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.
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.
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.
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.
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.