Yet Another Move

I don't know if anybody out there is still following, but I am now writing at The Lord Geekington (WordPress) and Biological Marginalia (tumblr). Some of these articles are getting very old and well overdue for revisiting; I'll put notifications on top when and if that happens.

A Change of Scenery

After blogging on Blogger since 2006 I've decided to give WordPress a try. There is a lot of juvenile baggage on this page and I think a move will help me distance from that and try and become a better and above all more consistent blogger. Anyways, the primary purpose of this post is to announce my first article on a new blog:

Lophenteropneusts and Beyond

Feresa: The Growling Wolf-Dolphin

The dolphin Feresa attenuata has been bestowed with dreadfully stupid common names. Feresa has been recognized as distinct from Orca since Gray (1871), which makes "Pygmy Killer Whale" both inappropriate and archaic. The alternative "Slender Blackfish" is actively misleading as the superficially similar Pseudorca is more slender (Reeves et al. 2002) and well-lit color photographs in Rossi-Santos et al. (2006) show that the species is actually brown, contra every illustration. I'll be calling the dolphin "Feresa" from here on out (I'm also not too fond of  "attenuata") as my alternate suggestion in the title is a tad verbose.

Feresea... maybe. This species can be distinguished from Pseudorca by having a proportionally larger dorsal fin (2 base lengths away from the blowhole vs. 2.5) and by having a clearly demarcated cape; Peponocephala can be distinguished by having pointed flipper tips, a pointed head when viewed from above, and no white extending around the face (Baird 2010). I think this is the case in the above photo, but I'm not entirely certain. Photo by Gary L. Friedrichsen from WoRMS.

Feresa is one of the most poorly-known toothed whales (McSweeney et al. 2009) and single sightings or strandings are still viewed as deserving publication (Baird 2010). Prior to 1954, the species was known from only two skulls (Reeves et al. 2002), making it extremely poorly-known even compared to beaked whales. What makes this absolutely shocking is that Feresa is not a cryptic species. They are known from the tropics and subtropics worldwide, are easy to detect in visual surveys, do not take extended dives, and (contra Leatherwood et al. 1982) do not avoid vessels (McSweeney et al. 2009). While their surface behavior is normally subdued compared to other dolphins, they have been observed jumping high above the surface and even riding on bow waves (Reeves et al. 2002). It appears that while a deep-water habitat and confusion with Pseudorca and Peponocephala can explain the lack of observations to a degree, the main factor is probably the species being rare (McSweeney et al. 2009).

In 1965 - a little over a decade after the external appearance of the animal became known - Feresa was held in captivity. Pryor (1991) remarked that one individual behaved "more like a wolf than a normal dolphin" would "growl and snap like as canid" and "not hesitate to attack people and other cetaceans". Since when are cetaceans capable of growling? This behavior has led some to presume that Feresa preys on mammals in the wild (Leatherwood et al. 1982) and aggression towards other dolphins has been observed whilst individuals were trapped in tuna seines (Reeves et al. 2002). Considering that both situations occurred in cramped and undoubtedly stressful environments, I think it is completely unfounded to conclude that Feresa is a pugnacious marine mammal-killing macropredator with the available evidence. Stomach contents have included squid and fish (Rodríguez-López and Mignucci- Giannoni 1999; Zerbini & Santos 1997)

Feresa skeleton. From Wikipedia Commons.

The skeleton of Feresa does appear superficially Orca-like, however, it is not a particularly close relative, hence my strong dislike of the "Pygmy Killer Whale". There is some disagreement as to how closely they are related; Slater et al. (2010) places Orcinus (and Orcaella) as the most basal delphinids, however Vilstrup et al. (2011) consider both to be both members of the clade Globicephalinae, but with Orca as the most basal member and Feresa as a derived member and close relative of Peponocephala and Globicephala. This seems like a very interesting group, and perhaps I'll give it some more coverage.


References:

Baird, R. W. (2010). Pygmy Killer Whales (Feresa attenuata) or False Killer Whales (Pseudorca crassidens) Identification of a Group of Small Cetaceans Seen off Ecuador in 2003. Aquatic Mammals 36(3), 326-327. Available.

