I have a creature that has a humanoid torso and abdomen except for it being larger to accommodate air sacs. It has a flexible spine; not rigid like birds. The pelvic bones are like a human's(except for it being longer front to back to accommodate larger torso/abdomen, and slightly wider), not a bird's. Human pelvis takes up more space in the abdomen than a bird's does. So the creature is basically an upright bird(ish) body shape with a human pelvis and back and lacking the big bird sternum.

The structure of the posterior air sacs is that it wraps around the organs; this means a good portion of it is in the front and sides of the midsection, and in front of the intestines. Unlike bird and human organs displacing outwards due to breathing, the air sacs on this would surround the organs in a way that there's no place for that outward displacement of other organs. If it wraps around the organs, would it squish them too much when it inflates/deflates?

Even with this differing structure, could it properly breathe with a bird respiratory system? More specifically, since the humanoid pelvis takes up more space in the abdomen than bird pelvis, can I have the air sacs wrapped around the other organs in the front and sides of the midsection plus in front of intestines without that causing issue?

  • $\begingroup$ Possible duplicate worldbuilding.stackexchange.com/questions/195073/… $\endgroup$
    – John
    Commented Sep 17, 2022 at 1:09
  • 1
    $\begingroup$ keep in mind while air sacs take up space, bird lungs are tiny, since they are stiff and only have to deal with gas exchange they don't need to be anywhere near the size of mammalian lungs. . $\endgroup$
    – John
    Commented Sep 17, 2022 at 1:11
  • $\begingroup$ @John that other one is talking about a humanoid body. Mine is humanoidish, the difference being mine having the larger body that's more bird shaped, and not having to stick to the completely human body shape. Made it a bit more clear. $\endgroup$ Commented Sep 17, 2022 at 2:25
  • $\begingroup$ @John I've been looking at bird anatomy diagrams to base it on. The problem is the human pelvis takes up more space than bird pelvis, so the air sacs have to be moved upwards. Where I've proposed the air sacs go, I don't know if their movement would squish the other organs too much. $\endgroup$ Commented Sep 17, 2022 at 2:34
  • $\begingroup$ the human pelvis does not take up more room, you are just looking at two completely different body plans. if it helps sauropods and therapods had the same breathing mechanism and the pelvis are very different than birds. again the lungs are going to be a lot smaller and there is no diaphragm. $\endgroup$
    – John
    Commented Sep 17, 2022 at 16:15

1 Answer 1


There is no issue at all with your wrap-around posterior air sacs

It seems you may be under the impression that the air-sacs themselves are what expand: However, the truth is that, just like mammals, ventilation is achieved by altering the size of the body cavity: Hence, there is no more risk of organs getting squished with your system than with that of regular mammals or birds (which is to say there is no risk at all)

Besides that, there is no other difference between the described system and that of regular birds, and hence it should be perfectly plausible


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