Mesoplodonts of the North Atlantic

The North Atlantic is a relatively well-studied area that is home to six species* of ziphiids, two (or three?) of which are endemic. It was previously discussed that ziphiids preferring large prey items (Ziphius and Hyperoodon) prefer different temperatures and as a result avoid direct competition through temporal and geographic separation. A widespread small prey consumer, (Mesoplodon densirostris) appears to prefer relatively shallow water and occupies a niche separate from potential competitors.

* Mesoplodon grayi once stranded in the Netherlands but this is around 80-90 degrees latitude higher than its normal range (MacLeod et al. 2006). I had previously mentioned a vague reference to an Indopacetus sighting in the Gulf of Mexico which MacLeod et al. 2006 did not mention (was it a misidentification?).


We're still left with three mesoplodonts which all appear to be small prey consumers with deep water preferences. How do they avoid direct competition?

Left: Sowerby's Beaked Whale (Mesoplodon bidens)
Middle: True's Beaked Whale (M. mirus)
Right: Gervais' Beaked Whale (M. europaeus)

From MacLeod et al. 2006

There does seem to be rough geographic separation between the mesoplodonts, although the amount of overlap (i.e. all species have been recorded from the UK) still needs explanation. MacLeod 2005 draws similarities between the H. ampullatus/Ziphius antagonistic pairing and the distributions of M. bidens and M. europaeus since there seem to be similar temperature preferences and seasonal migrations which further decrease any competition. Everything would be pretty neat and tidy if it wasn't for M. mirus occuring in between the species and apparently out-competing them only in a rather narrow band of water. It appears that we're going to need more information to figure out what sort of eco-geographic (and temporal?) separation is occuring or if there is some habitat segregation (MacLeod 2005).

True's (Wonderful) Beaked Whale
Mesoplodon mirus

Despite living in the North Atlantic, this species seems to be rather poorly known. For one thing, the first three confirmed sightings in the Northeastern Atlantic were recorded from 2001-2003 (two sightings may have occurred in 1997 and '99) and apparently the only earlier sighting(s) were from North Carolina (Weir et al. 2004). This ziphiid can be distinguished by other local species due to its apical teeth and resulting closely set parallel scars, a rounded melon that slopes steeply into a short rostrum, a head that is not dorsally or laterally compressed and an appearance overall similar to a big Tursiops ("bottlenose dolphin") (Weir et al. 2004). At least one decomposed specimen appeared to show both a blind accessory main stomach and a blind pyloric stomach in a condition similar to M. europaeus (Mead 2007). Animals in the sightings were estimated at 3.9, 4.5 (n=2), and 4.8 meters long (Weir et al. 2004) and stranding data (n=34) gave males a median length of 4.56 m and females a 4.87 m median length (4.8-4.9 mode) - however males were recorded with a somewhat larger maximum size (5.33 m vs. 5.26 m) (MacLeod et al. 2005 - Appendix I). Although the sample size is limited, M. mirus appears to be larger than the more southernly M. europaeus and slightly smaller (or similarly sized?) than the more northernly M. bidens - possibly echoing the larger size attained by H. ampullatus in comparison to Ziphius.

It was first realized in 1959 that M. bidens also occurred in the southern hemisphere (MacLeod 2005) and some have suspected that this antitropical population may represent a separate species (MacLeod et al. 2006). Molecular evidence suggests that the populations of M. mirus form a monophyletic clade with a deep divergence that suggests either separate species or subspecies (Dalebout et al. 2007). Apparently Dalebout et al. have some unpublished data on the subject so this may be a subject we'll hear about again.

Gervais' Beaked Whale
Mesoplodon europaeus

This species is roughly similar in appearance to M. mirus but it can be distinguished by proportionally smaller pectoral flippers, teeth located 1/3 the length of the mouth from the apex, rostral flattening and a less dolphin-like profile (Norman and Mead 2001). It has been suggested that the tooth placement suggests that this species is rather basal for a mesoplodonts (somewhat more derived than M. mirus) (Norman and Mead 2001) but I'm wary about a one character phylogeny. What's really weird is that one study placed this species in a clade with Hyperoodon to the exclusion of other mesoplodonts (Bianucci et al. 2007) but only 4 of 14 extant mesoplodonts were included and it looks like we're going to have to wait for a hybrid morphological/genetic analysis to sort out ziphiid phylogeny. It probably would tell us a great deal about why the mesoplodonts have the distributions that they do.

