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Upright; no tail. This is a creature that requires a more efficient respiratory system for longer intense physical activity. But it needs strong bones like mammals have. Bird air sacs extend into the bones, though, which would probably weaken the bones. It does not fly. By bird system efficiency I'm referring to the fact that birds have air constantly moving through the lungs, unlike mammals that have a pause between fresh air.Bird respiratory

Can this creature have an efficient bird like respiratory system without having the air sacs needing to extend into hollow bones(pneumatic)?

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    $\begingroup$ Not directly relevant to the question, but you might find that balancing a theropod without a tail is a challenge. Those tails played a major role in locomotion. If nothing else you're completely changing the hips. $\endgroup$
    – addaon
    Commented May 18, 2022 at 21:09
  • $\begingroup$ you don't need hollow bones to have air sacs... $\endgroup$
    – John
    Commented May 18, 2022 at 22:06
  • $\begingroup$ @John oh that's good. Animals with neither or both are all I'm aware of, so I wanted to make sure that hollow bones weren't required, like if air sacs needed so much space that it had to replace things. $\endgroup$
    – Chickenpeep
    Commented May 18, 2022 at 22:16
  • $\begingroup$ many dinosaurs had air sacs and no hollow bones, hollow bones evolved later, nothing living is like this but then only one living group has air sacs and they are the dinosaurs evolved for flight and thus extreme weight reduction. even in birds most of the air sacs are not inside bones. $\endgroup$
    – John
    Commented May 18, 2022 at 22:19
  • $\begingroup$ @John well my understanding is wrong then $\endgroup$
    – Chickenpeep
    Commented May 18, 2022 at 22:22

3 Answers 3

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You don't need pneumatic bone (hollow bone) to have air sacs.

air sacs evolved before hollow bones, pneumatic bones is just a useful extra thing you can do with air sacs once you have them, (you can make bone stiffer). pneumatic bone is not necessary for functional air sacs, most air sacs are not even in bone but in the body cavity.

see the six big air sacs in this image here, they are in the body cavity not bone.

enter image description here

you can see it in air sacs casts, in B the light green portion (LVD and CV) is the only portion actually inside bone.

enter image description here

Air sacs evolved outside bone, but get moved into bone fairly quickly evolutionarily because it has several benefits (stiffer lighter bones for a given size for a start) but it is not needed for the air sacs to work.

You may want to check this out (http://people.eku.edu/ritchisong/birdrespiration.html)

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  • $\begingroup$ Excellent. I know of creatures with either both or neither, so I wanted to make sure they weren't always needing to be together to function properly. $\endgroup$
    – Chickenpeep
    Commented May 20, 2022 at 2:06
  • $\begingroup$ Stronger bones? I thought that it was simply because a bone filled with air was lighter (not stronger) than a bone filled with marrow. $\endgroup$
    – TonyK
    Commented May 20, 2022 at 14:50
  • $\begingroup$ @tony that's my fault, Stiffer not necessarily stronger, stronger against loading but weaker against crushing. $\endgroup$
    – John
    Commented May 20, 2022 at 20:45
  • $\begingroup$ I think this misses the mark on addressing "efficiency". Hollow bones as a substitute for more body cavity space is part of the efficiency factor of using hallow bones. When you look at how much space air sacs actually take up, they are on their own no more efficient than just using larger lungs in terms of space (though they are a bit lighter). To fully answer the efficiency question, you need to address how to remove needed space from the body. $\endgroup$
    – Nosajimiki
    Commented May 23, 2022 at 14:15
  • $\begingroup$ @Nosajimiki actually air sacs are far more efficient, remember air sacs work on direct muscular control they don't need to give up a big chink of body cavity for negative void space and diagram movement for mammalian lungs, and the lung portion itself is actually much small in birds and air sacs can be fit around organs, which mammalian lungs cannot. the only real advantage mammals have is unnucleated red blood cells. bird lungs are about 2-3X as efficient as mammalian lungs per unit volume so they would need to take up twice toe volume before it is even comparable. $\endgroup$
    – John
    Commented May 23, 2022 at 20:16
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Sure.

