Indo-Pacific Mesoplodonts

Not to be confused for the Tropical Bottlenose whale, Indopacetus pacificus, previously classified as Mesoplodon pacificus.

This post won't cover every species in the Indian and Pacific oceans, mesoplodonts living in the higher latitudes of the southern hemisphere's oceans will be covered in a future posts. There is an article, either in press or unpublished ("known key areas for beaked whales around the world" by MacLeod et al.?) that apparently brought up the possibility of mesoplodonts having distributions affected by latitude, like the North Atlantic mesoplodonts discussed in the previous post. There appeared to have been prey preference and trophic level difference between the North Atlantic species but these may be due to small sample sizes; temperature preference and subsequent temporal/geographic distribution seems to be the most parsimonious explanation. The purpose of morphological differences (sexual size dimorphism, body size, stomach morphology, etc.) and potential physiological mechanisms for such temperature preferences aren't too clear at the moment.

Stejneger's Beaked Whale
M. stejnegeri
True, 1885

This is a sub-polar to cold-temperate species that lives on both sides of the Pacific from 57 to 35 degrees N (MacLeod et al. 2006). The numbers of specimens that have been measured (n=99) is second only to Hyperoodon ampullatus and Ziphius and is is similar to M. bidens (n=95) (MacLeod 2005), the most northerly Atlantic species. There is no obvious or great sexual dimorphism, females have a median length of 4.88 m and a mode of 4.8-4.9 compared to 4.76 and 4.6-4.7 and 5.0-5.1 in males - males max out at 5.77 m compared to the 5.44 m record for females (MacLeod 2005 - Appendix I). Body size in M. stejnegeri may be slightly larger than M. carlhubbsi which in turn may be slightly larger than M. ginkgodens, but size may not be the only factor in determining distribution. Wide-ranging M. densirostris does not appear to overlap this species (or M. bidens in the Atlantic) and M. stejnegeri shares a generalized stomach anatomy (unlike M. bidens) with it to the exception of M. carlhubbsi (Mead 2007). Stomach anatomy in M. ginkgodens is not known and the function of additional stomachs is not obvious.

I couldn't help but wonder why the deep water preferring mesoplodonts in the Atlantic have strait to sinusoidal mouthlines while the possibly analogous mesoplodonts in the Pacific all have stepped or arched mouthlines. M. stejnegeri can only open its mouth a few centimeters (Loughlin and Perez 1985) which limits prey size substantially, is this a side-effect of exaggerated sexual dimorphism between males or females or does ecology play a role? Is it possible that there was both an inter- and intra-specific "arms race" for male sexual dimorphism in these species?

The high number of strandings in a fairly small area (and without much hunting) would seem to indicate that this species is fairly common, but MacLeod et al. 2006 only appear to have two sightings on their distribution map. It was not possible to confirm the species status of north Pacific mesoplodonts until fairly recently (Loughlin and Perez 1985) although Mesoplodont spp. sightings in northern waters were assumed to represent this species (Loughlin et al. 1982). Whalers have stated that they are either found alone or in groups of two to three but possible M. stejnegeri sightings in the Bering Sea showed groups of 5-15 animals (Loughlin et al. 1982).

On the trivial side, it is speculated that mesoplodonts may dissipate heat by dilating their blood vessels and turn from gray/black to brown (Loughlin et al. 1982). This species also apparently darkens with age (Reeves et al. 2002) and this could provide an alternate explanation for observed color. Even more curiously, a paper that I can't access documents the milk of this species, which lacks lactose in its milk and is blue-green thanks to bilverdin. An abstract from Milk Science does not mention this pigment in M. ginkgodens.

Hubbs' Beaked Whale
M. carlhubbsi
Moore, 1963

This species has at least 26 stranding records (MacLeod et al. 2005 - Appendix I) ranging from 35 to 42 degrees off the coast of Japan and from 33 to 54 degrees north on the North American coast (MacLeod et al. 2006). There seems to be a considerable amount of overlap with M. stejnegeri (except off Alaska and northern Russia) and very little with M. densirostris (up to 37 degrees N in the East Pacific) so the possibility that M. carlhubbsi has a niche similar to the latter species should be addressed. There do not appear to be any confirmed sightings (MacLeod et al 2006 - Fig. 2f) and the presence of this species in the central Pacific (and the possibility of two separate populations) also needs to be addressed.

This species is similar in size to M. stejnegeri (and larger than M. densirostris) with a median male size of 4.72 m, a female median size of 4.85 m and maximum sizes of 5.3 and 5.32 m, respectively (MacLeod 2005 - Appendix I). The jawline of M. carlhubbsi is also rather similar to M. stejnegeri in appearance due to an arched mandible with very prominent teeth appearing to constrain the gape of the animal. The melon is moderately bulbous, unlike M. stejnegeri, and males have a white "beanie" on top of it (Reeves et al. 2002). Unlike M. stejnegeri and M. densirostris, M. carlhubbsi has a derived stomach anatomy similar to M. europaeus i.e. with an additional blind main and pyloric stomach but it differed by having trabeculae in the blind accessory main stomach (Mead 2007). Interestingly, Mead predicted that M. bowdoini would have a very similar stomach setup since the two species are "extremely similar". I'll discuss this in a following post.

