Tetrapod Zoology

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More on those rorquals: part I is required reading. To those who have seen this stuff before: sorry, am going through a busy phase and no time for new material (blame dinosaurs and azhdarchoid pterosaurs… and baby girls). Oh, incidentally, I recently registered Tet Zoo with ResearchBlogging: I haven’t done this before because ‘blogging on peer-reviewed research’ is the norm at Tet Zoo, not the exception. It seems to take ages for posts to be uploaded to ResearchBlogging – like, hours. Is this normal? Anyway…

This time we look at the basics of rorqual morphology and at their feeding behaviour. The rostrum in rorquals is long and tapers to a point (though it is comparatively broad in blue whales) and, in contrast to other mysticetes, a stout finger-like extension of the maxillary bone extends posteriorly, overlapping the nasals and abutting the supraoccipital (the shield-like plate that forms the rear margin of the skull). The dorsal surfaces of the frontals (on the top of the skull) possess large depressions while the ventral surfaces of the zygomatic processes (the structures that project laterally from the cheek regions) are strongly concave, again unlike the condition in other mysticetes [painting above by Valter Fogato].

Rorqual lower jaws are gigantic, beam-like bones that bow outwards along their length. The symphyseal area (the region where the jaw tips meet) is unfused, as is the case in all mysticetes (even the most basal ones) but not other cetaceans, meaning that the two halves of the jaw can stretch apart at their tips somewhat. Exceeding 7 m in blue whales, rorqual lower jaws are the largest single bones in history (ha! Take that Sauropoda).

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A section of blue whale jaw was once ‘discovered’ at Loch Ness and misidentified as the femur of a gigantic, hitherto undiscovered tetrapod. Occasionally rorqual skulls have been discovered in which the long lower jaws have been stuck wedged inside various of the skull openings and with their tips protruding like tusks. People unfamiliar with cetacean skulls have then naively assumed that the skull belonged to some sort of tusked prehistoric sea monster. Ben Roesch once discussed the case of the Ataka carcass of 1956: a giant beached animal possessing divergent ‘tusks’ that are in fact the separated halves of a rorqual’s lower jaw (see adjacent image).

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I’ve come across another case of this sort of thing. The accompanying newspaper piece, from The Telegraph of June 29th 1908, features a skull trawled up by the Aberdeen vessel Balmedie (sailing out of Grimsby), and thought by the article’s writer to be that of ‘some prehistoric monster’, apparently with tongue preserved. It’s clearly a rorqual skull, and the pointed, narrow rostrum and posterior widening of the mesorostral gutter indicates that it’s a minke whale skull [for other cases in which whale carcasses have been mistaken for ‘sea monsters’ see the Tecolutla monster article].

Moving back to the morphology of the rorqual lower jaw, a tall, well-developed coronoid process – way larger than that of any other mysticete – projects from each jaw bone and forms the attachment site for a tendinous part of the temporalis muscle, termed the frontomandibular stay.

All of these unusual features are linked to the remarkable feeding style used by rorquals. How do they feed? Predominantly by lunge-feeding (also known as engulfment feeding): by opening their mouths to full gape (c. 90º), and then lunging into a mass of prey. Those depressed areas on the frontals and zygomatic processes house particularly large temporalis and masseter muscles, the muscles involved in closing the jaw. The frontomandibular stay provides a strong mechanical linkage between the lower jaw and skull and primarily serves to amplify the mechanical advantage of the temporalis muscles.

ResearchBlogging.org

As a rorqual lunge-feeds, a huge quantity of water (hopefully containing prey) is engulfed within the buccal pouch, transforming the whale from ‘a cigar shape to the shape of an elongated, bloated tadpole’ (Orton & Brodie 1987, p. 2898). While a rorqual uses its muscles to open its jaws, the energy that powers the expansion of the buccal pouch is essentially provided by the whale’s forward motion, and not by the jaw muscles. In other words, the engulfing process is powered solely by the speed of swimming. Orton & Brodie (1987) noted that the engulfed water ‘is not displaced forward or moved backward by internal suction, but is simply enveloped with highly compliant material’ (p. 2905). Rorquals do not, therefore, set up a bow wave as they engulf (UPDATE: by complete coincidence, Paul Brodie told me in a recent email [28th Feb 2009] that he’s just completing a long-in-the-pipeline manuscript containing field data on Sei whale. Wow, I really look forward to seeing this).