The main advantage of the avian respiratory system over the mammalian one is that the airflow is unidirectional. This is a huge advantage for oxygen transfer, but it's a challenge to get there evolutionarily from a bidirectional lung system. Birds got there, mammals haven't.

There's another example of the same limitations of bidirectionality in systems. Many more primitive animals like cnidarians have a blind gut -- their digestive system is bidirectional in the same way that mammalian lungs are bidrectional. This isn't ideal -- to the extent that the formation of the gut happens remarkably early in most embryos of more advanced animals. Evolution "decided" that a unidirectional gut was the only solution to this problem for most animals.

It's perfectly reasonable to describe an avian-style respiratory system where the air sacs are discrete structures in the body, separate from bones. But the avian system has only gone most of the way towards unidirectionality -- quite a bit of its complexity is around allowing unidirectional flow over oxygen transfer surfaces while still using the same upper respiratory system for both inhaling and exhaling. If you want to describe an even more efficient system, and are not constrained by "easy" paths from current mammalian biology, a two-orifice unidirectional respiratory system fits the bill. A single lung for oxygen transfer (more if desired for redundancy), a single inhalation port, a single exhaust port, and either a peristaltic or heart-style chambered pump to push the air in one direction.

There's more than one solution possible here, of course; but this shows there's at least one viable path, which means that there's a way forward here. The evolutionary history of your creature (if any) might determine the details.

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  • $\begingroup$ This thing is genetically altered, so evolutionary history is irrelevant. Also I think an issue with more holes is that there are more access points for pathogens and other contaminants, and I'd assume they'd be damp and mucusy so more places to lose moisture. So not necessarily the most advantageous. $\endgroup$
    – Chickenpeep
    Commented May 18, 2022 at 22:09
  • $\begingroup$ Even in our own respiratory system we don't make any effort to minimize holes (nostrils, mouth), so I don't know how much of a concern this is in practice, but it definitely would be a non-conventional layout that may be awkward for other reasons. $\endgroup$
    – addaon
    Commented May 18, 2022 at 22:20
  • $\begingroup$ @addaon actually we do, animals have large sinuses just to minimize water loss, many organisms us included breath primarily through one nostril at a time to minimize losses as well. there just is no good way for us to reduce the number of holes entirely. separating the respiratory and digestive system however could be well worth the extra holes. $\endgroup$
    – John
    Commented May 19, 2022 at 21:43
  • $\begingroup$ Right... but one nostril can support our breathing. We have two, perhaps due to symmetry constraints of formation or perhaps for redundancy; but there's clearly been no major pressure to reduce that number to one. And that one nostril can support our breathing at a 50% intake / 50% exhaust duty cycle. So having two similarly sized nostrils, one dedicated to intake and one to exhaust, would not introduce a "too many holes" issue. $\endgroup$
    – addaon
    Commented May 19, 2022 at 22:02
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A bird's respiratory system is about 8x as powerful as ours, but not 8x as effecent.

  • A bird's lungs are about 2x the proportional size of a terrestrial mammal's.
  • Air sacs approximately double a bird's respiratory volume as compared to a terrestrial mammal's lungs making the total volume of their respiratory system about 4x that of a terrestrial mammal's.
  • Birds typically have about 1/2 of their air sacs inside of pneumatic bones. This means that by virtue of their bones, they can reduce how much body cavity space they need for their respiratory system by 25%.
  • While a bird's respiration is unidirectional thanks to its air sacs, this does not make their respiratory system inherently more efferent. What it does is normalizes the oxygen intake so that the bird can absorb oxygen on both the inhale and the exhale which means that it does not have to stop intaking oxygen to exhale. While this doubles the efficiency of the lungs, when you add in the extra volume the air sacs take up, it is kind of a moot point in terms of size efficient but they are more efferent in terms of weight since they are just empty sacs instead filled in organs like lungs In contrast, bats achieve the same levels of oxygen exchange as birds just by using bigger lungs.
  • Bird lungs have about 2x as much exchange surface for their size as mammal lungs.

So what all of this tells us is that the efficiency you see come directly out of pneumatic bones in terms of respiration is that it allows you to reduce the needed weight and body cavity space of your respiratory system by ~25%. Everything else that is beneficial about a bird's respiratory system either does not contribute directly to respiration efficiency or it could be easily replicated just by making lungs bigger, have more surface area, or by using air sacs purely inside the body cavity.

