Monday, July 28, 2008

Even More Pacific Mesoplodonts

This should have been posted a long time ago, but unfortunately my internet access was fried by lightning.

There is fairly convincing evidence that a triad of Atlantic mesoplodonts distribute according to temperature (and latitude) preferences; three species of Pacific mesoplodonts may mirror them, but further research is needed on those poorly-known species. Mesoplodon densirostris exists in both oceans and appears to avoid competition with other mesoplodonts by specializing in a shallow water niche, but it does not appear to inhabit the northernmost parts of the oceans (due to small size?) and it is not clear if other species "fill in". Things in the Pacific quickly get complicated thanks to a possible sister species/subspecies of M. ginkgodens, Pitman's Mesoplodon sp. B sightings* and two recently described species.

*These cetaceans, mistakenly referred to M. grayi by previous authors, had extremely long and narrow beaks with strait mouthlines, a dark gray rostrum contrasting with a white lower jaw, and a pale patch behind the eye and similar coloration in both the mother and calf (Pitman and Morgan 2001). The last feature was previously known only from Indopacetus, a species Pitman is familiar with. This is the only mention of M. sp. "B" that I am aware of.

Bandolero Beaked Whale or Pygmy/Lesser/Peruvian Beaked Whale
Mesoplodon peruvianus

Reyes, Mead and Van Waerebeek, 1991

Much like the tropical bottlenose/Indopacetus situation, it wasn't until quite recently that sightings (and one 1955 stranding) of "Mesoplodon sp. A" were found to be synonymous with M. peruvianus. The 65 "Mesoplodon sp. A" sightings are highly concentrated off central Mexico and range from the tropical waters of the Gulf of California to central Peru corresponding strongly with M. peruvianus strandings (Pitman and Lynn 2001), except for a northern Chile and New Zealand specimen (MacLeod et al. 2006). The latter record is probably a stray since M. sp. A. appears to be parapatric with M. densirostris, which replaces it to the south and west (Pitman and Lynn 2001).

It could be possible that M. peruvianus and M. densirostris occupy the same niche, with the former able to out-compete the latter in very warm waters thanks to its small body size. M. sp. A was originally estimated at 5-5.5 m, but this was from a considerable distance (rarely under 3.5 km); subsequent sightings estimated animals with chevrons (mature males) at 3.52 m average and 3.79 m maximum (some were still developing at 3.68 m) and unpatterned animals over 3 m (mature females) at 3.74 m average and 4.07 m maximum (Pitman and Lynn 2001). The median length of 15 M. peruvianus specimens (including a 1.59 m animal) was 3.385 m but there were modes of 3.3-3.4, 3.6-3.7 and 3.7-3.8 (MacLeod 2005 - App. I). M. densirostris has modes of 3.9-4.0 and 4.3-4.4 m (4.15 m avg.) (MacLeod 2005 - App. I) and even though it is smaller than the previously discussed Pacific mesoplodonts, there seems to be a considerable size difference. However, MacLeod 2005 suggests that the smallest beaked whales (including M. perrini, M. bowdoini and M. hectori) may have their own distinctive guild of very small prey consumers, but this needs further investigation.

Unfortunately, the original description of this species is not available online (to me anyways) so I'll have to piece together what I can. M. peruvianus has rather diminutive teeth (Dalebout et al. 2002) which are not laterally flattened like other mesoplodonts but are oval in cross-section and Ziphius-like (and M. mirus-like) (Dalebout et al. 2003) and are placed behind the mandibular symphysis. A male M. peruvianus carcass that washed up in Peru in 1955 (classified then as M. bowdoini) showed that scarring was very heavy behind the dorsal fin and may indicate that the male ziphiids are aiming for their opponent's genitals with their battle teeth (M. carlhubbsi may do this as well) (Pitman and Lynn 2001). Judging from photographs in Pitman and Lynn 2001, it looks like the teeth are elevated enough to cause parallel scarring, and they're apparently as elevated as those of M. carlhubbsi. At least one molecular analysis seemed to indicate that M. peruvianus was a sister species to the parapatric M. perrini (Dalebout et al. 2002) and later mtDNA control region and cytochrome b put them reasonably close together (Dalebout et al. 2007).

Adult males have a distinct three-part coloration with orangish brown on the anterior end followed by a white chevron starting a bit behind the blowhole and then blackish brown (Pitman and Lynn 2001), Fig. 2 in that publication shows the coloration very clearly. I really appreciate it that Pitman et al. gave this species an informal name ("Bandolero beaked whale") that's informative about the coloration and pretty witty at the same time, as opposed to somebody's last name. Unfortunately, other common names have gotten pretty ingrained and the only instances of this name's usage are from Darren Naish, but I'll go edit Wikipedia later.

