Sunday, August 3, 2008

Mesoplodonts of the Southern Oceans

The huge expanse of water made up of the southern Indian, Atlantic and Pacific Oceans is home to six mesoplodonts (and 4 other ziphiids) including a Mesoplodon mirus subspecies or sister species. That species is known from strandings off South Africa, southern Brazil and southern Australia and sightings off the coast of Madagascar occuring within 33 to 38 degrees S, compared to 26.7 to 53.7 degrees N for the northern population (MacLeod et al. 2006). It should be warned that records for many of these species are uncommon and they may not entirely or accurately portray the range. It is certainly puzzling how these species coexist niche-wise and future studies will undoubtedly clarify or completely revise the information in this post.



Andrews' Beaked Whale
Mesoplodon bowdoini

Andrews, 1908

This species was regarded to be an antitropical population of M. stejnegeri by some, despite being more similar to M. carlhubbsi morphologically, and fortunately there has been a recent review (Baker 2001) that solidified the distinctiveness of this species. M. bowdoini has smaller teeth with a more forwards and upwards facing denticle than M. carlhubbsi; blunter antorbital tubercles that do not extend (anteriorly) past the maxillary prominences and are made of the maxilla, frontal, jugal and lacrimal (contra: maxilla and jugal); and prominential notches that are shallow and curved (and not deep and "v"-shaped). M. bowdoini was known as the splay-toothed whale to some due to the 20 degree splay in the type specimen, but it turns out that adult males have splays from 0 to <16 style="font-style: italic;">M. bowdoini
and M. carlhubbsi to be "extremely similar" anatomically and predicts that the species will share similar stomach anatomy (Mead 2007). If stomach anatomy has some correlation to niche then there may be some differences.

Mesoplodon bowdoini is known from 35 strandings mostly from New Zealand (22) and Australia (8) but there are recent records from Tristan da Cunha, the Falkland Island and Tierra del Fuego (MacLeod et al. 2006, Baker 2001). This species may be circumpolar (all ziphiids in this area probably are) in waters roughly between 35 and 55 degrees S, but it is not known if the huge gaps in the Indian and Pacific oceans represent breaks in the range or not (MacLeod et al. 2006). This species is one of the four smallest mesoplodonts and may be in the "very small prey consumer guild" (MacLeod 2005), males (n= 7) have a median length of 4.22 m and a maximum of 4.41 m while females (n= 6) have a median length of 4.075 m and a maximum length of 4.36 m and the modes for all specimens (n= 15) were 3.9-4.0 and 4.2-4.3 (MacLeod 2005 - App. I). Does this M. bowdoini represent the same niche as M. carlhubbsi? Information is of course rather limited, but if M. carlhubbsi and M. mirus have analogous niches in different oceans (small prey consumer, deep water, moderate temp. preference) it would seem quite odd that similar species both inhabit the southern hemisphere. Unless M. mirus or a similar species are much smaller than their northern relatives, it would not make sense for the smaller of the species to inhabit higher latitudes.

Before I get too entangled, I should mention another piece of the puzzle.




Hector's Beaked Whale
Mesoplodon hectori

Gray, 1871


Not much is known about M. hectori and older sources are unreliable since the information presented mixes this species with M. perrini. Gales 2002 has an incredibly close-up photograph of M. hectori (considering how boat shy it probably is) that looks quite different from normal depictions (e.g. Reeves et al. 2002) that were based off of M. perrini. As far as I can tell this page has the only depiction of M. hectori that actually depicts the species.

One juvenile specimen (2.8 m) from New Zealand curiously had a single indentation on its throat instead of the pair of throat grooves present in all other ziphiids which are believed to aid in suction feeding (Baker 2001a). The specimen was in good physical condition and had stomach parasites which indicated that it did prey on organisms and wasn't weaned (Baker 2001a). It is theorized that this specimen was able to create suction by tongue retraction (Baker 2001) and I guess it shows that the throat grooves aren't critical in feeding, but they still should be offering a significant advantage.

M. hectori stranded from 32 to 55 degrees S (Tierra del Fuego) off South America and 35-42 degrees S off New Zealand, showing a range very similar to M. bowdoini. M. hectori is the final potential member of the "very small prey consumer" guild (MacLeod 2005) with a male median length is 3.73 m (3.7-3.8 mode, 4.34 max) and a female median length of 4 m (4.15 max) (MacLeod 2005 - App. I). While the other two possible members of the guild, M. peruvianus and M. perrini, are apparently parapatric it would seem that these species occupy separate niches. There is a chance that the stranding records give an inaccurate impression of distribution and these species are temporally separated. As discussed in the previous post, M. hectori and M. perrini do not appear to be sister species and are morphologically similar, possibly suggesting that M. hectori is also capable of out-competing M. densirostris. The latter species only appears to go down to 41 S and a smaller species out-competing it would be very unlikely. Maybe there are some fine-scale differences which allow for this to occur.

