Tetrapod Zoology

Dissecting an emu

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ResearchBlogging.org

Back in 2006 my good friend Matt Wedel – who you may know better as one of the three SV-POWsketeers or as plain old Dr Vector – produced a short article on an emu dissection he participated in at the University of California at Santa Cruz [adjacent image: an Emu Dromaius novaehollandiae... though note that this one is very much alive and not involved in any way in the photos you are about to see].

Because I like talking about ratite anatomy, I’ve been meaning for ages to steal borrow showcase Matt’s photos here at Tet Zoo. Finally, I’m doing it now (with his permission), better late than never. I’ve dissected a few birds in my time, but never a palaeognath: here are some of the highlights. If you dislike gore, this is probably not for you…

First off, here is the skinned bird. Note the muscular thigh and tiny, twig-like arm (read on). The neck is tightly coiled (such that the dorsal surface of the head is almost resting on the dorsal surface of the neck-base) but has been detached from the oesophagus and trachea.

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Emus have 17 or 18 cervical vertebrae (Mivart 1877), and of course I’m sure you’ll be wondering whether the bird can bend its neck to such an impressive degree during life. I’m not sure, though it probably can. I’m resisting the urge here to talk about neck flexibility in birds as this is a subject we’ll be coming back to in a reasonably short time. Anyway, note for now that what looks like a big sac is located at the base of the trachea: we’ll come back to that in a minute.

You’ll notice the big clumps of orange fat on parts of the body and piled on trays at the back of the shot: this bird, which weighed about 46 kg, carried 8 kg of fat. Captive animals typically carry a lot more fat than wild ones but, even so, it’s apparently normal for emus to have large amounts of fat stored on the back. While on the subject of mass, the feathers of this individual accounted for about 7.5% of its total mass. That seems about comparable to what you might expect for a normal terrestrial mammal, but it’s quite different from a flying bird, where the much larger, longer and more complicated feathers account for about 20% of the total mass.

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This picture shows one of the best known (or is it one of the most poorly known?) features of emu: the pendulous throat pouch mentioned a moment ago. While cartilaginous rings support the trachea for most of its length, a group of 7-12 incomplete rings down at the base of the neck form a ventral cleft that’s covered by a membrane [the tracheal cleft is shown below: note that you're looking down onto the cleft from an incision made in the dorsal surface of the trachea]. This membrane bulges outwards (particularly during the breeding season, and particularly in females) to form a sac that extends in parallel to the trachea and functions in the production of loud, booming calls. This is all unique to emus and not seen in other ratites (Cho et al. 1983). Incidentally, neural data indicates that emus have specialised hearing that allows them to detect low-frequency, and even infrasonic, noises. It’s now known that cassowaries can produce infrasonic noises (Mack & Jones 2003), and given that they’re the closest relatives of emus it seems plausible that emus might actually be able to make, and not just hear, infrasonic noises too. In fact, I predict that emu will eventually be shown to be capable of making infrasonic noises: so far as I can tell, no-one has tested this possibility yet.

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While on the subject of ratite tracheae, some other particularly impressive tracheal specialisations are seen elsewhere in the group: in the moa Euryapteryx a 1-m-long loop in the trachea (it effectively doubled the length of the trachea) extended along the left side of the chest and across the thorax. Like the super-long tracheal loops of swans, cranes, certain birds-of-paradise and some other birds, this was almost certainly used in making loud noises with low formant frequencies: I must discuss this stuff some other time.

Let’s finish with a de-feathered arm, or wing if you want. Note how gracile and slender it is. And you’ll also note that it bears a pretty decent, curved claw. It doesn’t seem to be widely known that hand claws are widespread and in fact wholly normal in extant birds. That’s right: hand claws are not unique to hoatzins and to fossil birds like Archaeopteryx. Digit I claws are generally present in ratites, galliforms, anseriforms, ciconiiforms, charadriiforms and others (Fisher 1940) – a distribution which strongly suggests that they are primitively retained throughout neornithines, and lost selectively here and there, especially among so-called ‘higher landbirds’. Digit II claws are rarer but they are present in some basal neornithines (like juvenile anseriforms), so might also be primitive for the clade.

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Well, that’s that, thanks again to Matt, and to his former lab-mates at UC. This article should be seen as part of a long-running Tet Zoo series on ratites that I still haven’t gotten close to finishing. For the previous instalments see…

Refs – –

Cho, P., Brown, R. & Anderson, M. 1983. Comparative gross anatomy of ratites. Zoo Biology 3, 133-144.