Interestingly, Carl Hubbs thought that M. carlhubbsi was rather tasty and recommended that it should be roasted or fried for tenderness - but other people have complained of diarrhea after eating mesoplodonts (Loughlin and Perez 1985). Okay....

Ginkgo-toothed Beaked Whale
M. ginkgodens
Nishiwaki and Kamiya, 1958

It was recently discovered through cytochrome b and mtDNA control sequences that mesoplodonts from Kiribati and Palmyra (in the central Pacific) are a deeply divergent sister group to M. ginkgodens from California, New Zealand, Japan and Taiwan (Dalebout et al. 2007). There does not seem to be any information on the distribution of M. ginkgodens in the central Pacific (it does not appear to have any confirmed sightings) (MacLeod et al. 2006) and exactly what is going on here in terms of eco-geography is rather unclear. There will be future investigations on this subject before too long (Robert Pitman pers. comm.).

MacLeod et al. 2006 report 23 strandings of this species with most (15) from Japan and others from China, Taiwan, Malaysia, Guam, Sri Lanka, SE Australia, California, Mexico and the Galapagos - genetic data from Dalebout et al. 2007 also included a couple specimens from New Zealand. There have been unconfirmed sightings in the Arabian sea (MacLeod et al. 2006) although since the only other ziphiids in the area are M. densirostris, Ziphius and Indopacetus it would seem likely that this species occupies the niche of a deep-diving small prey consumer in that area and other across the Indo-Pacific. Only 16 accurate measurements appear to have been taken, male median size (n=4) was 4.86 m compared to 4.45 m (n=9, median = 4.8-4.9 m) for females; maximum male size was 5.1 m compared to 4.9 m (MacLeod 2005 - App. I) and it does not appear that there is any sexual size dimorphism.

What's really curious about this species is that while males have erupted tusks (or "battle teeth") like any other ziphiid, they have been reported to lack (Reeves et al. 2002) or almost lack linear scarring (MacLeod 1998). The sample size was limited for MacLeod 1998 (n=16) and it has been suggested that observations of live animals may yield a different and more complex color pattern (Reeves et al. 2002) and will presumably clear up this matter. It would be interesting for a mesoplodont to have (possibly quite recently) abandoned a near-ubiquitous behavior and there could be a reason why it retained the anatomy anyways. Well, that is assuming the loss wasn't very recent or an artifact of the limit data. The following paragraph may contain speculation:


It has been suggested that large sharks are the only natural predators of ziphiids (Loughlin and Perez 1985) but mesoplodonts have an average length similar to Carcharodon carcharias and may be unfeasibly large prey. Sleeper sharks (and Hexanchus?) will attack ~4 meter elephant seals (Mirounga) but echolocation and similar size may prevent predation in deep waters. The only evidence of shark attacks that I know of are scars from cookiecutter sharks (Isistius). Scars from either Orcinus or Pseudorca have been observed on a live mesoplodont and there is one record of Orcinus consuming a Ziphius individual that it may have killed (Tyack et al. 2006). Ziphiids do not appear to make sounds above 200 m and it appears that this is done to avoid encounters with Orcinus (Tyack et al. 2006). Since it appears that at least one mesoplodont escaped from Orcinus or Pseudorca alive, how did it manage that? The maximum reported swimming speed of M. stejnegeri is 6 knots (~11 km/h) (Loughlin and Perez 1985) is not fast enough for a "flight" response (15-20 km/h) (Ford and Reeves 2008) unless the ziphiid took a crash dive. Male Right whales (Eubalaena) use encrustations on their heads in intraspecific confrontations but females posses them to a lesser degree as well and it is possible that both genders use this as a weapon against Orcinus (Ford and Reeves 2008). Is it possible for the ossified rostrums of male and female ziphiids to be analogous to this? The fighting style would have to be different from normal male encounters, and possible more risky, but it could provide explanations for why "battle" characteristics are retained in individuals who do not appear to otherwise have a use for these resource-consuming features. An alternative explanation is that females have ossified rostrums because of some sort of ontogenetic "requirement" and that M. ginkgodens does engage in "battle", although possibly less commonly than other species for unknown reasons.

I think I'll cut off this post here for the time being. While the mesoplodonts discussed so far roughly mirror those in the Atlantic, there is a major departure: two more species discovered in the past two decades!

References:


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

Ford, John K. B. and Reeves, Randall R. 2008. Fight or flight: antipredator strategies of baleen whales. Mammal Rev. 38 (1) pp.50–86.

Loughlin, Thomas R. and Perez, Michael A. 1985. Mesoplodon stejnegeri. Mammalian Species No. 250, pp. 1-6.

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

MacLeod, Colin D. 2005. Niche Partitioning, Distribution And Competition In North Atlantic Beaked Whales. Doctoral Thesis. Available

MacLeod, Colin D. 1998. Intraspecific scarring in odontocete cetaceans: an indicator of
male `quality' in aggressive social interactions? J. Zool., Lond. 244, 71-77

Mead, James G. 2007. Stomach Anatomy and Use in Defining Systemic Relationships of the Cetacean Family Ziphiidae (Beaked Whales). The Anatomical Record. 290: 581–595

Tyack, Peter L. et al. 2006. Extreme diving of beaked whales. The Journal of Experimental Biology 209, 4238-4253

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