Gray, J. E. (1871). Supplement to the Catalogue of seals and whales in the British Museum. Available.

Leatherwood, S., Reeves, R. R., Perrin, W. F., & Evans, W. (1982). Whales, dolphins, and porpoises of the eastern north pacific and adjacent arctic waters (NOAA Technical Report NMFA Circular 444). Washington, DC: U.S. Department of Commerce, National Oceanic and Atmospheric Administration, National Marine Fisheries Service. Partially Available.

McSweeney, D. J., Baird, R. W., Mahaffy, S. D., Webster, D. L., and Schorr, G. S. (2009). Site fidelity and association patterns of a rare species: Pygmy killer whales (Feresa attenuata) in the main Hawaiian Island. Marine Mammal Science 25(3), 557-572. Available.

Pryor, K. (1991). Mortal remains: Studying dead animals. In: Pryor, K. & Norris, K. S. (eds.) Dolphin Society: Discoveries and Puzzles. University of California Press: Berkeley. Available.

Reeves, R. R., Stewart, B. S., Clapham, P. J., & Powell, J. A. (2002). National Audubon Society Guide to Marine Mammals of the World. Alfred A. Knopf: New York.

Rodríguez-López, M. A. & Mignucci-Giannoni, A. A. (1999). A stranded pygmy killer whale (Feresa attenuata) in Puerto Rico. Aquatic Mammals 25(2), 119-121. Available.

Rossi-Santos, M., Baracho, C., Neto, E. S., & Marcovaldi, E. (2006). First sightings of the pygmy killer whale, Feresa attenuata, for the Brazilian coast. JMBA2 - Biodiversity Records. Available.

Slater, G. J., Price, S. A., Santini, F., and Alfaro, M. E. (2010). Diversity versus disparity and the radiation of modern cetaceans. Proceedings of the Royal Society B 277(1697), 3097-3104. Available.

Vilstrup, J. T., Ho, S. Y. W., Foote, A. D., Morin, P. A., Kreb, D., Krützen, M., Parra, G. J., Robertson, K. M., de Stephanis, R., Verborgh, P., Willerslev, E., Orlando, L., & Gilbert, M. T. P. (2011). Mitogenomic phylogenetic analyses of the Delphinidae with an emphasis on the Globicephalinae. BMC Evolutionary Biology 11(65). Available.

Zerbini, A. N. & Santos, M. C. O. (1997). First record of the pygmy killer whale Feresa attenuata (Gray, 1874) for the Brazilian coast. Aquatic Mammals 23(2), 105-109. Available.

Eocetus, "Eocetus", and Friends

Update (January 28, 2014): "Eocetus" wardii is now Basilotritus wardii. More on my new post, The Third King.


I was shocked that Uhen (2010) remarked that Basilosaurus drazindai and Basiloterus hussaini "probably represent protocetids... akin to Eocetus". This would place the whales outside Pelagiceti and imply that the now-questionable basilosaurids were potentially capable of walking on land, despite being enormous. Unfortunately, other mentions of this revised placement give no further details (Uhen 2008, Uhen et al. 2011) and Uhen (2010) further states the placement is "difficult to determine with certainty" due to scarce materials. I suspect the hypothesis will not be officially discussed until further material is found and/or described... which won't stop me from wildly speculating.

Lumbar vertebrae in right lateral view. From left to right: "Eocetus" wardii (from Uhen 1999), Basiloterus hussaini, and Basilosaurus drazindai - note that the latter-most may be an anterior caudal (from Gingerich et al. 1997). For comparison: Basilosaurus isis vertebrae.