Fisher, H. I. 1940. The occurence of vestigial claws on the wings of birds. American Midland Naturalist 23, 234-243.

Mack, A., & Jones, J. (2003). LOW-FREQUENCY VOCALIZATIONS BY CASSOWARIES (CASUARIUS SPP.) The Auk, 120 (4) DOI: 10.1642/0004-8038(2003)120[1062:LVBCCS]2.0.CO;2

Mivart, S. G. 1877. On the axial skeleton of the Struthionidae. Transactions of the Zoological Society 10 (1), 1-52.

Comments

  1. #1 Tim Morris
    April 27, 2009

    Anyone who has eaten a roast chicken, and knew where to look, would know that they do have claws.

  2. #2 MartinB
    April 27, 2009

    “it seems plausible that emus might actually be able to make, and not just hear, infrasonic noises too. ”

    I am surprised because I thought this was known. Lasty ear I visited a small zoo here in germany where I suddenly got a very eery, unwell feeling. Then I heard a very deep, rumbling sound and finally found that some emus were responsible forthe latter. Since it is known that infrasound can cause feelings of discomfort, I immediately assumed that they were capable of it.

  3. #3 Michael P. Taylor
    April 27, 2009

    My favourite one-page paper is Brodkorb (1955), which is basically just a table showing the weights of various parts of a bald eagle. The headline figures, for me anyway, were the feathers totalling 16.59% Of total body mass (14.36% contour feathers + 2.23% down) while the skeleton provided a feeble 6.66%. So the feathers of a bald eagle weigh TWO AND A HALF TIMES AS MUCH as the skeleton. That came as quite a shock. (Most of the mass — 57.81% — was accounted for by muscles and nervous system)

  4. #4 Hai~Ren
    April 27, 2009

    Very nice indeed, although I’m suddenly feeling a little hungry.

    I suddenly have this bizarre mental image of a ratite anatomy gallery, where there is this room in which an ostrich, emu, cassowary, rhea and kiwi are being dissected simultaneously. If it ever happens I guess it would an excellent way to compare the similarities and differences between these various ratite species.

  5. #5 Graydon
    April 27, 2009

    Feathers are configurable, sectional exoskeleton. Of course the mass is significant. :)

  6. #6 Darren Naish
    April 27, 2009

    YAY – blogroll is back!

  7. #7 Dr Vector
    April 27, 2009

    Ha, cool! Thanks for bringing some of my disgusting adventures to a wider audience. A couple of notes: almost all of the fat on the bird was located over the synsacrum (i.e., baby got back), and came off as big thick pads, sort of like a skillet-full of congealed scrambled eggs. I assume that fat is stored there because it is close to the center of gravity, has a broad bony support, and doesn’t hinder the respiratory movements of the ribcage.

    Also, in the third photo down you can see the big sternal callosity, on which the bird rests its torso when sitting.

    I pulled together all my ratite posts, including more dissection pictures, and linked to them from here.

  8. #8 Omphaloskepsis
    April 27, 2009

    Do emus do anything with their claws, or are they just vestigial?

  9. #9 Zach Miller
    April 27, 2009

    I second Omphaloskepsis’ question, but with “extent birds” in place of “emus.” Seems like they could be useful in grooming downy feathers. But they could just be like non-avian theropod first toes–don’t really have a purpose. They’re just there.

    My old piano teacher lived next to a couple who raised emus (yes, up here in AK). I visited their “farm” one time, but they only had one emu when I visited. It was a large female, and she was very curious about us. She did a bizarre “prance” around the backyard, lifting her feet comically high and bobbing her head around. The whole time this was going on, she made these deep, rumbling “drum-beats” which the breeder told me were produced in her throat pouch. She eventually calmed down enough that we went into her pen, and she just kind of watched us from afar. Beautiful bird.

    Another question: Does the throat pouch have an osteological correlate? I ask because I wonder if this sort of structure could ever be detected in fossil animals (like dinosaurs).

  10. #10 Dr Vector
    April 27, 2009

    Another question: Does the throat pouch have an osteological correlate? I ask because I wonder if this sort of structure could ever be detected in fossil animals (like dinosaurs).

    Nope! The trachea is completely outside the skeletal muscles that enclose the vertebral column, and the pouch is anterior to the trachea, under the skin. It’s a good question, and a frustrating answer, because there is a lot of cool stuff that we are simply unlikely to ever detect in the fossil record.

