Monday, July 7, 2008

Ziphius

Ziphius was first described as an extinct genus* by Cuvier from a "petrified" partial skull and it took several decades to establish that it was extant (Heuvelmans 1968). While the holotypic skull (and a paratype) were from the Mediterranean, the first complete specimen was found in New Zealand (Heuvelmans 1968) at a near-antipodal point; it is now known that Ziphius has a range from the tropics to polar waters and is the most widely distributed ziphiid (MacLeod et al. 2006) as well as the most frequently stranded (Hardy 2005). It has been observed that Z. cavirostris varies considerably across its range and while some have expressed uncertainty as to whether Ziphius is monotypic or not (Hardy 2005) recent mtDNA analysis over most of the species' range supported that it is a single species (Dalebout et al. 2005).

* The genus originally included several extinct species until Huxley whittled it down to Z. cavirostris - he was uncertain if it actually was fossilized or not (Huxley 1864). It has been suggested that this name is somehow related to the swordfish Xiphias (in the family Xiphiidae) (Hardy 2005) but I'm not clear on how. As you can see, this is the type genus for Ziphiidae, but it wasn't the first one described.


It has been proposed that Ziphius shares a subfamily "Ziphininae" (sic?) with Berardius and Tasmacetus, but molecular evidence does not support such a clade (
May-Collado and Agnarsson 2006, Dalebout et al. 2004, Dalebout et al. 1998). Fossil evidence also suggests that Ziphius is alone in Ziphiinae, but the subfamily seems to have been more diverse in the past. Members of the subfamily are defined by a transverse premaxillary crest directed anterolaterally and reduced contact between the nasal and the premaxillary crest (Bianucci et al. 2007) and it has also been defined as including the common ancestor of Ziphius and Ziphirostrum and all of its descendants (Lambert 2005). Fuller and Godfrey 2007 suggest that Messapicetus is the most basal species and outside of Ziphius/Ziphirostrum but oddly their Fig. 5 puts Messapicetus + Ziphius in the tribe (i.e. sub-sub-family) Ziphiini. Nitpicking aside, it is worth pointing out that Messapicetus has a very elongated rostrum with multiple pairs of teeth (Fuller and Godfrey 2007) quite unlike the modern Ziphius. Members of Ziphiinae did not appear to loose alveoli until the Choneziphius + Ziphius clade (Fuller et al. 2007) and despite having reduced maxillary alveoli reduced and unlikely to support teeth, Ziphirostrum still showed tooth wear and may have supported maxillary teeth mostly by gum (Lambert 2005). Ziphius has replaced alveoli with an alveolar trough but specimens with vestigial teeth in the gums of both jaws have been documented (Gomerčić et al. 2006) - apparently this is not unusual for ziphiids but further documentation is in an article ("Rows of small teeth in ziphioid whales") that is not accessible (it was cited in Fordyce et al. 2002). The presence of functional teeth would seem to suggest that extinct species had a much different niche from the extant one (less teuthophagous?) but a structure called a prenarial basin could hint at some behavioral similarities.

Hardy's thesis, previously discussed in the Hyperoodon post, also discussed the implications of the prenarial basin as a sexually dimorphic trait. Extinct members of Ziphiinae reported not to have this feature (Izikoziphius, Messapicetus) could thus be female specimens. Anyways, Hardy 2005 describes that as a male Ziphius ages, the melon changes shape and differentiates into high and low density regions - the latter of which is functional, occupies the prenarial basin and appears to be homologous with the spermaceti organ of the sperm whale (Physeter). While the development of maxillary crests in Hyperoodon ampullatus appeared to be protection from head-butting, the prenarial basin appears to protect the melon from head-on "jousts" in Ziphius (Hardy 2005). Mesoplodonts do not appear to need such a protective basin thanks to the teeth being raised and located more posteriorly (Hardy 2005) but how some mesoplodonts with more apical teeth (plus Indopacetus and Tasmacetus) and no obvious protection avoid melon damage is a good question. It should also be pointed out that in species that joust, larger male size is probably a hindrance (thanks to a larger turning radius) and the median lengths for male and female Ziphius are pretty much identical (5.5 and 5.47 m) (MacLeod 2005 - Appendix 1). If the prenarial basin was used for protection from head-butting, we would anticipate a considerably larger size in some male Ziphius - as is the case for H. ampullatus (MacLeod 2005).