In the description of Basilosaurus drazindai, Gingerich et al. (1997) note a number of "primitive retentions" which resemble the morphology of "generalized archaeocetes": long neural spine and arch; broad, almost-horizontally placed, anterior-projecting metapophyses which project beyond the anterior edge of the vertebral centrum; and paired, posterolateral processes of the neural arch. Aside from the last trait (which I can't confirm without a dorsal view), all of these traits are present in "Eocetus" (Uhen 1999). Additionally, "Eocetus" has elongated transverse processes, unlike the condition of Basilosaurus (Uhen 1999); however, B. drazindai has processes with a 15.5 cm long base (they broke off) relative to the 30 cm centrum (Gingerich et al. 1997), and so probably had a similar, albeit slightly less extreme, condition. The only criterion for placing B. drazindai in the genus Basilosaurus was the size and shape of the centrum (Gingerich et al. 1997), and while they are uncannily similar in shape, everything else seems to be pointing towards a relationship with "Eocetus".

Lumbar vertebrae in anterior view. Ditto order.

As for awkward middle-child Basiloterus, it appears to have a centrum which is slightly more elongated than that of "Eocetus", however the neural arch and maybe the neural spine appear to be narrower. The metapophyses are upwardly-angled (Gingerich et al. 1997), less broad, less anterior-projecting, but still appear to extend past the centrum. The posterolateral processes are absent (Gingerich et al. 1997). The base of the transverse process is 9.3 cm long relative to a 19.5-20 cm centra (Gingerich et al. 1997), proportionally similar to Basilosaurus drazindai. The placement of Basiloterus is thus not clear, and perhaps it was a basilosaurid or an even more derived protocetid.

Maiacetus inuus, a basal "protocetid" (Uhen 2011). From Wikipedia Commons.

Protocetidae is a blatantly paraphyletic "family" of extinct cetaceans from Eocene coastal marine deposits with hip and femur morphology indicating amphibious capabilities (most of the time) and no evidence of flukes (Uhen 2010). Uhen (1999) appears to think that "Eocetus" wardii had weight-bearing hips, however Uhen (2010) refers to them as "moderately reduced" and regarded the species as possibly non-amphibious. This is perhaps not surprising since Eocetus, "Eocetus", and an unnamed Pisco Formation species are the sister group of Pelagiceti (Uhen et al. 2011). This could make them closer relatives of Dorudon than Maiacetus, and raises the question of how many typical protocetid traits they actually exhibited. Perhaps they were entirely aquatic tail-based swimmers which just happened to have fairly large vestigial legs.

Dorudon atrox. From Wikipedia Commons.

The scare quotes around "Eocetus" hint at a taxonomic misadventure. "E." wardii was assigned to its genus by Uhen (1999) based on comparisons of its skull and vertebrae to Eocetus schweinfurthi; the problem is, the holotype of E. schweinfurthi is an isolated skull and it is not possible to determine whether the vertebrae referred to it actually represent the species (Geisler et al. 2005). There is overlapping skull material (Uhen 1999), but Geisler et al. (2005) apparently regarded it as too incomplete to warrant unambiguous placement in the genus. Somehow, "Eocetus" and Eocetus formed a clade in phylogenetic analyses (Geisler et al. 2005, Uhen et al. 2011), making it probable that future discoveries will confirm their close relationship.

"Eocetus" wardii is clearly related to unnamed Pisco Formation material which exhibits the same distinctive traits (moderate centrum elongation, elongated neural arches and spines and transverse processes, strange pock-marked texture) with the only difference being that the unnamed material is 35% smaller (Uhen et al. 2011). The Egyptian vertebrae dubiously assigned to Eocetus schweinfurthi (figured in Uhen 1999) also seem quite similar (including the pock-marks), and if it is also a member of this clade, it would indicate a sizable trans-oceanic range. This in turn could be taken as evidence of the whales being largely pelagic... of course this is quite speculative.

There of course remains much to be known about these cetaceans, and perhaps future discoveries will be enlightening as to how similar they were to the pelagic cetaceans, as well as the origins of Pelagiceti. I really hope it turns out that a Basilosaurus-sized animal could walk on land.


References:

Geisler, J. H., Sanders, A. E., and Luo, Z-X. (2005). A New Protocetid Whale (Cetacea: Archaeoceti) from the Late Middle Eocene of South Carolina. American Museum Novitates 3480, 1-65. Available.