    It is likely that subcutaneous air sacs were present in many ornithodirans. Evidence comes from distal wing pneumaticity in pterosaurs, pneumatic gastralia in Aerosteon, and, uh, something I can’t tell you about yet. ;-)

  11. #11 Dr Vector
    April 27, 2009

    Anyone who has eaten a roast chicken, and knew where to look, would know that they do have claws.

    To my extreme embarrassment, I did not know this. We had a faculty seminar today with roast chicken for lunch. As usual I collected everyone’s leftover bones, and sure enough, every single manus had a claw. A timely tip–thanks!

  12. #12 Nathan Myers
    April 27, 2009

    I saw a claim elsewhere that the long trachea of the swan was necessary to maintain tolerable blood pH, or lung CO2 concentration, or something, despite a very high respiration rate. Could its use in calling be secondary?

  13. #13 Darren Naish
    April 27, 2009

    Thanks to all for brilliant comments…

    — On the function of neornithine manual claws, so far as I know they are functionless, though juvenile hoatzins and turacos climb with them. Claims that the claws of some neornithines became re-enlarged and then functioned in predation are, sadly, not supported.

    — On osteological correlates for weird tracheal structures: what Matt said, but… if you have the tracheal rings preserved (and you sometimes do, as the rings are sometimes ossified), it’s quite conceivable that a structure like the emu’s tracheal pouch could at least be suggested.

    — On the long, looping trachea of swans, I’ve heard (informal) suggestions that they might play a role in respiration, but I think this is unlikely and I haven’t seen it supported in the literature: tracheal elongation is distributed in diverse birds, and the only thing they have in common is that they produce particularly loud vocalisations for their size. I’m inspired to produce an article on this subject, stay tuned.

  14. #14 Owlmirror
    April 27, 2009

    I searched for the original paper on hand-claws, and could not find it, before realizing that copy-and-pasting titles sometimes leaves spelling errors. Ahem. Scholar.google has it, though.

    The Occurrence of Vestigial Claws on the Wings of Birds

    I’d like to know more about them (I don’t have full access to the journal articles).

    This (found searching on the misspelled title) looks like a good summary, though.

    http://dml.cmnh.org/1998Jul/msg01020.html

    “little or no taxonomic significance” — really?

  15. #15 John Scanlon FCD
    April 27, 2009

    Those really long tracheae in swans and some other large birds… has anyone done an analysis of airflow and applied info from these birds to sauropods? Of course the problem with a long trachea is that its volume approaches that of the lungs (+ air sacs if present), beyond which it cannot go (because then you’d never get ANY fresh air). There’s a fun experiment they do in undergrad physiology labs where the students see how long they can breathe through a hose of various lengths… how we laughed. Making the diameter smaller doesn’t help much because it increases resistance, again limiting the volume of fresh air that can be inhaled per unit time or muscular effort. The best solution might be to split the trachea into incurrent and excurrent pipes to eliminate the dead space, but no vertebrate is known to have done this. Maybe sauropods did, I dunno.
    The variation in throat pouch size with sex and season supports the idea that it’s heavily involved in social signalling, but it might be primarily an adaptation for desert living. When it’s hot, emus spend a lot of the time standing in little patches of shade, gaping slightly, fluttering the throat; evaporative cooling, like panting in mammals. What’s the role of the throat pouch in this?

  16. #16 mus
    April 27, 2009

    I find the subject of wing claws very interesting. It’s just one of those cool things that really brings back the fact that birds are extant dinosaurs.

    I used to have two lovebirds, and while they never let me look, I am pretty sure that they had wing claws. I took a photo once in which a small gray thing on a wing was EXTREMELY suggestive of a claw. It’s incredible that I had two adorable little dinosaurs flying about in my living room.

  17. #17 Carlos
    April 28, 2009

    As Mus suggested, do Psitaciformes have wing claws? I know not all birds have them (seriemas, for instance, which have been proposed as parrot/passerine/falcon relatives), and I’d also like to know if pelagic soarers like frigate birds lost them as well.

  18. #18 David Marjanović
    April 28, 2009

    “little or no taxonomic significance” — really?

    Reading Fisher (1940) right now. (We have JSTOR access here in the museum! Yabbadabbadoo!!!) He did not find a wing claw in any of the 241 alcoholic specimens of 68 genera (genera of 1940, that is; might be three times as many today) of passeriforms he had available… though near the beginning of the paper he mentions Owen describing a claw or spur on a Syrian blackbird.