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A rorqual may engulf nearly 70% of its total body weight in water and prey during this action, which in an adult blue whale amounts to about 70 tons (Pivorunas 1979). In order to cope with this, the tissues of the buccal pouch must be highly extensible and able to cope with massive distortion. The ventral surface of the pouch is covered by grooved blubber, on which the 50-90 grooves extend from the jaw tips to as far posteriorly as the umbilicus. The ventral grooves can be extended to 4 times their resting width, and to 1.5 times their resting length. Internal to the grooved blubber is the muscle tissue of the buccal pouch, and this is unique, containing large amounts of elastin, and consisting of an inner layer of longitudinally arranged muscle bands and an outer layer where the bands are obliquely oriented (Pivorunas 1977).

When a rorqual lunges, delicate timing is needed, otherwise the buccal pouch will rapidly fill with seawater and not with prey. How then do rorquals get their timing just right? It seems that rorquals possess batteries of sensory organs within and around the buccal pouch: there are laminated corpuscles closely associated with the ventral grooves that might serve a sensory function, and located around the edges of the jaws, and at their tips, are a number of short (12.5 mm) vibrissae. Long assumed to be vestiges from the time when whale ancestors had body hair, it now seems that these structures have a role in sensing vibrations.

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Once a mass of prey is engulfed, a rorqual then has to squeeze the water out through its baleen plates while at the same time retaining the prey. Rorqual baleen plates number between 219 to 475 in each side of the jaw (the number of plates is highly variable within species, with sei whales alone having between 219 and 402), and each plate ranges in length from 20 cm (in the minkes) to 1 m (in the blue). As the whale stops lunging forward, the pressure drops off, allowing deflation of the buccal pouch. Passive contraction of the blubber grooves and active contraction of the muscle layer within the buccal pouch also occurs at this time [adjacent image, showing engulfment process in Fin whale, by Jeremy Goldbogen and Nicholas Pyenson and taken from the UC Berkeley news site. Goldbogen et al.’s research is discussed in the next article. See also Pyenson’s site and Goldbogen’s site].

For an outstanding sequence of photos illustrating engulfment in action, see Randy Morse’s photos of a feeding blue whale.

So that’s the basics. But there’s so much more to the subject than this. How is it that, during lunge feeding, agile, highly reactive prey remain within the mouth cavity prior to the mouth’s closure? Why do some rorquals make loud noises during lunge-feeding? Why, given their giant size and theoretical high aerobic dive limit, do big rorquals not spend more time lunge-feeding beneath the surface? Why do some rorquals exhibit strongly asymmetrical patterns of pigmentation? And don’t forget that not all rorquals lunge-feed. More on these issues in the following post.

Refs – –

Orton, L. S., Brodie, P. F. (1987). Engulfing mechanisms of fin whales Canadian Journal of Zoology, 65, 2898-2907

Pivorunas, A. 1977. The fibrocartilage skeleton and related structures of the ventral pouch of balaenopterid whales. Journal of Morphology 151, 299-314.

– . 1979. The feeding mechanisms of baleen whales. American Scientist 67, 432-440.

Comments

  1. #1 Sordes
    March 3, 2009

    There is something about rorqual anatomy which irritates me since years. We all know that the ventral grooves run from the tip of the lower jaw to about the middle of the body, and that this pouch can bloat up to this region. But now my question is, where does the inside of the mouth actually ends? What happens with the tongue when the mouth is closed but the whole pouch still full of water? And how is the oesophagus protected against the masses of water and the pressure?

  2. #2 Mike Keesey
    March 3, 2009

    opening their mouths to full gape (c. 45º)

    Looks more like 90º in the figures.

    Crazy picture with the “tusks”.

  3. #3 Darren Naish
    March 3, 2009

    Looks more like 90º in the figures.

    I agree, text now changed.

  4. #4 TEO
    March 3, 2009

    Sordes said: But now my question is, where does the inside of the mouth actually ends? What happens with the tongue when the mouth is closed but the whole pouch still full of water? And how is the oesophagus protected against the masses of water and the pressure?

    Good questions… any ideas? would be interesting to know.

  5. #5 Boesse
    March 3, 2009

    I thought I’d mention this as an aside – some Cetotheriidae sensu stricto (small bodied, weird mysticetes from the late Neogen) have a very large coronoid process that is larger/taller/longer than balaenopterids (proportionally to the size of the dentary). Oddly enough, most of these are very small bodied critters (3-6m).

    However, among modern mysticetes, balaenopterids win.