I've mentioned before that Ziphius has a huge range and while it presumably overlaps with just about every other ziphiid, it does appear to segregate with H. ampullatus (MacLeod 2005). Ziphius does not occur above 60 degrees N in the Atlantic and H. ampullatus does not occur below 40 degrees N and in the area where they are sympatric it appears that there is spatio-temporal segregation (MacLeod 2005). Both species consume large prey items (relative to mesoplodonts) and it seems probable that they occupy the same niche - Ziphius also overlaps with Berardius and Indopacetus and presumably these large prey consumers occupy different niches, perhaps related to their large group sizes (MacLeod 2005). MacLeod also made mention of body size and water temperature potentially influencing niche separation (e.g. smaller occupies warmer water) but distribution maps in MacLeod et al. 2006 have sightings of off the coast of Antarctica - overlap with H. planifrons and the accuracy of these sightings probably needs to be addressed.

Even though a lot of work has been done on Ziphius recently, it is still regarded as a poorly known species (Moulins et al. 2007). For instance, while Ziphius is well known to be widespread and strands very frequently, little is known about local populations and their abundance (which could be quite considerable) (Dalebout et al. 2005). Dalebout et al. 2005 estimated that there are 456-916,000 breeding adults worldwide (Hawai'i alone has over 12,000) and there is a population in the Mediterranean which appears to be small in population and unique. Genetically it appears that geographical location shapes genetic diversity much more in Ziphius than in the Sperm whale (occupying a roughly similar niche) and it appears that the social structure is not matrifocal such as in that species (it is not known what the social structure is) (Dalebout et al. 2005). While there don't appear to be multiple species in Ziphius, a lot more analysis needs to be done and it seems likely that more Evolutionary Significant Units (like the Mediterranean population) will become apparent.

While apparently numerous, Ziphius still does have a number of threats. They have been hunted in Japan and a few other countries and it is one of the species affected by sonar testing (Reeves et al. 2002, Dalebout et al. 2005). Reeves stated that the relatively common occurrence of the species could mask how vulnerable it is, and the status (and existence!) of small regional populations needs investigating. Well, a lot of stuff needs investigating but fortunately we know a lot more now than we did even a few years ago.


Next, ugh, mesoplodonts. This could get ugly...


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.

Dalebout, M. L. et al. 2004. A Comprehensive and Validated Molecular Taxonomy of Beaked Whales, Family Ziphiidae. Journal of Heredity 95 (6): 459–473

Dalebout, M. L. et al. 1998. Molecular genetic identification of southern hemisphere beaked whales. Molecular Ecology 7, 687-694

Fordyce, R. Ewan et al. 2002. Australodelphis mirus, a bizarre new toothless ziphiid-like fossil
dolphin (Cetacea: Delphinidae) from the Pliocene of Vestfold Hills, East Antarctica. Antarctic Science 14 (I) , 37-54

Fuller, Anna J. and Godfrey, Stephen J. 2007. A Late Miocene Ziphiid. Journal of Vertebrate Paleontology 27 (2), 535–540

Gomerčić, Hrvoje et al. 2006. Biological aspects of Cuvier’s beaked whale (Ziphius cavirostris)
recorded in the Croatian part of the Adriatic Sea. Eur J Wildl Res 52, 182–187

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

Heuvelmans, Bernard. 1968. In the Wake of the Sea Serpents. Hill and Wang, New York

Huxley, Thomas H. 1864. On the Cetacean Fossils termed "Ziphius" by Cuvier, with a Notice of a New Species (Belemnoziphius compressus) from the Red Crag. Quarterly Journal of the Geological Society 20, 388-396

Lambert, Olivier. 2005. Systematics and phylogeny of the fossil beaked whales Ziphirostrum du Bus, 1868 and Choneziphius Duvernoy, 1851 (Mammalia, Cetacea, Odontoceti), from the Neogene of Antwerp (North of Belgium). Geodiversitas 27 (3) : 443-497.

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

May-Collado, Laura and Agnarsson, Ingi. 2006. Cytochrome b and Bayesian inference of whale phylogeny. Molecular Phylogenetics and Evolution 38, 344–354

Moulins, Aurélie et al. 2007. Aspects of the distribution of Cuvier’s beaked whale (Ziphius cavirostris) in relation to topographic features in the Pelagos Sanctuary (north-western Mediterranean Sea). J. Mar. Biol. Ass. U.K. 87, 177–186

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

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