This is the second post in the “bird neural canals are weird” series (intro post here), and it covers the first of five expansions of the spinal cord or meninges in the lumbosacral regions of birds.

The lumbosacral expansion of the spinal cord is not unique to birds and doesn’t require any special explanation. As noted in the slide, all limbed tetrapods and some fishes with sensitive fins have adjacent segments of the spinal cord correspondingly expanded. These expansions house the extra afferent neurons needed to collect sensory inputs from the limbs, the extra efferent neurons needed to provide motor control to the limbs, and the extra interneurons needed for sensory and motor integration (including reflex arcs) – ‘extra’ here meaning ‘more than are required for non-limb neck, trunk, and tail segments’.

Humans have these, too, in our lower cervical vertebrae to run our forelimbs, and in our lower thoracic vertebrae to run our hindlimbs. Recall that the segmental anatomy of the adult human spinal cord corresponds increasingly poorly to the vertebrae the farther we are from the head because of our child-sized spinal cords (see this post for more).

So if the lumbosacral expansion is present in all tetrapods with hindlimbs, why bring it up? My goal is to develop a set of criteria to distinguish the various spinal and meningeal specializations in birds, in part because it’s an interesting challenge in its own right, and in part because doing so may help illuminate some unusual features in sauropods and other non-avian dinosaurs. If we want to be able to detect whether, say, a glycogen body is present, we need to know how to tell the impression left by a glycogen body from the more generalized lumbosacral expansion present in all limbed tetrapods. The key characteristics of the lumbosacral expansion are that the cord (and hence the canal) expands and contracts gradually, over many segments, and that the expansion is in all directions, radially, and not biased dorsoventrally or mediolaterally.

Numbering reflects spinal nerve count – 8 cervical, 12 thoracic, 5 lumbar, and 5 sacral spinal spinal nerves. Cervical expansion for the forelimbs is roughly C5-T1, and lumbosacral expansion for hindlimbs is L2-S3. Gray (1918 image 665).

The one way in which the lumbosacral expansion of birds is weird, at least compared to mammals, is that the magnitude of the change is so great in hindlimb-dominant flightless birds like the ostrich. Here’s a graph from Gray’s Anatomy showing the cross-sectional area of the human spinal cord in square mm, with the head on the left. Note that the swellings for the limbs bump up the cross-sectional area by a quarter to a third, relative to adjacent non-limb areas.

Streeter (1904: fig. 4)

Here’s the same diagram for an ostrich, again in square mm, again with the head to the left. The lines here are a little different – the “substantia grisea” is the gray matter (mostly neuron cell bodies), and the white matter (axons, mostly myelinated) is divided into the large ventrolateral funiculi (descending motor, ascending pain, temperature, and unconscious proprioception) and the much smaller dorsal funiculi (ascending touch and conscious proprioception). Here the lumbosacral expansion maxes out at more than double the cross-sectional area of the cord in the inter-limb torso segments – and this is just the white and gray matter, and does not include the glycogen body (which is proportionally small in the ostrich, as we’ll see in a future post).

Note that the ostrich does have a much smaller expansion of the spinal cord associated with the forelimbs, but one glance at the graph will tell you that the hindlimbs are a lot more important. This too has implications for fossils. Because the cross-sectional area of the neural canal tends to track the cross-sectional area of the spinal cord (despite the cord not filling the canal), it is possible to make inferences about limb use in fossil taxa based on the relative cross-sectional area of the neural canal along the vertebral column. Emily Giffin published several papers about this in the 1990s (e.g., Giffin 1990, 1995), all of which are worth reading.

Next in this series: the glycogen body.



Dorsal vertebra of a rhea from the LACM ornithology collection. Note the pneumatic foramina in the lateral wall of the neural canal.

If you’ve been here for very long you know I have a bit of a neural canal fixation. Some of this is related to pneumaticity, some of it is related to my interest in the nervous systems of animals, and some of it is pure curiosity about an anatomical region that seems to receive very little attention in proportion to its weirdness – especially in birds.

Human thoracic vertebrae in midsagittal section showing vertebral venous plexus. Gray (1918, image 579), available from

The neural canals of mammals are pretty boring. The canal is occupied by the spinal cord and its supporting layers of meninges, and the rest of the volume is padded out by adipose tissue and blood vessels, notably an extra-dural venous plexus. Aaand that’s about it, as far as I know. (If there are weird things inside mammalian neural canals that I’ve missed, please let me know in the comments – I’m a collector.)

But not so in birds, which have a whole festival of weird stuff going on inside their neural canals. Let’s start with pneumaticity, just to get it out of the way. Many birds have supramedullary diverticula inside their neural canals, and these can leave osteological traces, such as pneumatic foramina, in the walls of the neural canal. That’s cool but it’s a pretty well-known system – see Muller (1908) on the pigeon, Cover (1953) on the turkey, and these previous posts – and I want to get on to other, even stranger things.