Like the tropical bottlenose/Indopacetus, an adult male M. peruvianus clearly demonstrating the bandolero coloration (or any coloration) has yet to wash up, but it seems very likely that Mesoplodon sp. A and M. peruvianus are one in the same.

I couldn't find a place to put this in the article, but it is worth mentioning that M. sp. A is still considered a cryptid by some. Bille 1995 does not directly cite Pitman et al. 1987 (well, I couldn't either) and described mysterious beaked whales seen off Mexico as 16 feet long (~4.9 m) with a flattened head, wide low-based dorsal fin and white "racing stripes" (scars?) on black adult males, which were larger than females. Some of these traits may be due to observational difficulties or the third-hand nature of this description and others are non-distinctive. Reading it initially, I wasn't even sure it was M. peruvianus. I seem to recall somebody actually arguing that M. sp. A was not M. peruvianus at some point, but I completely forgot where.

Perrin's Beaked Whale
Mesoplodon perrini
Dalebout, Mead, Baker, Baker and van Helden 2002

This species became the newest member of the beaked whale family when it was discovered to be distinct from M. hectori , now recognized as a Southern hemisphere species (Dalebout et al. 2002). This species is known from five strandings in California (MacLeod et al. 2006) although figure 2n in that publication has a black dot indicating a sighting, which may be a typo. It is not known how much more (if any?) the range is. Since this species is parapatric with M. peruvianus it may occupy a similar niche, albeit one more specialized for somewhat warmer temperatures. The limited data on length gives males a maximum size of 3.9 m (n=3) and females 4.43 m (n=1) (MacLeod 2005 - Appendix I) possibly allowing it to out-compete M. peruvianus at slighter colder temperatures. It does not seem to be obviously parapatric with M. densirostris judging by distribution maps in MacLeod et al. 2006, but seasonality may need to be taken into account.

Mesoplodon perrini is superficially very similar to M. hectori, and field guides (e.g. Reeves et al. 2002) illustrating the latter species actually take the coloration pattern from the former. After Dalebout et al. 2002 showed with mtDNA that the California specimens were not M. hectori, they thoroughly documented the many morphological differences between these two superficially similar species. M. perrini differs by having slightly posterior (1-2 cm) rather than apical teeth, a synvertex that narrows upwards (upper skull triangular in frontal view) as opposed to one that is flatter (box-like in frontal view), much narrower premaxillaries and many many others. It doesn't seem like there's any unique information on this species that isn't published in Dalebout et al. 2002.

So why does the Pacific ocean have a greater mesoplodont diversity than the Atlantic? Perhaps it is related size to the former body of water or the (apparently) unique nature of its eastern shores. Or maybe the Atlantic has sub-populations of a known species (M. densirostris?) that occupy an analogous niche - or maybe dwarf and/or pygmy sperm whales (Kogia spp.) do. Dalebout et al. 2002 point out that we only learned about M. peruvianus and M. perrini through chance strandings and fishing bycatch, so I suppose that there's the chance that analogous mesoplodonts have yet to be discovered. The problem is, of course, that the only "leads" that I know about for new mesoplodonts (besides M. mirus) occur in the Pacific (the M. ginkgodens-like species and M. sp. B).

Things get more difficult from here on out.


Bille, Matthew A. Rumors of existence. Hancock House Publishers, 1995.

Dalebout, Merel L. et al. 2007. A divergent mtDNA lineage among Mesoplodon beaked whales: Molecular evidence for a new species in the Tropical Pacific? Marine Mammal Science 23 (4): 954–966

Dalebout, Merel L. et al. 2003. Appearance, distribution, and genetic distinctiveness of Longman's beaked whale, Indopacetus pacificus. Marine Mammal Science 19 (3) 421-461

Dalebout, Merel L. et al. 2002. A New Species of Beaked Whale Mesoplodon perrini sp. n. (Cetacean: Ziphiidae) discovered through phylogenetic analyses of mitochondrial DNA sequences. Marine Mammal Science. 18 (3), pp. 577-608

MacLeod, Colin D. et al. 2006. Known and inferred distributions of beaked whale species (Cetacea: Ziphiidae). J. Cetacean Res. Manage. 7(3):271–286

Pitman, Robert L. and Lynn, Morgan S. 2001. Biological observation of an unidentified mesoplodont whale in the Eastern tropical Pacific and probable identity of Mesoplodon peruvianus. Marine Mammal Science. 17(3), pp. 648-657

Reeves, Randall R. et al. 2002. National Audubon Society Guide to Marine Mammals, Alfred A. Knopf, New York.

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