Can a humanoid that doesn't have ribs but instead has solid bone plates function properly ? Imagine plates starting from upper trapezius to the lower back and the obliques but leaving open the belly for pregnancy, obviously the chest area is covered.
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$\begingroup$ Welcome to WorldBuilding Ergeis! You might be interested in some of our existing questions, such as A ribcage capable of surviving the bite of a Tyrannosaurus Rex. If you have a moment please take the tour and visit the help center to learn more about the site. Have fun! $\endgroup$– SecespitusFeb 17, 2018 at 16:09
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$\begingroup$ Turtles come to mind, but they're not nearly as flexible as people. I would imagine that to be one of your biggest problems. $\endgroup$– LenFeb 17, 2018 at 18:46
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2 Answers
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Turtles do fine with solid bone plates instead of ribs.
https://reptilis.net/tag/turtles/ The thing about vertebrates is that we need to change the size of our thorax to raise and lower pressure, thus moving air in and out. You could not have a circumferential rib cage like a pipe because you could not change the diameter. You would have to have some silly deal like the head and abdomen shrinking into the tube then popping back out, changing the volume by changing vertical extent of the thoracic cavity. Your humanoids' heads (and shoulders) would slowly bob up and down at all times with their breathing. Actually there is a lot to be said for that setup.
The turtles, though - the top and bottom shells are not connected with bone at the sides and so they can expand and contract the thoracic cavity and breath in a way similar to how we do. If you do not like the head bobbing humanoid (think about it, now!) you could have a turtle humanoid.
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2$\begingroup$ Having the abdomen moving in and out, moving the internal organs up and down to change the size of the chest cavity is a good enough mechanism. One of the symptoms to look for when assessing if someone is having trouble breathing is the use of the abdominal muscles for breathing rather than the chest muscles. Also, horses use the pendulum motion of their gut while galloping to improve the efficiency of their breathing. If the abdomen is unarmoured, or has a separate armoured plate, then that would be a definite (if less efficient) option. $\endgroup$ Jun 21, 2018 at 10:59
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1$\begingroup$ @Ynneadwraith - / pendulum motion of their gut while galloping to improve the efficiency of their breathing/ I assert that I personally am doing exactly that, and so my particular pendulous build is in service of efficient breathing. $\endgroup$– WillkNov 25, 2022 at 17:56
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The problems with a fixed/solid ribcage are many. For instance:
1- It will immensely reduce the mobility of your creatures. Our (mostly empty) ribcages don't hinder our motion in most directions simply because they are mostly empty. A solid ribcage creature would have extreme trouble bending beyond a certain degree. Touching the ground without crouching would be simply impossible.
2- It will also increase the weight of your creature. Bones tend to be heavy and a solid cylinder of bone would be really heavy, further decreasing the mobility of your creatures. They would require a lot of energy to run, and then again, they would have little stamina.
3- If your humanoids are any technologically developed race, it will be extremely difficult for them to perform chest surgeries, considering that they would have to either cut open the ribcage (which would trigger a host of its own traumas) or operate very very carefully through an abdominal incision.
4- As Willk has already mentioned, your creatures would have extreme trouble breathing quickly. A solid ribcage would have zero flexibility and hence would be impossible to increase or decrease in diameter as the creature inhales and exhales. You would indeed require a different mechanism for allowing change in volume of the lungs.
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1$\begingroup$ If the rib-case were larger than the lungs then there would be room to expand and contract inside the rib-case without the rib-case having to flex. You'd have some big burly barrel chested people, They wouldn't be very fast but would probably be a lot stronger and more resilient than regular humans at the expense of agility. Probably similar to neanderthals in build. $\endgroup$– TCAT117Feb 18, 2018 at 2:05
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2$\begingroup$ @TCAT117: it is not so easy. Of course the lungs are going to be located inside the ribcage, but you are forgetting that you cannot have vacuum in the ribcage. That is to say, when the creature exhales, the lungs would get smaller in size, creating vacuum in the ribcage. This would prevent the creature from breathing properly and at most, they will only get a few gulps of air during each cycle. $\endgroup$ Feb 18, 2018 at 16:17
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$\begingroup$ @YoustayIgo, then don't do tidal lungs; use a true one-way system where muscles at the top act as air pumps pushing air through the lungs located in the ribcage and then out through a separate opening. $\endgroup$ Dec 9, 2022 at 15:54