Gingerich, P. D., Arif, M., Bhatti, M. A., Anwar, M., & Sanders, W. J. (1997). Basilosaurus drazindai and Basiloterus hussaini, new Archaeoceti (Mammalia, Cetacea) from the middle Eocene Drazinda Formation, with a revised interpretation of ages of whale-bearing strata in the Kirthar Group of the Sulaiman Range, Punjab (Pakistan). Contributions from the Museum of Paleontology, University of Michigan 30 (2), 55-81. Available.

Uhen, M. D., Pyenson, N. D., Devries, T. J., Urbina, M., and Renne, P. R. (2011). New middle Eocene whales from the Pisco Basin of Peru. Journal of Paleontology 85(5), 955-969. doi: http://dx.doi.org/10.1666/10-162.1

Uhen, M. D. (2010). The Origin(s) of Whales. Annual Review of Earth and Planetary Sciences 38, 189–221. Available.

Uhen, M. D. (2008). Basilosaurids. In: Perrin, W. F., Würsig, B., and Thewissen, J. G. M. (eds.) Encyclopedia of Marine Mammals, Second Edition. Elsevier: Burlington, Massachusetts. Available.

Uhen, M. D. (1999). New Species of Protocetid Archaeocete Whale, Eocetus wardii (Mammalia: Cetacea) from the Middle Eocene of North Carolina. Journal of Paleontology 73(3), 512-528.

Weems, R. E., Edwards, L. E., Osborne, J. E., and Alford, A. A. (2011). An occurrence of the protocetid whale "Eocetus" wardii in the Middle Eocene formation of Virginia. Journal of Paleontology 85(2), 271-278. Available.

Billfish Bills - What Are They Good For?

In the prior article, I discussed long-beaked "dolphins" (Eurhinodelphidae) and noted that I couldn't find hypotheses on the function of their uneven jaws in the literature... aside from a weird proposal involving Skimmers. The Theatrical Tanystropheus mentioned a couple ("digging for small, sand-dwelling organisms or as a bat with which to stun fish") which are plausible, but I don't know where they are from or what lines of reasoning are behind them. There are extant species with a superficially similar condition - billfishes - and it could be relevant to review what they do with their bills.

Atlantic White Marlin (Tetrapturus albidus) from Wikipedia Commons.

Swordfish (Xiphiidae) and Marlins/Sailfishes/Spearfishes (Istiophoridae) are living sister taxa¹ in the clade Xiphioidea; while traditionally included in Scombroidei, billfishes are presently regarded as phylogenetically distinct (Orrell et al. 2006) and possibly close relatives of jacks and... flatfishes (Little et al. 2010). Fish phylogenetics is scary business, and I suspect billfish relations will undergo further revisions as the monstrosity known as "Perciformes" is reasoned into pieces. Anyways, while xiphiids² and istiophorids look superficially similar, they actually have rather distinctive morphology. Swordfish have a bill which is flat in cross-section, toothless, blunt-tipped, and with central chambers (compared to rounded, denticulated, pointed, and chamber-less for istiophorids), a weak mandible much shorter than the rostrum, no scales, and no pelvic fins (Collette et al. 2006; Fierstine 2006; Fierstine and Voight 1996 citing Nakamura 1983). Strangely, most extinct billfishes have jaws of equal length, and if the proposed (Istiophoridae + Hemingwayidae) and (Xiphias + Xiphiorhynchus) clades (Fierstine 2006) are correct (see note 1), this would mean the unequal jaws of extant billfishes evolved twice. 

¹ A detailed cladistic analysis with the fossil members of the group has yet to be undertaken (Fierstine 2006).
² As for what the deal with them and ziphiids is, I have no idea.

Swordfish (Xiphias gladius) from Wikipedia Commons.

One infamous use of the billfish bill is impaling unexpected objects. One Blue Marlin was found with rostrum fragments from two other, different billfish species (Fierstine 1997). Other unfortunates include large fish, whales, bales of rubber, boats, ships, deep-diving vessels, people, and turtles (Frazier et al. 1994 - citing various). The billfish-on-billfish impaling has been interpreted as defense against predators (Fierstine 1997) and in the case of the turtles, it was hypothesized that the billfish accidentally impaled them when aiming for fish aggregated nearby (Frazier et al. 1994). Istiophorids can survive with a foreshortened rostrum (Fierstine 2006) so apparently these accidents are survivable. But this raises another question - do they need an elongated rostrum at all?

