It is sometimes claimed the two species of Giant Beaked Whale (Berardius bairdii, B. arnuxii) are practically identical, and may thus be disjunct populations rather than distinct species (e.g. Reeves et al. 2002, Wikipedia). This seems increasingly unlikely to be true. The species were estimated to have diverged 0.68-5.81 million years ago, which probably makes them younger than the similarly anti-tropical Hyperoodon species (2.37–11.53 mya), but likely older than other cetacean pairs universally considered distinct (Lissodelphis borealis/L. peronii, Globicephala macrorhynchus/G. melas, etc) (McGowen et al. 2009). Morphologically, the Berardius species differ in flipper size and in the shape of the nasal bones and vomer (Kasuya 2009); the Beaked Whale Identification Guide states that literature describing cranial differences is not yet available, so this list is probably incomplete.
Berardius bairdii is far from uniform itself, as Japanese populations from the Pacific Ocean, Sea of Japan and Sea of Okhotsk reportedly differ morphologically (Kitamura et al. 2012 – citing Kishiro 2007). How, or whether, these purported populations differ genetically has yet to be determined. There is a fourth Japanese population, however, which is unambiguously distinct. In the Sea of Okhotsk (off Hokkaido) slate-gray specimens observed year-round appear to be vanilla B. bairdii, but there are also black individuals present from April to June (Kitamura et al. 2012). These individuals are quite rare, with Kitamura et al. only managing to acquire three in archives. The only specimen with a determined length measured 6.6 meters yet remarkably, it was mature (Kitamura et al. 2012). Comparatively, B. bairdii is believed to first ovulate at 9.8-10.7 m (Kasuya 2009). To put how small 6.6 meters is (for a B. bairdii) in perspective, of the 64 ‘normal’ specimens Kitamura et al. examined, only three were smaller than 6.6 m (5, 4.2, 1.8 m) and they were likely a pre-weaned juvenile (Huang et al. 2009), a newborn (Kasuya 2009) and a fetus, respectively.
Small size on its own is of course not proof of divergence – particularly when only one individual was measured – so that’s where DNA comes in. Nuclear DNA weakly distinguished the black specimens, however mtDNA had high bootstrap values and placed them outside the B. bairdii–B. arnuxii clade. Kitamura et al. suggested that the black Berardius individuals could very well be a cryptic species, although their study was too preliminary to formally describe it. There still remains a lot of genetic and morphological work to sort out just what’s happening with the Berardius variants and maybe some day we’ll see a third species added to the field guides.
As beaked whales gradually become less mysterious, it seems likely additional cryptic subspecies or even species will be described. This is, of course, a story for another day
Huang, S-L. et al. (2009) Comparable length at weaning in cetaceans. Marine Mammal Science. Available.
Kasuya, T. (2009) Giant Beaked Whales IN: Perrin, W. et al. (eds.) Encyclopedia of Marine Mammals.
Kishiro, T. (2007) Geographical variations in the external body proportions of Baird’s
beaked whales (Berardius bairdii) off Japan. Journal of Cetacean Research and
Management 9, 89–93.
Kitamura, S. et al. (2012) Two genetically distinct stocks in Baird’s beaked whale (Cetacea: Ziphiidae) Marine Mammal Science doi: 10.1111/j.1748-7692.2012.00607.x
McGowen, M. et al. (2009) Divergence date estimation and a comprehensive molecular tree of extant cetaceans. Molecular Phylogenetics and Evolution 53, 891–906. Available.
Reeves, R. et al. (2002) National Audubon Society Guide to Marine Mammals of the World.