Herons don’t just lie, they cheat (or at least their tracheas do)

January 7, 2021

This beautiful image is bird 52659 from Florida Museum, a green heron Butorides virescens, CT scanned and published on Twitter.

(The scan is apparently from MorphoSource, but I can’t find it there.)

There is lots to love here: for example, you can see that the long bones of the arm are pneumatic, because the margins of the bones show up more strongly than the cores. But you won’t be surprised that I am interested mostly in the neck.

As you can see, while the vertebrae of the neck are pulled back into a strong curve, the trachea doesn’t bother, and just sort of hangs there from the base of the head to the top of the lungs, cheerfully crossing over (i.e. passing to the side of) the vertebral sequence. So the trachea here is not much more than half the length of the vertebral sequence.

Now this is the opposite of what we see in some birds. Here, for example, is a trumpet manucode Phonygammus keraudrenii (a bird-of-paradise) as illustrated in Katrina van Grouw’s book The Unfeathered Bird:

Yes, all those coils visible in the torso are the trachea, which is many times longer than it needs to be to connect the head to the lungs. Birds-of-paradise do this sort of thing a lot (Clench 1978).

And they are not alone: cranes and others also have elongated and contorted tracheal trajectories. So it’s odd that herons seem to do the opposite.

But the heron is even odder than that. As we have noted before, herons can stretch their necks out to the point where you would scarcely believe the unstretched and stretched animals are the same thing. But they are:

The CT-scanned heron at the top of this post is in a pose intermediate between the two shown here. But since it can adopt the long-necked pose on the right, it’s apparent that the trachea can become long enough to connect the head and lungs in that pose. Which means it must be able to stretch to nearly twice the length we see in the CT scan.

Don’t try this at home, kids!

References

  • Clench, Mary H. 1978. Tracheal elongation in birds-of-paradise. The Condor 80(4):423–430. doi:10.2307/1367193

10 Responses to “Herons don’t just lie, they cheat (or at least their tracheas do)”

  1. Matt Wedel Says:

    What an amazing image. Going from the skeleton alone, it’s hard to credit that such a long and feeble-looking neck can not only support the head, but propel it forward fast and hard enough for the beak to serve as a spear for hunting.

    Some days I worry that all of my intuitions about animals are wrong.

  2. Mike Taylor Says:

    “Some days I worry that all of my intuitions about animals are wrong.”

    I think that may be the most important high-level insight I’ve picked up from the last two decades of working in this field.

  3. Dale M Says:

    WOW!!! We really don’t know dick about some fossil archosaurs, do we ?!? U think this might apply to sauropods/ prosauropods? Or is this only a development in birds?

  4. Mike Taylor Says:

    I don’t think it’s directly applicable to sauropods, because their cervical vertebrae filled much more of the space in the neck than those of birds so, do they necessarily define the structure of the neck more and leave less “slop” for the trachea to flop around — see the Extent of soft-tissue relative to size of vertebrae section of Taylor and Wedel (2013a) or more informally Sauropods were corn-on-the-cob, not shish kebabs.

  5. Casey Tucker Says:

    Here is the specimen on MorphoSource, but that image doesn’t seem to be associated with that data entry: https://www.morphosource.org/Detail/SpecimenDetail/Show/specimen_id/46849

  6. LeeB. Says:

    Also not only must the heron’s trachea stretch when they thrust their heads out when hunting, but it must stretch very rapidly.
    I wonder if it is elastic enough that it helps to withdraw the neck afterwards.
    The length of the trachea makes it look as if the birds of paradise may be producing subsonic sounds when they call.
    Given the length of a sauropods trachea they may also have been able to produce very low frequency sounds to accompany the whip cracks of their tails.
    On the other hand their ears are not far apart so they may have been able to hear the sounds but not able to accurately judge the direction they came from.
    Perhaps like elephants they may have been able to communicate by sounds transmitted through the ground and detected by their feet.

    L

  7. Matt Wedel Says:

    Also not only must the heron’s trachea stretch when they thrust their heads out when hunting, but it must stretch very rapidly.
    I wonder if it is elastic enough that it helps to withdraw the neck afterwards.

    I haven’t dissected a heron specifically, but from the other long-necked-bird tracheas I’ve seen, I doubt it. The cartilage rings can stretch apart from each other and to some extent slide over adjacent ones when they stack up. The trachea is elastic enough to collapse down to the shorter version when it’s not being stretched, but it’s more like a slightly-stretchy accordion joint than a rubber band.

    Which reminds me, the trachea runs alongside the esophagus, and the recurrent laryngeal nerves lie in the tracheo-esophageal grooves. So they must accommodate the big changes in trachea length as well. I’m assuming by having a single fixed length, and squiggling up accordion-style when needed, since nerves don’t do great with getting quickly stretched beyond their max axon length. (Although by slowly stretching nerves you can get them to grow several millimeters per day–and in fact our bodies do just that while we are growing.)

  8. llewelly Says:

    compare to Arambourgiania .


  9. All of your content is just amazing :)

  10. Mike Taylor Says:

    Thank you! But it’s not really our content, that amazing photo comes to us from Zach Randall’s tweet.


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