I have to admit that more information is needed to even get a basic understanding of how these species fit together niche-wise. Does similar morphology indicate similar niches or common ancestry? How accurate are the ranges? Do species occupy niches that have no northern counterparts? How is there all this confusion with three species left?


Strap-toothed whale
Mesoplodon layardii
Gray 1865

This is the first of three species which appear to be ecologically similar and overlap substantially in the southern oceans. Unexpectedly, this species seems to be fairly well studied for a mesoplodont but articles are either lacking or unaccessible for me. Fortunately I can at least get basic information from other sources.

M. layardii
is the largest mesoplodont with a male median of 5.145 m (max= 5.84, n= 10) and a female median length of 5.765 m (max= 6.25 m, n=8), it is also one of two species (M. europaeus being the other one) where females appear to be consistently larger than males (MacLeod 2005 - Appendix I). M. europaeus also had many more measured specimens (72 vs. 18) so we'll have to see if this patterns continues (and to what degree) with additional information from M. layardii. What is known for certain about the strap-toothed whale is that it has enormous (30 cm) tusks that cross the upper jaw and has cutting areas reduced to small points (MacLeod 2003). It is commonly cited that this incredible tooth development limits the gape of male M. layardii to about an inch, but not so frequently mentioned is the fact that M. carlhubbsi (and apparently M. stejnegeri) has a similarly reduced gape thanks to mandible and tooth development*. It seems interesting that M. layardii, M. carlhubbsi and M. stejnegeri all tended to consume a greater number and a greater variety of cephalopods than other mesoplodonts and the prey tended to be quite small (under 500 g) (MacLeod et al. 2003). Local Ziphius and Hyperoodon can take prey five times larger than M. layardii can despite being only 10 and 20% percent longer, respectively (MacLeod et al. 2003) and I'd be curious if there are any differences between male and female M. layardii. It seems very odd that three of the four consistently largest mesoplodonts (MacLeod et al. 2005 - Appendix I, M. mirus is the fourth) would severely reduce their gape and potential prey size, the pressures for this don't seem overly clear. I was thinking about competition with large prey consumers (Indopacetus, Ziphius, Hyperoodon, Ziphius, Tasmacetus?) but a reduced gape does not seem to occur in Atlantic mesoplodonts.

* The reason for increased mandibular height/tooth development seems to be tied in with more posterior placement of the teeth and appears to be related to a fighting style that bypasses the reinforced melons (Hardy 2005). Other species with mandibular arches seem to have teeth placed in a way that won't reduce the gape much, if at all - M. bowdoini, M. densirostris, M. ginkgodens, M. peruvianus.


This species seems to range from 32 to 63 degrees S (for the most part) judging by records from strandings and sightings (MacLeod et al. 2006). Male M. layardii can of course be distinguished at sea by their strap-like tusks plus they have a bold coloration of white on black, including lighter coloration on the dorsal surface that at least one authority views as cape-like (Reeves et al. 2002). The lighter dorsal surface is reminiscent of Ziphius and the cape is reminiscent of Tasmacetus (where it seems to be better defined). As distinctive as this species is, there is an incredibly poorly known mesoplodont that may be confused for it.



Spade-toothed Whale
Mesoplodon traversii

Gray, 1874


This species is only known from a damaged mandible and teeth from Pitt Island New Zealand in 1872 (synonymized with M. layardii), a calvaria from White Island New Zealand from the 1950's (classified as M. ginkgodens) and a calvaria from Chile in 1986 (type specimen for M. bahamondi); these specimens have been assigned to M. traversii on the basis of molecular phylogenetics and morphology (van Helden et al. 2002). This gives the species a range of 33 to 44 degrees S although future investigation is needed to determine if any prior M. layardii records actually represent M. traversii (MacLeod et al. 2006)

Mesoplodon traversii of course lacks strap teeth, theirs look somewhat reminiscent of a whaler's flense (compare to the teeth). These teeth are very large (233 and 238 mm for right and left, respectively), are angled posteriorly at 140 degrees, have a large apically placed denticle that faces outwards, the teeth are sinusoidal in the sagittal place and offset the root by 20 degrees (van Helden et al. 2002). An anterior view of the mandible makes it look like the teeth could reduce the gape and van Helden et al. 2002 documented severe erosion just above the gum-lines on the anterior ends of the teeth, a trait shared with M. stejnegeri (and others) where the teeth impinge upon the rostrum. Somebody (probably Reyes et al. 1995 - which I can't access) estimated "M. bahamondi" at 5-5.0 m (according to this page and this page); if M. traversii occupies the same niche as M. layardii but at a higher latitude, a somewhat smaller body size would be likely. In cases of very low specimen counts (like fossils) it seems at least reasonable to assume that the specimen on hand represents the average size and range, but we'll have to see about that.

