavily hunted population, and subsequently speculated that Narwhals in other populations could potentially reach "considerably higher" ages. As for methodology, Garde et al. (2007) used aspartic acid racemization rate in the eye; this method was also used to calculate the extreme age estimate for Bowheads (George et al. 1999), and ages of over a hundred years have subsequently been supported by bomb lance fragments (George and Bockstoce 2008) and ovarian corpora counts (George et al. 2011). So it looks probable that aspartic acid racemization does not provide grossly inaccurate estimates of old age - why would Narwhals live to be centenarians? Garde et al. (2007) note that Narwhals and Bowheads are both year-round Arctic residents and speculate that their extreme longevity is an adaptation to drastic changes is climate. While an interesting idea, there does not seem to be much data available on cetacean longevity (Table 2 in Garde et al. 2007 has only 12 out of ~80 species) and Narwhals are apparently not far older than other cetaceans (for instance, Orcas apparently live to be 90). It could be possible that further investigation into cetacean longevity will reveal that lifespans of over a hundred years are perfectly normal.



References:

Fontanella, J. E., Fish, F. E., Rybczynski, N., Nweeia, M. T., & Ketten, D. R. (2010). Three-dimensional geometry of the narwhal (Monodon monoceros) flukes in relation to hydrodynamics. Marine Mammal Science 27(4), 889-898. Available.

Garde, E., Heide-Jørgensen, M. P., Hansen, S. H., Nachman, G., and Forchhammer, M. C. (2007). Age-specific growth and remarkable longevity in Narwhals (Monodon monoceros) from West Greenland as estimated by Aspartic Acid Racemization. Journal of Mammalogy 88(1), 49-58. Available.

George, J. C., Follmann, E., Zeh, J., Sousa, M., Tarpley, R., Suydam, R. Horstmann-Dehn, L. (2011). A new way to estimate the age of bowhead whales (Balaena mysticetus) using ovarian corpora counts. Canadian Journal of Zoology 89(9), 840-852. doi: 10.1139/z11-057

George, J. C., and Bocktoce, J. R. (2008). Two historical weapon fragments as an aid to estimating the longevity and movements of bowhead whales. Polar Biology 31(6), 751-754. Available.

George, J. C., Bada, J., Zeh, J., Scott, L., Brown, S. E., O'Hara, T., & Suydam, R. (1999). Age and growth estimates of bowhead whales (Balaena mysticetus) via aspartic acid racemization. Canadian Journal of Zoology 77, 571-578.

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