Does the Hero Shrew have a Functional Analogue?

Scutisorex somereni* (or the hero/armored shrew) is a member of the order Soricomorpha and family Crocidurinae which has derived vertebrae morphology quite unlike even its close relatives** (Allen 1917). The book Axial Character Seriation in Mammals by Aaron G. Filler (p. 65) proclaims that these vertebrae represent the most bizarre and unusual axial morphology from an amniote and the Culliane et al. 1998./Culliane and Aleper 1998 papers cite East African Mammals II by J. Kingdon who is of the opinion that the spinal morphology is probably the most derived among vertebrates. Statements like this made a blog post all but inevitable, but there was at least one fascinating claim I wasn't aware of. Darren Naish's post on his dead mole and a post on Emile's blog alerted me to the North American Upper Oligocene hedgehog Proterix (order Erinaceomorpha) which may have some similarities with Scutisorex.

* S. somereni = S. congicus. The former was erroneously thought to be a distinct species by Allen 1917.
** There is a chance that the hero shrew is a member of Sylvisorex.

From Fig. 6 of Gawne 1968. These appear to be lumbar vertebrae (there is no indication of ribs) arranged so the anterior-most one is on the left. A is lateral, B is ventral and C is dorsal.

The massive vertebrae of the Proterix species (P. loomisi and P. bicuspis) have their sides covered by large vertical plates which extend above and below the body of the vertebrae and overlap posteriorly (Gawne 1968). It doesn't seem clear in the illustration but the spines extend about 3 mm above the plates (Gawne 1968). The vertebrae allow for free vertical movement but the plates allow for very little lateral movement and twisting (Gawne 1968). Emile's blog informs us about Dougal Dixon's portrayal of Proterix as a mole-like animal with compressed and reinforced vertebrae as an adaptation for burrowing*, plating on the head and, oh, limblessness. Gawne doesn't mention any of this and since her paper is the most recent that I'm aware of, where is this information coming from? Skulls and teeth (and in this case, massive vertebrae) fossilize readily thanks to their density and it needs to be said that Proterix also lacks fossilized cervical and dorsal vertebrae; absence of fossilization in such incomplete specimens cannot be taken as evidence of actual absence. Maybe there are informally undescribed Proterix specimens out there which show vestigial limb bones and conform to Dixon's portrayal, but until they're formally published I will continue to regard the limb morphology and lifestyle of Proterix as an open question.


That aside, why did Gawne think that Proterix was similar to Scutisorex? Similarities are not obvious in a superficial comparison:

Vertebrae in anterior view.
The uppermost vertebrae are the 1st and 6th lumbars of Scutisorex, modified from Allen 1917, Fig. 4
The bottom vertebrae are lumbars(?) from Proterix loomisi, modified from Gawne 1968, Fig. 6

Asking why Gawne though the vertebrae were similar is also a question to myself, she never makes it terribly clear. Functionally, she interprets that the vertebrae of Proterix will allow for free vertical motion but lateral movement and twisting will be restricted. Filler's book notes that while Scutisorex can have more than one hundred processes (!) on some lumbars, few of these articulate and the hero shrew has free lateral and dorsal movement. However, Filler notes that the shape of the vertebrae allows for little rotation along the long axis of the column, i.e. twisting. Culliane et al. 1998 suggest that the spinal morphology resists large internally generated loads and compressive forces during flexion, bending and torsion but the circumstances of needing this are not obvious. While the spine of Scutisorex may be a sort of morphological "breakthrough" with unique functional circumstances (Culliane et al. 1998) perhaps the rotationally restricted spine of Proterix is a sort of alternate "low tech" analogue. Levels of lateral flexion may be the result of the differing methods to resist torsion but it could also indicate that Proterix and Scutisorex aren't quite functional analogues. Gawne 1968 also mentions that Scutisorex and Proterix both have expanded mastoids, a strong posterior lambdoid crest and roughening of the skull roof - these appear to be related to a strong bite in both animals. There's always the chance that Proterix really is some weird-ass limbless mammalian mole that evolved torsion resistance and a strong bite for some completely different reasons than Scutisorex, but I wouldn't bet on it.