One study of 227 Blue Marlins (Makaira nigricans) stomach contents found that 38% of prey items showed evidence of damage from the bill, 11% of which were speared and 81% of which were slashed, and the rest of which were in multiple pieces (Shimose et al. 2007). Bizarrely, another study with 226 Blue Marlins found no evidence of prey being struck or speared (Vaske et al. 2011). Vaske et al. (2011) offered no explanation for this anomaly, and I can't see an obvious one either. Both populations (from Japan and Brazil, respectively) even primarily preyed on Skipjack Tuna (Katsuwonus pelamis), which were normally killed with the bill in the former population. I'm stumped.

Fierstine (2006) hypothesized that unequal jaw length in billfishes may have evolved to avoid suffocation when impaling large objects (predator or prey) and to avoid damage to the mandible. I don't buy the mandibular reasoning since extant billfishes get by just fine with them naturally foreshortened. The available evidence suggests impaling is a rather rare event and thus unlikely to be the main factor in the evolution of the characteristic billfish bill. An alternate hypothesis could be that the mandible was shortened so the rostrum could be "weaponized" (sword-like flattening in xiphiids and denticles in istiophorids³) to slash at prey. However, the population which apparently doesn't use bills to feed and healthy individuals with damaged rostra are problematic for both of these hypotheses. Perhaps future studies will show that the bill is generally important for feeding in the group and that the counterexamples are just freaks, but either way, it seems premature to make any conclusions about why billfish have their striking morphology.


³ The ichthyosaur Eurhinosaurus has teeth on the upper jaw which could be a similar instance of "weaponization". 

I really have no idea how eurhinodelphids fit into this framework since Fierstine's hypothetical suffocation would not be an issue (if they could impale at all) and the rostrum does not seem particularly dangerous (no teeth, denticles, or flattening). I wonder if this morphology evolved for different reasons, or if it evolved for reasons that have yet to be hypothesized.

References:

Collette, B. B., McDowell, J. R., and Graves, J. E. (2006). Phylogeny of Recent Billfishes. Bulletin of Marine Science 79(3), 455-468. Available.

Fierstine, H. L. (2006). Fossil history of Billfishes (Xiphioidea). Bulletin of Marine Science 79(3), 433-453. Available.

Fierstine, H. L. (1997). An Atlantic Blue Marlin (Makaira nigricans), impaled by two species of billfishes (Teleostei: Istiophoridae). Bulletin of Marine Science 61(2), 495-499. Available.

Fierstine, H. L., and Voight, N. L. (1996). Use of Rostral Characters for Identifying Adult Billfishes (Teleostei: Perciformes: Istiophoridae and Xiphiidae). Copeia 1996(1), 148-161. Available.

Frazier, J. G., Fierstine, H. L., Beavers, S. C., Achaval, F., Suganuma, H., Pitman, R. L., Yamaguchi, Y., and Prigioni, C. M. (1994). Impalement of marine turtles (Reptilia, Chelonia: Cheloniidae and Dermochelyidae) by billfishes (Osteichthyes, Perciformes: Istiophoridae and Xiphiidae). Fisheries Science 39(1), 85-96. Available.

Little, A. G., Lougheed, S. C., and Moyes, C. D. (2010). Evolutionary affinity of billfishes (Xiphiidae and Istiophoridae) and flatfishes (Plueronectiformes): Independent and trans-subordinal origins of endothermy in teleost fishes. Molecular Phylogenetics and Evolution 56(3), 897-904. doi:10.1016/j.ympev.2010.04.022

Nakamura, I. (1983). Systematics of billfishes (Xip­hiidae and Istiophoridae). Publications of the Seto Marine Biological Laboratory 28, 255-396.

Orrell, T. M., Collette B. B., and Johnson, G. J. (2006). Molecular data supports separate scombroid and xiphioid clades. Bulletin of Marine Science 79(3), 505-519. Available.