    He also didn’t find wing claws in Piciformes, Coraciiformes, Trogoniformes, Columbiformes, and “Colymbiformes” ( = Podicipediformes = Podicipedidae), nor in Hemiprocnidae (the tree swifts – the true swifts, “Micropodidae” = Apodidae, have them).

    There’s also a lot of individual variation, and apparently some ontogenetic variation (claws being found more often, and “better developed”, in hatchlings than in adults).

  19. #19 Monado
    April 28, 2009

    I’ve often wondered, given dinosaurs’ upright stance and implied fast metabolism, why reconstructions always give them spindly thighs instead of muscular haunches. Can you enlighten me?

    Ditto a point made long ago, possibly in The Hot-blooded Dinosaur, if ceratopsians were chomping cycad trunks with their beaks (and somebody was, because when they died out so did plants with their seeds embedded in their trunks), why aren’t they portrayed with muscular cheeks like a rodent’s instead of that gaunt look?

    Have you ever noticed that emus have twinned, pliable, Dr.-Seuss-style feathers?

  20. #20 Carlos
    April 28, 2009

    Dinosaurs could have had cheeks, but appearently they weren’t muscled like mammalian cheeks; just skin flaps like those of condors.

  21. #21 Wilbert
    April 28, 2009

    Emu’s rule ! But has anybody out there have some information about an enigmatic holocene creature like Mullerornis ?

    Also the Ergilornithidae (Gruiformes actually which resembled the ostriches) are sorely missed.
    I believe that in Asia Struthio lived sympatric with Amphipelargus.
    But as usually there’s hardly any information available.

    But still
    Emu’s kick ass !

  22. #22 Nathan Myers
    April 28, 2009

    Who ate it afterward?

  23. #23 John H
    April 29, 2009

    I should post my photos of our dissection of “Britain’s largest ostrich” – 130kg male. He was awesome, of course.

  24. #24 John Jackson
    April 29, 2009

    [from Darren: sorry, delayed by spam filter due to all those links]

    Screamers have pretty useful wing claws. The Southern Screamer at the bottom of this webpage appears to have at least two on its wing.
    Here’s one in real life, also with two on each wing.
    This page tells a little of what they use them for, and other things.
    Here’s a screamer in flight, showing the claws.

  25. #25 Rosel
    April 30, 2009

    @ John H
    Please do! Sounds fascinating.

  26. #26 David Marjanović
    April 30, 2009

    Mullerornis is a close relative of Aepyornis, and about 1/3 of its mitochondrial genome was sequenced in 2001 and used in a “ratite” phylogeny paper in Nature…

    Screamers have pretty useful wing claws.

    No, these are spurs – bone spikes that are outgrowths of the metacarpals –, not claws. They sit at a right angle to the fingers.

  27. #27 John Jackson
    April 30, 2009
    Screamers have pretty useful wing claws.

    No, these are spurs – bone spikes that are outgrowths of the metacarpals –, not claws. They sit at a right angle to the fingers.

    Oh.

  28. #28 John Jackson
    April 30, 2009

    Someone called Sidney Hugh Reynolds writes in his 1897 “book “The Vertebrate Skeleton” Google throws up:

    “Claws should not be confounded with spurs, which are conical horny structures developed on bony outgrowths of the radial side of the carpus, metacarpus, or metatarsus. They occur in a number of birds, but are most commonly developed in gallinaceous birds, by which they are used for fighting. A single spur occurs on the metacarpus in Megapodius[that mound brooder], in Palamedea[a screamer], in Parra jacana and in Hoplopterus spinosus, the Spur-winged plover. The Derbian Screamer, Chauna derbiana, has two metacarpal spurs, borne on the first and second metacarpals. The Spur-winged goose, Plectropterus gambensis, has a carpal spur borne on the radial carpal. Metatarsal spurs are quite common.

    The male Solitaire (Pezophaps) has large bony excrescences on the wrist which may, like spurs, have been sheathed in horn and used for fighting.”

    (Oh, BTW, thanks for tidying up my urls earlier.)

  29. #29 tammie ann rolfe
    May 15, 2009

    i think you so much for the info it helped me so much with my sons school work. children should be able to look up all there home work

  30. #30 Cody Blais
    October 5, 2009

    your head is sick only a retarded sicko would put this on the internet who dropped u on your head when you were just a wee lad(or lasse)

  31. #31 Hai~Ren
    October 10, 2009

    Cody Blais: Oh seriously, GROW UP.

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