The Spade-Toothed Whale

It is almost beyond belief that in the 21st Century there was a whale so obscure it was known only from three partial skulls. But no more, the Spade-Toothed Whale (Mesoplodon traversii) has finally turned up in the flesh:

For such a reticent species, the Spade-Toothed Whale has a surprisingly long history. The first known remains consisted of a mandible washed up on the Chatham Islands discovered by a Henry H. Travers (Esq.) in 1872 (van Helden 2002). Hector (1873) classified the mandible as ‘Dolichodon layardii‘, now known as Mesoplodon layardii, the Strap-Toothed Whale. Gray (1874) argued that the Chatham Islands specimen was sufficiently different to warrant status as a distinct species, ‘Dolichodon‘ traversii. While the species are superficially similar in having large teeth with a denticle at the apex, M. layardii specimens consistently have tapering strap-like teeth whereas the M. traversii specimen has shorter, whaler’s spade-shaped teeth with a prominent denticle (van Helden 2002). Hector (1878), without explaining why, stood by his original classification, and for over a century the significance of the jaw from the Chatham Islands went unrecognized.

The first known _Mesoplodon traversii_ remains, from Gray (1873).

_M. layardii_ mandible, from Van Beneden & Gervais (1880).

In 1986, a damaged calvaria (skull sans mandible) was discovered on Robinson Crusoe Island and, despite lacking the typically diagnostic teeth, it was considered distinctive enough to be classified as a new species, Mesoplodon bahamondi (Reyes et al. 1996). Baker (2001) discovered that another calvaria collected in the 1950’s from Whakaari, New Zealand previously thought to be M. ginkgodens was in fact a second specimen of Bahamonde’s Beaked Whale. Analysis of mtDNA revealed that the calvariae were indeed from the same species… as was the mandible from the Chatham Islands (van Helden et al. 2002). Thus a complete picture of the Spade-Toothed Whale’s skull was revealed, but every other aspect of the species’ biology remained a tantalizing blank. It’s a little less blank now.

In December 2010, a female and male calf stranded on Opape Beach, New Zealand and after being initially identified as Mesoplodon grayi, mtDNA revealed they were in fact M. traversii (Thompson et al. 2012). The confusion with M. grayi appears to have resulted from similar mouth-lines, dorsal fin and flipper shape but it is distinctive in having a more prominent melon, and dark rather than white rostrum (Thompson 2012). Thompson (2012) mentions a dark eye patch, white belly, and dark flippers as “notable”, but it appears female M. grayi arguably exhibit these traits as well.

Reyes et al. (1996) estimated that the Robinson Crusoe Island calvaria belonged to a 5.0-5.5 meter whale, and it is curious that the stranded female was close to the midpoint at 5.3 meters long (Thompson et al. 2012). If the Spade-Toothed Whale exceeds 5 meters on average, it would be among the largest mesoplodonts, second only to the Strap-Toothed Whale (MacLeod 2005). Since the two species appear to be similar in size and tooth morphology, it could be possible they occupy a similar niche and avoid competition with spatiotemporal segregation, similar to the North Atlantic ziphiids (MacLeod 2005).

And for even more rampant speculation – has Mesoplodon traversii been seen before? In 1980 and 1983 Bob Pitman observed unknown cetaceans in the East Pacific with extremely long and narrow rostrums, strait gapes, a white mandible contrasting with a dark gray rostrum, and a light patch behind the eyes present in both an apparent mother and calf (Pitman & Lynn 2001). Pitman & Lynn (2001) notably speculated that it may be M. bahamondi, suggesting the ~30 degrees of latitude from Robinson Crusoe Island is not necessarily hugely problematic. Looking at the above photograph of the stranded female, I could see how the described coloration fits, but confusingly the reconstruction in Thompson et al. (2012) has a white area somewhat ahead of the eyes and the beak is entirely dark. Post-mortem color interpretation in whales is notoriously tricky to interpret, and it will probably take photographs of live specimens under clear lighting to document all the subtle aspects of the coloration. While the beak of M. traversii is long, those of female M. grayi are ridiculously so, so this is also problematic. Additionally, the juvenile M. traversii had a “color pattern characteristic of most juvenile mesoplodont beaked whales” (Thompson et al. 2012), apparently meaning it did not have the same coloration as the female. ‘Mesoplodon species B.’ remains an enigma, but I wouldn’t bet against regional M. traversii color variants.

Have I mentioned that this is the marine biological discovery of the century?

References:

Baker, A. (2001) Status, relationships, and distribution of Mesoplodon bowdoini Andrews, 1908. Marine Mammal Science 17, 473-493. DOI: 10.1111/j.1748-7692.2001.tb00999.x

Hector, J. (1873) On the Whales and Dolphins of the New Zealand Seas. Transactions of the New Zealand Institute 5, 154-170. Available.

Hector, J. (1878) Notes on the Whales of the New Zealand Seas. Transactions of the New Zealand Institute 10, 331-343. Available.

MacLeod, C. (2005) Niche Partitioning, Distribution And Competition In North Atlantic Beaked Whales. Doctor of Philosophy Thesis at University of Aberdeen. Available.

Pitman R. & Lynn, M. (2001) Biological observations of an unidentified mesoplodont whale in the eastern tropical Pacific and probable identity: Mesoplodon peruvianus. Marine Mammal Science 17, 648-657. Available.

Reyes, J. C., et al. (1996) Mesoplodon bahamondi sp. n. (Cetacea, Ziphiidae), a new living beaked whale from the Juan Fernández Archipelago, Chile. Boletin del Museo Nacional de Historia Natural, Chile 45, 31-44. Available.

Thompson, K., et al. (2012) The world’s rarest whale. Current Biology 22(1). Available. Supplement.

van Beneden, P.-J. & Gervais, P. (1880) Ostéographie des Cétacés, vivants et fossiles. Available.

van Helden, A., et al. (2002) Resurrection of Mesoplodon traversii (Gray, 1874), senior synonym of M. bahamondi Reyes, van Waerebeek, Cárdenas and Yáñez, 1995 (Cetacea: Ziphiidae). Marine Mammal Science 18(3), 609-621. Available.