The lumbosacral spinal cord of a 3-week-old chick in dorsal view. The big egg-shaped mass in the middle is the glycogen body. Watterson (1949: plate 1).

The spinal cords of birds have several gross morphological specializations not seen in mammals, as do their meninges, and most of these apomorphic structures can also leave diagnostic traces on the inner walls of the neural canal. In fact, birds have so many weird things going on with their spinal cords – at least five different things in the lumbosacral region alone – that I spent a week back in January just sorting them out. To crystalize that body of knowledge while I had it all loaded in RAM, I made a little slideshow for myself, and I’ll use screenshots of those slides to illustrate the morphologies I want to discuss. We’ll cover the vanilla stuff in the next post, and the really weird stuff in subsequent posts.

Stay tuned!


ostrich peeing

cormorant peeing

alligator peeing

Stand by . . . grumpy old man routine compiling . . . 

So, someone at Sony decided that an Angry Birds movie would be a good idea, about three years after the Angry Birds “having a moment” moment was over. There’s a trailer for it now, and at the end of the trailer, a bird pees for like 17 seconds (which is about 1/7 of my personal record, but whatever).

And now I see these Poindexters all over the internet pushing their glasses up their noses and typing, “But everyone knows that birds don’t pee! They make uric acid instead! That’s the white stuff in ‘bird poop’. Dur-hur-hur-hurrr!” I am reasonably sure these are the same people who harped on the “inaccuracy” of the peeing Postosuchus in Walking With Dinosaurs two decades ago. (Honestly, how I didn’t get this written and posted in our first year of blogging is quite beyond my capacity.)

Congratulations, IFLScientists, on knowing One Fact about nature. Tragically for you, nature knows countless facts, and among them are that birds and crocodilians can pee. And since extant dinosaurs can and do pee, extinct ones probably could as well.

So, you know . . . try to show a little respect.

So, you know . . . try to show a little respect.

Now, it is true that crocs (mostly) and birds (always?) release more of their nitrogenous waste as uric acid than as urea. But their bodies produce both compounds. So does yours. We mammals are just shifted waaaay more heavily toward urea than uric acid, and extant archosaurs – and many (but not all) other reptiles to boot – are shifted waaaay more heavily toward uric acid than urea. Alligators also make a crapload of ammonia, but that’s a story for another time.

BUT, crucially, birds and crocs almost always release some clear, watery, urea-containing fluid when they dump the whitish uric acid, as shown in this helpful diagram that I stole from International Cockatiel Resource:

International Cockatiel Resource bird pee guide

If you’ve never seen this, you’re just not getting to the bird poop fast enough – the urine is drying up before you notice it. Pick up the pace!

Sometimes birds and crocs save up a large quantity of fluid, and then flush everything out of their cloacas and lower intestines in one shot, as shown in the photos dribbled through this post. Which has led to some erroneous reports that ostriches have urinary bladders. They don’t, they just back up lots of urine into their colons. Many birds recapture some water and minerals that way, and thereby concentrate their wastes and save water – basically using the colon as a sort of second-stage kidney (Skadhauge 1976).

Rhea peeing by Markus Buhler

Many thanks to Markus Bühler for permission to post his well-timed u-rhea photo.

[UPDATE the next day: To be perfectly clear, all that’s going on here is that the birds and crocs keep their cloacal sphincters closed. The kidneys keep on producing urine and uric acid, and with no way out (closed sphincter) and nowhere else to go (no bladder – although urinary bladders have evolved repeatedly in lizards), the pee backs up into the colon. So if you’re wondering if extinct dinosaurs needed some kind of special adaptation to be able to pee, the answer is no. Peeing is an inherent possibility, and in fact the default setting, for any reptile that can keep its cloaca shut.]

Aaaanyway, all those white urate solids tend to make bird pee more whitish than yellow, as shown in the photos. I have seen a photo of an ostrich making a good solid stream from cloaca to ground that was yellow, but that was years ago and frustratingly I haven’t been able to relocate it. Crocodilians seem to have no problem making a clear, yellowish pee-stream, as you can see in many hilarious YouTube videos of gators peeing on herpetologists and reporters, which I am putting at the bottom of this post so as not to break up the flow of the rant.

ostrich excreting

You can explore this “secret history” of archosaur pee by entering the appropriate search terms into Google Scholar, where you’ll find papers with titles like:

  • “Technique for the collection of clear urine from the Nile crocodile (Crocodylus niloticus)” (Myburgh et al. 2012)
  • “Movement of urine in the lower colon and cloaca of ostriches” (Duke et al. 1995)
  • “Plasma homeostasis and cloacal urine composition in Crocodylus porosus caught along a salinity gradient” (Grigg 1981)
  • “Cloacal absorption of urine in birds” (Skadhauge 1976)
  • “The cloacal storage of urine in the rooster” (Skadhauge 1968)

I’ve helpfully highlighted the operative term, to reinforce the main point of the post. Many of these papers are freely available – get the links from the References section below. A few are paywalled – really, Elsevier? $31.50 for a half-century-old paper on chicken pee? – but I’m saving them up, and I’ll be happy to lend a hand to other scholars who want to follow this stream of inquiry. If you’re really into the physiology of birds pooling pee in their poopers, the work of Erik Skadhauge will be a gold mine.