In the previous post I had mentioned Pitman's long-beaked "Mesoplodon sp. B" sightings which were tenuously attached to "M. bahamondi" because the wide rostrum base of the Chilean calvaria was taken as possible evidence for a long beak (Pitman and Lynn 2001). A year after that was written, we can now see that the jaw of M. traversii is proportionally similar in length to that of M. layardii and nowhere near as long as that of M. grayi, a long-beaked species "M. sp. B" was confused for by some. The external appearance of M. traversii is currently unknown, but I suppose it could be possible for it to strongly resemble M. layardii in external coloration.


Gray's Beaked Whale
Mesoplodon grayi

van Haast, 1876


This appears to be the widest ranging of the southern mesoplodonts, M. grayi has been seen summering off the coasts of Antarctica and for the most part appears to be circumpolar in waters below 30 degrees S (MacLeod et al. 2006). A stranding off Peru (at 13.8 S) may indicate that this species extends its range north in the cold Humboldt current, but another stranding in the Netherlands is probably a stray (MacLeod et al. 2006). Mesoplodon grayi is a moderately sized mesoplodont with a male median length of 4.5 m (n= 8) and a female median length of 4.67 m (n= 5) - I'd be curious about the reported length of "M. sp. B" which was reported from waters near the equator, if it occupies the same niche then we would expect it to be somewhat smaller.

Curiously, stomachs of M. grayi individuals have been found to contain only fish, which digest much faster than cephalopods (MacLeod et al. 2003). M. grayi is a long beaked species which often retains small maxillary teeth embedded in the gum, although I'm not sure if they're functional. Two other species which reportedly have fish as a major part of their diet, M. bidens and M. mirus (MacLeod et al. 2003); the former species also has a long beak, M. mirus on the other hand has a plesiomorphic dentition with simple apical teeth. Perhaps the presence of a long beak is best suited for a piscivorous diet while one with teeth impinging on the rostrum is better suited for teuthophagy. The theory of mesoplodonts selecting prey items on factors besides size still needs a lot of testing, but perhaps it could clarify how there are so many species in the southern oceans.




This ends an experiment in blogging. At the beginning of the summer I set out with the intention of seeing how deep I could get into a group. As extensive as these posts became, they're still only a moderately detailed look into ziphiids. I had dramatically underestimated how much reading I had to do, oh, and plus my writing process is painfully slow. Even though the odds are that I'll never even get to see a ziphiid, I'm still glad I did this. As interesting as a group is, there is a time to move on, at least for now.



References:

Baker, Alan N. 2001. Status, relationships, and distribution of Mesoplodon bowdoini Andrews 1908 (Cetacean: Ziphiidae). Marine Mammal Science 17 (3) pp. 473-493

Baker, Alan N. 2001a. A Juvenile Hector's Beaked Whale Mesoplodon hectori (Gray, 1871), without functional throat grooves, plus notes on parasites (Cetacea: Ziphiidae) Marine Mammal Science 17 (1) pp. 171-175

Gales, N. J. 2002.Genetic identification and biological observations of two free-swimming beaked whales: Hector's beaked whale (Mesoplodon hectori, Gray 1871), and Gray's Beaked whale (Mesoplodon grayi, von Haast, 1876). Marine Mammal Science 18 (2) pp. 551-557.

Hardy, Mathew T. 2005. Extent, Development and Function of Sexual Dimorphisms in the Skulls of the Bottlenose Whales (Hyperoodon spp.) and Cuvier’s Beaked Whale (Ziphius cavirostris). Available

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. et al. 2003. Review of data on diets of beaked whales: evidence of niche separation and geographic segregation. J. Mar. Biol. Ass. U.K. , 83, pp. 651-665

MacLeod, Colin D. 2003. Species recognition as a possible function for variation in position and shape of the sexually dimorphic tusks of mesoplodon whales. Evolution, 54(6), pp. 2171–2173

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

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.

van Helden, Anton L. et al. 2002. Resurrection of Mesoplodon traversii (Gray, 1874), Senior synonym of M. bahamondi Reyes, van Waerebeek, Cardenas and Yanez, 1995 (Cetacea: Ziphiidae). Marine Mammal Science 18 (3), pp. 609-621

1 comment:

"the Dude" said...

Wow Cameron, I had no idea you are so into the beaked whales. I know almost nothing about them. I have a question on pygmy right whales and cetaceans in general, regarding laryngeal air sacs. First, please see the whale autopsy picture at my blog: link
showing the small lar. air sac of the pygmy right whale, then see what I wrote here: link

I just don't know for sure about cetacean lar. air sacs and their functions, do you know?
(Anyone else know?)
I'd appreciate any info., the net doesn't have much. Sorry for OTing!
DDeden