M. europaeus is also unusual since it is the only ziphiid where sexually dimorphic size seems to be consistent (and considerable) in both maximum and median sizes (MacLeod 2005 - Appendix I). Males have a median length of 4.09 m compared to 4.32 m for females (mode = 4.5-4.6 m) and males reach a maximum length of 4.57 m compared with 4.85 m in females (MacLeod 2005 - Appendix I). MacLeod brings up the possibility that strandings may have some bias towards size and of course the implications of such sexual dimorphism are not known. This species does appear to be larger than more northernly mesoplodonts and is roughly the same size as M. densirostris.

I should point out that this species is also not endemic to the North Atlantic either and while its southern hemisphere distribution is not clear, if the water temperature preferences are consistent it should range south to Uruguay and Angola.

While Norman and Mead 2001 have photographs of a stranded whale being held in captivity, up until recently this species has never been positively identified at sea (Reeves et al. 2002) and judging by the distribution maps in MacLeod et al. 2006 both populations now have a single definite sighting.

I believe this is another specimen that also happened to wash onshore in Florida.

Sowerby's Beaked Whale
Mesoplodon bidens

This species can be distinguished by other north Atlantic mesoplodonts by tooth projection in the middle of an arch-less mandible coupled with a concave forehead (Carlstrom et al. 1997). M. bidens only appears to have around a dozen sightings to its name (MacLeod et al. 2006) so these features are probably not very distinctive for an open-ocean animal that avoids ships (many sightings were probably categorized as "Mesoplodon spp."). The presence of an "ossicular dental support" is an autapomorphy for this species and not only does it let us know what forces are being put on the teeth, it could provide evidence as to how the incredible stepped jaw of M. densirostris evolved. M. bidens does appear to occupy areas where M. densirostris is absent (i.e. very high latitudes) (MacLeod 2005) but it doesn't seem obvious why one mesoplodont species would begin to develop more extreme sexual dimorphism.

Sowerby's beaked whale does not seem to have sexual size dimorphism, males and female medial lengths are 4.5 and 4.49 m respectively and the modes are 4.7-4.8 for males and 4.8-4.9 for females (MacLeod 2005 - Appendix 1). There was a reported 5.5 m maximum for males, but it appears that the max is likely 4.95 m, in comparison to 5.1 m for females (MacLeod 2005 - Appendix 1). This species does appear to reach larger sizes than the warmer water-inhabiting M. europaeus but there isn't any obvious size difference in comparison to M. mirus which inhabits somewhat lower latitudes. It is possible that the sample size was insufficient to see the pattern (or it was biased somehow) but is there a chance than something other than proportionate bodily surface area plays a role in distribution? M. mirus and M. europaeus both have a derived stomach anatomy with secondary blind main and pyloric stomachs but M. bidens has a second main stomach in a direct series (in addition to a blind pyloric stomach) (Mead 2007). Analysis of stomach contents showed that this species seems to be heavily reliant on benthopelagic fish, but M. mirus also appeared to be heavily reliant on fish (MacLeod 2005). It could very well be possible that this was due to some localized abundance of prey items since sample size is limited of course. Analysis of a nitrogen isotope in order to determine trophic level also curiously predicted that this species preyed on much larger items than stomach content shows, also this could also possibly be related to a fish-heavy diet (MacLeod 2005). For now it seems safe to assume that mesoplodonts are generalists, but if there are some prey preferences this would make the whole situation even more complicated...

It seems amazing how little we know about animals that live in the North Atlantic but incredibly this appears to be one of the best studied area for mesoplodonts. Things get murkier and more complicated from here, and hopefully the picture that is beginning to form here will be useful trying to make sense of everything.

References:

Bianucci, Giovanni et al. 2007. A high diversity in fossil beaked whales (Mammalia, Odontoceti, Ziphiidae) recovered by trawling from the sea floor off South Africa. A high diversity in fossil beaked whales (Mammalia, Odontoceti, Ziphiidae) recovered by trawling from the sea floor off South Africa. Geodiversitas 29 (4) : 561-618.

Carlstrom, Julia et al. 1997. Record of a new northern range of Sowerby’s beaked whale
(Mesoplodon bidens). Polar Biol 17: 459±461

Dalebout, Merel L. 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

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. and Herman, Jerry S. 2004. Development of tusks and associated structures in Mesoplodon bidens (Cetaceae, Mammalia). Mammalia 68 (2-3) pp. 175-184.

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

Norman, Stephanie A. and Mead, James G. 2001. Mesoplodon europaeus. Mammalian Species. No. 688, pp. 1-5.

Weir, Caroline R. et al. 2004. Three sightings of Mesoplodon species in the Bay of Biscay: first confirmed True’s beaked whales (M. mirus) for the north-east Atlantic? J. Mar. Biol. Ass. U.K. 84, 1095-1099