So exactly what functional circumstances does Scutisorex put itself in? There is the possibility that the hero shrew's spinal morphology evolved with no related behaviors or ecology, suggesting that "complex morphologies can evolve without selection driving their adaptive trajectory" (Culliane and Bertram 2000). I really can't wrap my head around an animal with a spine 7 standard deviations more massive than predicted for its size (Culliane et al. 1998) obtaining it with no selection - the relative energy put into building up such a structure must be phenomenal. Culliane and Bertram 2000 sort-of indicate that the spinal morphology could be related to the general problem of torsion injury; selection is of course involved but unique behaviors and ecological niches may not be. The limited sample size of Churchfield et al. 2007 showed that Scutisorex was a partially subterranean opportunist with a preference for earthworms - this doesn't exactly set it apart from other shrews and can't explain the anatomical adaptations. Oddly, one of Darren Naish's posts stated that hero shrews exploit aquatic environments and on my previous hero shrew post it was suggested that their spinal morphology may be related to levering up stones. The appendicular skeleton of the hero shrew is not adapted for load bearing (Culliane et al. 1998) which I think would be quite problematic for a shrew trying to lever something up.

Absence of evidence for a unique niche isn't of course proof of its absence in such a poorly known animal and claims of unique stone prying behavior need to be addressed. Hopefully with further understanding of Scutisorex it can be determined if Proterix was trying to do something similar. Who knows, maybe more fossil skeletons of "large" (~20 cm) insectivorous mammals will show up with different methods of reducing torsion. And then there's whole witnessing major structural changes thing.


Refs.

Allen, J. A. 1917. The Skeletal Characters of Scutisorex Thomas. Amer. Mus. Nat. Hist., vol. 37, pp. 769-784.

Churchfield, S. et al. 2007. Feeding ecology of the armoured shrew, from the north-eastern Democratic Republic of Congo.

Culliane, Dennis M. and Bertram, John E. A. 2000. The mechanical behaviour of a novel mammalian intervertebral joint. J. Anat. 197, pp. 627-634

Culliane, Dennis M. et al. 1998. The functional and biomechanical modifications of the spine of Scutisorex somereni, the hero shrew: skeletal scaling relationships. J. Zool., Lond. Vol. 244 pp. 447-452

Culliane, Dennis M. and Aleper, D. 1998. The functional and biomechanical modifications of the spine of Scutisorex somereni, the hero shrew: spinal musculature. J. Zool., Lond. Vol. 244 pp. 453-458

Gawne, Constance Elaine. 1968. The Genus Proterix (Insectivora, Erinaceidae) of the Upper Oligocene of North America. American Museum Novitates No. 2315, pp. 1-26. Available

Coyotes of New England

While I was writing about ziphiids, I couldn't help but notice that coyotes (Canis latrans) established themselves in my neighborhood. I've seen the occasional individual around before but the chorus howls only started a couple of months ago. The coyotes seem to avoid me (probably because I make a lot of noise running at night) although they seem to have been within 50 feet of my residence judging by tracks and my neighbors reported them exhibiting curious behavior. Coyotes are still rather recently established in the Eastern US (the eastward expansion started c. the late 1800's) but they don't seem to be quite like their Western relatives.

Eastern coyotes differ from their Western relatives by preying on deer more frequently, living at lower densities, in larger home ranges and smaller group sizes (Way 2007). The largest reliably recorded* extant coyote was a 25.1 kg (55.3 lb), 1.57 m long female ("Casper") from Barnstable, Cape Cod, Massachusetts (Way and Proietto 2005); E. Mass./Cape Cod has the second largest average coyote sizes (17.9 kg M, 16 kg F) and is exceeded by New Hampshire (20.4 kg M, 17.9 kg F) (Way 2007). Weight can vary a lot due to condition, hydration, season and so forth (as opposed to skeletal length) and while average size figures will undoubtedly be revised, the difference between Eastern and Western coyotes (overall average: 11.4 kg M, 10.5 kg F) is consistent and significant enough for them to be placed in different size categories (Way 2007). So how did these coyotes get significantly larger all the sudden and apparently occupy a distinct niche?

*Other records of 25 kg+ coyotes are secondhand and questionable (Way and Proietto 2005).