  6. #6 Darren Naish
    March 3, 2009

    Seeing you leave comments reminds me that I must add your blog to my blogroll. I would have done this a while ago (sorry) – unfortunately, the new publishing platform we have at ScienceBlogs has made changing/updating the blogroll nigh-on impossible. It takes ages to actually locate the blog lists in the first place (due to thousands of lines of empty space), and ages and ages to add a new url, which is insane. Am I the only one suffering from this?

  7. #7 Alton Dooley
    March 3, 2009

    Boesse, if you’re talking about herpetocetines (or strict cetotheriids, if you prefer), I have wondered for a while if they converged on the balaenopterid feeding mode. Besides their mandibular oddities, herpetocetines seem to also have some modifications on the cranial vertex that are somewhat similar to balaenopterids (possibly strengthening the connection between the rostrum and the cranium). Of course, they’re tiny, which presents something of a problem for this hypothesis if lunge feeding led to gigantism in balaenopterids.

  8. #8 Zach Miller
    March 3, 2009

    Okay, potentially stupid question, here. I should probably know this already. How do baleen plates grow, and what are they made of? I know the short answer is “keratin,” but there’s gotta be more to it than that. I liked that paper awhile back about how Aetiocetus had both teeth and small baleen plates, but do modern baleen whales show any vistage of this transition, either in the embryo or as babies?

  9. #9 Darren Naish
    March 3, 2009

    On cetotheres, coronoid process and lunge-feeding, etc, I’m pretty sure Bent Lindow (of Stenfugle) did some work on this, though I don’t recall it being published except in abstracts. Plus there’s also…

    Kimura, T. 2002. Feeding strategy of an Early Miocene cetothere from the Toyama and Akeyo Formations, central Japan. Paleontological Research 6, 179-189.

    … which I haven’t seen.

    Spent the last HOUR playing with the Tet Zoo blogroll and trying to add The Coastal Paleontologist. Sorry, the blogroll is totally broken and, unless it gets fixed, I am not ever going to be able to change it again.

  10. #10 Alan Kellogg
    March 3, 2009

    Darren,

    Funny, but the last time I used MT adding links was one of the easier things to do. Are you using the admin interface or trying to edit the source code directly?

  11. #11 Dartian
    March 4, 2009

    Darren:

    It seems that rorquals possess batteries of sensory organs within and around the buccal pouch: there are laminated corpuscles closely associated with the ventral grooves that might serve a sensory function, and located around the edges of the jaws, and at their tips, are a number of short (12.5 mm) vibrissae. Long assumed to be vestiges from the time when whale ancestors had body hair, it now seems that these structures have a role in sensing vibrations.

    Very, very interesting. I hadn’t heard of that before; could you give me a reference?

  12. #12 Mark Lees
    March 4, 2009

    Zach,

    You asked if baleen whales have teeth at an early stage. The answer is that at least some species do (and I presume all do). There was an interesting paper I read sometime ago, I don’t have the author’s names, but they were Japanese, that discussed research on minke whale embryos. I had mixed feelings about the paper since I assume (based on the authors being Japanese and the limited opportunity for sampling minke embryos) that this research was based on Japanese ‘scientific whaling’, which I don’t approve of.

    Minke whale embryos do develop deciduous teeth (milk teeth) that degenerate before birth.

    Apparently the embryonic teeth are not functionless vestiges as the study showed they seem to be related to the development of the baleen.

    Minke embryos apparently develop many decidious teeth in the upper jaw (where the baleen will develop) and much fewer in the lower jaw.

  13. #13 Mark Lees
    March 4, 2009

    Zach,

    You asked if baleen whales have teeth at an early stage. The answer is that at least some species do (and I presume all do). There was an interesting paper I read sometime ago, I don’t have the author’s names, but they were Japanese, that discussed research on minke whale embryos. I had mixed feelings about the paper since I assume (based on the authors being Japanese and the limited opportunity for sampling minke embryos) that this research was based on Japanese ‘scientific whaling’, which I don’t approve of.

    Minke whale embryos do develop deciduous teeth (milk teeth) that degenerate before birth.

    Apparently the embryonic teeth are not functionless vestiges as the study showed they seem to be related to the development of the baleen.

    Minke embryos apparently develop many decidious teeth in the upper jaw (where the baleen will develop) and much fewer in the lower jaw.

  14. #14 Mark Lees
    March 4, 2009

    Sorry for double posting. The content filter on my web proxy said it blocked it the first time (for no obvious reason), so I resent.