Solution: Give them a higher Myoglobin based Oxygen Intake Mechanism

All this said, both mammals and birds tend towards hemoglobin based oxygen intake. Most animals prefer hemoglobin because it forms weaker oxygen bonds than myoglobin making it require less energy to release the oxygen when needed. However, the stronger bonding force of myoglobin means you can uptake oxygen several times more quickly and completely. In fact, some mammals have been shown to boost how much myoglobin they use for oxygen transport and absorption when injured, living at higher altitudes, or requiring a more energetic lifestyle to help compensate for higher oxygen needs; so, the mechanism required is already in place. You just need to activate it.

While hemoglobin only allows animals to absorbs ~15% of the oxygen it breaths in, mammals which use primarily myoglobin based intake (like Whales) are able to absorb up 90% of the oxygen they inhale. So, by increasing your myoglobin dependence by just a little bit, you can increase your oxygen intake by 25% to compensate for not using pneumatic bones as part of your respiratory system. In fact, if you were to switch to a purely myoglobin based exchange system, and increase the surface area of your lungs to the same density as a bird's you could achieve the same total oxygen exchange as a bird gets without even needing to make your lungs any bigger.

Other reasons to favor pneumatic bones.

The big downside, and thus the reason most organisms don't do this, is that it will take more calories to release the oxygen when needed. While inefficient calorie use is normally selected against, your creatures may be in a situation where they are genetically engineered and/or fulfill a niche where the extra calorie needs are not a major constraint.

Furthermore, most terrestrial body plans would overheat using this kind of respiratory system. Part of what a bird uses its pneumatic bones for is temperature control. Burning more oxygen means more body heat; so, birds use these bones as heat sinks to help remove this extra heat from their bodies.

So, if you are going for a body plan that prefers solid bones, you should consider that your organism may need to live in a colder environment, and/or have specialized structures that act as heat sinks like an elephant's ears or a bat's wings.

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  • $\begingroup$ actually unidirectional flow does make the system more efficient because it allows the actual gas exchange surface to almost an order of magnitude thinner, making for far more efficient diffusion. $\endgroup$
    – John
    Commented May 23, 2022 at 20:28
  • $\begingroup$ @John Birds have thinner lung membranes then mammals. They also have unidirectional airflow. But correlation does not prove causation here. Birds have on average a 0.2 μm blood barrier vs mammals which average about 0.43 μm. This sort of difference was taken into account when I said "Bird lungs have about 2x as much exchange surface for their size as mammal lungs." That said, mammals are also on average much larger than birds and larger animals on average have thicker air-blood barriers. Mammals also have a lot more terrestrial ancestry which also favors thicker air blood barriers. $\endgroup$
    – Nosajimiki
    Commented May 23, 2022 at 21:22
  • $\begingroup$ None of this proves how thin bi-directional lungs would be if evolved under different evolutionary pressures, but we do know that flying mammals (bats) have evolved thinner membranes than other mammals with an average thickness of 0.31μm. A bat's respiratory system makes up about 12-15% of its total volume. A bird's LUNGS are slightly smaller than a bats, but its whole respiratory system together makes up about 20% of its total volume indicating that they get the same job done slightly more mass efficiently thanks to air sacs, but less space efficiently. $\endgroup$
    – Nosajimiki
    Commented May 23, 2022 at 21:22
  • $\begingroup$ the difference in thickness holds even for birds and mammals of the same size. birds and mammals split from a terrestrial ancestor, they have equal terrestrial ancestry. the bird breathing system evolved before flight. finally where are you getting your volume number, because I am willing to bet they are only including the lungs of mammals not the thoracic cavity and its change in volume during mammalian breathing. $\endgroup$
    – John
    Commented May 24, 2022 at 20:59
  • $\begingroup$ @John There is no "empty space" in the thoracic cavity to make room for breathing. Your other internal organs and bones move with your diaphragm just like a birds organs move with the air sacs. I did not write down where I got the percentages from, but they were both from the same source; so, they were presumably both referring to the inflated states of each respiratory system $\endgroup$
    – Nosajimiki
    Commented May 24, 2022 at 22:13

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