It has been suggested that there are three possible reasons for large size in Eastern coyotes: introgression of wolf genes, genetic selection due to larger prey size/food supply or phenotypic response to enhanced food supply (Thurber and Peterson 1991). Thurber and Peterson thought the size difference was phenotypic* due to the time frame and because recently arrived coyotes in Alaska did not show larger size than their southern cousins. The authors suggest that this is due to less available food in Alaska, but it needs to be pointed out that the Eastern wolf (C. lycaon) may be a closer relative of the coyote than the wolves that live in the west (C. lupus). North American "higher canids" (C. lupus, C. lycaon/C. rufus, C. latrans) have a rather ambiguous phylogeny thanks to possible hybridization and introgression (Zrzavy and Ricankova 2004) and an article documenting eastern coyote genetic ancestry has yet to be published. Jon Way's website mentions that there is a study awaiting publication on this subject which concludes that the eastern coyote is distinctive and has heritage from both the western coyote and eastern wolf (C. lycaon). Apparently the paper will consider the eastern coyote to be a distinct species, although I'm not sure if eastern coyotes should qualify as anything more than a distinct population. If the admixture really is as extensive as it appears, maybe the coyotes and the eastern wolf (C. lycaon) should be considered some "higher Canis complex" or maybe even the same species. I'll definitely have to comment upon this when it eventually gets published.

* Larivière and Crête 1993 noted that New Hampshire coyotes reached a larger size in captivity than western ones. However, Thurber and Peterson 1991 didn't entirely rule out hybridization in some localized areas.


The idea of having a considerably sized* wild canid roaming around suburban areas is undoubtedly going to make some people uncomfortable, but at the moment there doesn't seem to be much to be concerned about. Well, in New England at least, California has 111 attacks on record (vs. 7 for New England) including one fatality (Timm and Baker 2007). These attacks are the result of habituation caused by increased coyote presence in the area and human activities towards them, such as intentional feeding (Timm and Baker 2007). Domestic dogs are of course responsible for far more fatalities (4 in California from 95/96, see footnotes) and attacks, but we should still try and prevent this sort of situation developing in New England. Much larger coyotes with tendencies to prey on larger animals could be trouble...

*The coyotes that I've seen didn't seem to be much smaller than a greyhound, but the average weights in RI are 16.6 kg (36.5 lbs) M and 15.3 kg (33.6 lbs) F (Way 2007). The record was a 21.1 kg (47 lbs) female (Way 2007). Unexpectedly seeing a canid at night undoubtedly makes them seem much larger.



I'll have to see if I can manage to get a picture of one of these canids, geez has my summer disappeared.

References:

Larivière, S., and M. Crête. 1993. The size of eastern coyotes (Canis latrans): A comment. Journal of Mammalogy 74 (4):1072–1074.

Murray, D. L. and Waits, L. P. 2007. Taxonomic status and conservation strategy of the endangered red wolf: a response to Kyle et al. (2006). Conserv. Genet. 8, pp. 1483–1485

Peterson, Rolf O. and Thurber, Joanne M. 1993. The size of eastern coyotes: A rebuttal. Journal of Mammalogy 74 (4): 1075-1076

Thurber, Joanne M. and Peterson, Rolf O. 1991. Changes in Body Size Associated with Range Expansion in the Coyote (Canis latrans). Journal of Mammalogy, 72 (4) pp. 750-755

Timm, Robert M. and Baker, Rex O. 2007. A History of Urban Coyote Problems. Proceedings of the Wildlife Damage Management Conference, Available

Way, Jonathan G. 2007. A Comparison of Body Mass of Canis latrans (Coyotes) Between Eastern and Western North America. Northeastern Naturalist. 14 (1) pp. 111- 124.

Way, Jonathan G. and Proietto, Robert L. 2005. Record Size Female Coyote, Canis latrans. The Canadian Field-Naturalist v. 119, pp. 139-140

Zrzavy, Jan and Ricankova, Vera. 2004. Phylogeny of Recent Canidae (Mammalia, Carnivora): relative reliability and utility of morphological and molecular datasets. Zoologica Scripta, 33 (4), pp. 311–333

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