Now, to be fair, I seriously doubt that any bird has ever peed for 17 seconds. But the misinformation abroad on the net seems to be more about whether birds and other archosaurs can pee at all, rather than whether a normal amount of bird pee was exaggerated for comedic effect in the Angry Birds trailer.

ostrich excreting 3

In conclusion, birds and crocs can pee. Go tell the world.

And now, those gator peeing videos I promised:


Jan. 30, 2016: I just became aware that I had missed one of the best previous discussions of this topic, with one of the best videos, and the most relevant citations! The post is this one, by Brian Switek, which went up almost two years ago, the video is this excellent shot of an ostrich urinating and then defecating immediately after:

…and the citations are McCarville and Bishop (2002) – an SVP poster about a possible sauropod pee-scour, which is knew about but didn’t mention yet because I was saving it for a post of its own – and Fernandes et al. (2004) on some very convincing trace fossils of dinosaurs peeing on sand, from the Lower Cretaceous of Brazil. In addition to being cogent and well-illustrated, the Fernandes et al. paper has the lovely attribute of being freely available, here.

So, sorry, Brian, that I’d missed your post!

And for everyone else, stand by for another dinosaur pee post soon. And here’s one more video of an ostrich urinating (not pooping as the video title implies). The main event starts about 45 seconds in.


I’m scrambling to get everything done before I leave for England and SVPCA this weekend, so no time for a substantive post. Instead, some goodies from old papers I’ve been reading. Explanations will have to come in the comments, if at all.

Streeter (1904: fig. 3). Compare to the next image down, and note that in birds and other reptiles the spinal cord runs the whole length of the vertebral column, in contrast to the situation in mammals.

Streeter (1904: fig. 3). Compare to the next image down, and note that in birds and other reptiles the spinal cord runs the whole length of the vertebral column, in contrast to the situation in mammals.

Nieuwenhuys (1964: fig. 1)

Nieuwenhuys (1964: fig. 1)

Butler and Hodos (1996: fig. 16.27)

Butler and Hodos (1996: fig. 16.27)

For more noodling about nerves, please see:


  • Butler, A.B., and Hodos, W. 1996. Comparative Vertebrate Neuroanatomy: Evolution and Adaptation. 514 pp. Wiley–Liss, New York.
  • Nieuwenhuys, R. (1964). Comparative anatomy of the spinal cord. Progress in Brain Research, 11, 1-57.
  • Streeter, G. L. (1904). The structure of the spinal cord of the ostrich. American Journal of Anatomy, 3(1), 1-27.


Illustration talk slide 58

Illustration talk slide 59

Illustration talk slide 60

The rest of the series.


Illustration talk slide 47

Illustration talk slide 48

Illustration talk slide 49

Illustration talk slide 50

That last one really hurts. Here’s the original image, which should have gone in the paper with the interpretive trace next to it rather than on top of it:

Sauroposeidon C6-C7 scout

The rest of the series.

Papers referenced in these slides:

Bird vertebra diagrams

January 10, 2014

bird neck note sheet

I made these back in the day. The idea was that you could print them out and have them along while dissecting bird necks, so you could draw on the muscles.

bird neck note sheet - LEFT - all three views

It’s basically one drawing of an ostrich vertebra, morphed in GIMP and stacked to simulate articulation. All of the ones in this post show the vertebrae in left lateral view. If you need right views, flip ’em in GIMP or heck, I think even Windows Explorer will do that for you. The one above has dorsal views in the top row, lateral view in the middle row, and ventral views in the bottom row.

bird neck note sheet - LEFT - double lateral

Here’s a sheet with two rows in lateral view, the idea being that you draw on the more superficial multi-segment muscles on one row, and the deeper single- or two-segment muscles on the other row.

bird neck note sheet - LEFT - 12 cervicals

A version with 12 vertebrae, so you can map out the often complicated patterns of origins and insertions in the really long muscles. How complicated? Well, check out this rhea neck with the M. longus colli dorsalis and M. longus colli ventralis fanned out.

Rhea neck muscles fanned - full

That’s all. Have fun!