Shimose, T., Yokawa, K., Saito, H., and Tachihara, K. (2007). Evidence for use of the bill by blue marlin, Makaira nigricans, during feeding. Ichthyological Research 54(4), 420-422. DOI: 10.1007/s10228-007-0419-x

Vaske, T., Travassos, P. E., Pinheiro, P. B., Hazin, F. H. V., Tolotti, M. T., and Barbosa, T. M. (2011). Diet of the Blue Marlin (Makaira nigricans, Lacepède 1802) (Perciformes: Istiophoridae) of the southwestern equatorial Atlantic Ocean. Brazilian Journal of Aquatic Science and Technology 15(1), 65-70. Available.

Picture of the Indiscriminate Interval #000008 - Eurhinodelphis longirostris

Eurhinodelphis longirostris at the American Museum of Natural History.

The most striking trait of Eurhinodelphidae is a toothless extension of the rostrum beyond the mandible (Lambert 2005), superficially similar to the bills of Billfish and Swordfish. Oddly, this morphology was speculative for a period of time (Kellogg 1925) although it has apparently been confirmed in several species as of Lambert (2005). Unfortunately, information on eurhinodelphids is scant and/or difficult to access and, among numerous other basics of their biology, I really don't know what the function of the extended rostrum would be. The only suggestion I could find is from one professor Abel who speculated that the cetaceans "swam on the surface of the sea, where they captured food - probably fishes - in much the same manner as does the skimmer (Rhynchops) [sic] among birds" (Anonymous 1909). Somehow, I find this even less plausible than azhdarchids-as-skimmers. On a curious note, there is a cetacean with the reverse of eurhinodelphid morphology (mandible extending past rostrum) unofficially known as the... skimmer porpoise.

Phylogenetically, eurhinodelphids have bounced around from being considered stem-ziphiids, the sister group to Delphinida, and the sister group to Squalodontidae + Squalodelphidae (Geisler et al. 2011 - citing various); within Geisler et al. (2011), they were placed outside crown-Odontoceti¹ in an unconstrained analysis and as the sister group of platanistoids in a constrained analysis. The authors regarded the latter position as more probable and placed eurhinodelphids within the new group Synrhina, which includes most odontocetes except for Sperm Whales and assorted extinct taxa. Whatever their placement, eurhinodelphids are certainly close relatives of living toothed whales, despite that whole extinct thing.

¹ It actually states they "did not fall inside crown Cetacea", but this is a typo. Otherwise, they'd be Miocene Archaeocetes. 

Eurhinodelphis longirostris seems to have an unusually long neck for a cetacean. The cervical vertebrae are not fused (Kellogg 1925), however this is a surprisingly common trait shared with river dolphins, monodontids, rorquals, and gray whales (Tinker 1988). The neck of E. longirostris appears to be proportionally longer than those of the baleen whales and Narwhal and is probably comparable to those of the Beluga and Dorudon. River dolphin skeletons are hard to find, but it seems likely they have similarly proportioned necks. It seems that Eurhinodelphis wasn't a total freak, well, except for the snout.

The Theatrical Tanystropheus covered Eurhinodelphis as well, and it doesn't even overlap that much!

References:

Anonymous. (1909). Notes. Nature 2088 (82), 16. Available.

Geisler, J. H., McGowen, M. R., Yang, G., Gatesy, J. (2011). A supermatrix analysis of genomic, morphological, and paleontological data from crown Cetacea. BMC Evolutionary Biology 11 (112). Available.

Kellogg, R. (1925). On the occurrence of fossil porpoises of the genus Eurhinodelphis in North America. Proceedings of the U. S. National Museum 66(26), 1-40. Available.

Lambert, O. (2005). Les dauphins longirostres et les baleines à bec du Néogène de la région d’Anvers: systématique, phylogénie, paléo-écologie et paléo-biogéographie. Doctoral Thesis. Partially Available.

Tinker, S. W. (1988). Whales of the World. E. J. Brill Publishing Company: New York. Partially Available.

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.