  15. #15 Boesse
    March 4, 2009

    Hey Butch,

    I’ve been thinking along similar lines recently as well. Particularly herpetocetines… The combination of the rather extreme interdigitation of the rostral/cranial bones, and the highly derived mandibular construction. Bouetel (2005) mentioned that both balaenopterids and herpetocetines have similar rostral bone articulations, and a laterally oriented hooklike coronoid process.

    Goldbogen et al. 2007 mention that lunge feeding is more or less a modification of raptorial pursuit predation. So, as long as smaller balaenopterids practiced lunge feeding (and we have dwarf minke whales today, that presumably lunge feed) prior to attaining gigantic sizes, it might not be a problem. I’m interested in doing some work in the future on the functional morphology of Herpetocetine jaw movement/feeding.

    Darren: Ah, whatever. If you get it to work, great! If not, maybe your blogroll is just too long! Keep up the rorqual posts; I wasn’t reading this blog when these were first posted, so its all new reading for me, and a good refresher on extant balaenopterids.

  16. #16 David Marjanović
    March 4, 2009

    Minke whale embryos do develop deciduous teeth (milk teeth) that degenerate before birth.

    That’s apparently the case in all baleen whales. The Aetiocetus paper has more on that, including a photo.

    ————————-

    Baleen is basically hair that grows on the palate.

  17. #17 Boesse
    March 4, 2009

    Hey Darren,

    That paper by Kimura refers to the jaw of a cetothere sensu lato – or a “cetothere”. For the last 100 years or so, “cetotheres” were a wastebasket group that all fossil mysticetes lacking apomorphies of extant families were lumped into.

    Recently, Bouetel and Muizon (2006) recognized a monophyletic group within this grade which they termed the cetotheriidae sensu stricto, which includes Herpetocetus, a critter near and dear to my heart.

    So, Kimura’s title is referring to the cetotheriidae sensu lato, and the jaw he describes is very balaenopterid-like, but not very similar to those of cetotheriidae sensu stricto.

    Bouetel, V. and Muizon, C. de, 2006. The anatomy and relationships of Piscobalaena nana (Cetacea, Mysticeti), a Cetotheriidae s.s. from the early Pliocene of Peru. Geodiversitas 28(2): 319-395.

  18. #18 Bent Lindow
    March 5, 2009

    Darren:

    “On cetotheres, coronoid process and lunge-feeding, etc, I’m pretty sure Bent Lindow (of Stenfugle) did some work on this…”

    I’m afraid I will have to disappoint you, Darren. My Master’s thesis work did not involve any work on cetothere feeding. It was just a (as yet unpublished) description of a new mysticete taxon from the Miocene of Denmark and a very preliminary (and rather wrong) cladistic analysis of extinct and extant mysticetes. Luckily, the latter has been superseded by far more well-analysed works such as:

    Steeman, M.E. (2007): Cladistic analysis and a revised classification of fossil and recent mysticetes. Zoological Journal of the Linnean Society 150, pp 875-894

    However, my colleague Mette Steeman did some work on the morphology of the coronoid process and mandibular cavity with regards to feeding habits in extinct mysticetes as part of her Ph.D.-thesis.

    Unfortunately, this rather neat work is still unpublished, so light harassment of Mette might be in order, until she gets her act together and publishes it ;-)

  19. #19 Markus
    October 11, 2010

    Just stumbled over an similiar article about the Balmedie skull:

    Found Sea Monster’s Skull

    The skull of a huge unknown sea monster was brought up by the trawler nets of the Aberdeen steamer Balmedie, while fishing in the Atlantic, north of Scotland, the other day. The skull was subsequently landed there, amid great excitement.
    Mystery surrounds ist classification.
    The skull is of immense size, ist dimensions suggesting that the animal it belonged to was as large as an elephant.
    It is in a wonderful state of preservation, and but for a protuberance from a pair of bell-shaped jawbones there is no flesh covering. This protuberance is leathery to the touch, three feet long, eight inches in circumfence and tapering to a point.
    A little above and to the edge of the tongue space were two huge cavities, quite a foot across, suggesting eye sockets.
    It is thought the relic is part of some pre-historic monster from the Arctic regions, which, having been preserved in ist ice bed for ages, may have been washed down into warmer latitudes on an ice flow before being deposited in the Atlantic.

    Crossfield Chronicle 18. Februar 1909

    BTW after this article the picture of the skull now is shown in various websites together with the pic of the egypt carcass – without any further explanations and so indicating the skull belongs also to the carcass of Ataka. Darren has added a new internet-myth. ;)

  20. #20 David Marjanović
    October 12, 2010

    Markus, turn your spellchecker off. It turned every its into ist, which makes sense in German, but…

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