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I ask this because I’ve been looking at alien designs for inspiration and on rare occasions found peculiar bone structures. The hexagonal rib cage in particular intrigues me. We know for one that honeycombs are the optimal shape for storage and are quite sturdy due to their geometry. What’s interesting is that they form from circles squished together. Even foam bubbles take this shape when close together so theres no issue growing bones in this way.

enter image description here

Here’s an example, just focus on the ribs and ignore the rest. (Though it looks pretty rad.)

Would this be more or less viable than regular ribs? In what context would hexagon ribs be better?

Depending on the size of the openings it may endanger vital organs, although our ribs aren’t full proof either. The interconnected ribs would also change the way the aliens breathe.

I’m curious to see what your thoughts are on the subject.

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    $\begingroup$ Are the ribs hard bone or cartilage? $\endgroup$
    – Hearsay
    Oct 11, 2021 at 13:36
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    $\begingroup$ I wouldn't call that hexagonal; that would mean its overall shape is a hexagon. You seem to mean it's made of hexagons. I'd call the thing in the picture geodetic. It seems to have more pentagons than hexagons, BTW. $\endgroup$ Oct 12, 2021 at 22:08
  • $\begingroup$ @Bloke Down The Pub The title has been edited as per your feedback. Thank you. $\endgroup$ Oct 13, 2021 at 9:24
  • $\begingroup$ That skeleton's designers: "allright so if we put some bones here they can move compared to each other. But movement means its unstable! So we add some bone connections, completely immobilizing the bones! Sounds like an awesomely stable structure! That done we still have some time left... bone spikes?". Other designer: "oh yes bone spikes". $\endgroup$
    – Demigan
    Oct 13, 2021 at 9:30
  • $\begingroup$ I really like the mesh skull. "Are you sure nothing will ever attack its head?" - "yeah, totally". $\endgroup$ Oct 13, 2021 at 12:25

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Ladder ribs

Pros:

  1. Vertical and horizontal Flexibility.
  2. Expansion space and elasticity for lung-like organs.
  3. The breaking of a single rib piece will not affect the other ribs too much.
  4. Bow-like shape allows more compressive stresses before breaking.
  5. Rib cage can be compressed to decrease space between ribs and increase organ protection.
  6. Easier to mend should it break, requiring at most two connective points to be stabilized and maintained.

Cons:

  1. Two-point connection system makes disconnect from skeleton more likely.
  2. Lateral slashes or penetrative attacks can slide past ribs and into organs.
  3. Overpowering the rib connections from the outside via blunt means can make the ribs break and stab inward and puncture the organs they're meant to protect

Hexa ribs

Pros:

  1. Greater structural integrity from all stress sources and directions
  2. Greater protection for vital organs from both slashes and blunt impacts.
  3. Rib breakage is less likely to damage internal organs due to force distribution lessening kinetic energy imparted into one spot

Cons:

  1. Inflexible. Amount of inflexibility may vary from hard bone to cartilage, but the inflexibility will remain an issue and may impact breathing.
  2. Harder to mend should it break. Requiring anything from two to six connective points to stabilize and maintain.
  3. Organ protection is static. Protective range cannot be altered to further protectiveness by movements of rib owner.
  4. Penetrative attacks will slide past these ribs like it's nobody's business, unless the attack happened to get stuck on the zenith of a bone which is relatively unlikely.

As for the circumstances that would lead to such ribs, I can imagine either mobile plant-like or insectoid lifeforms getting such ribs due to their breathing essentially being different as well as their structural stability requirements being less necessary to be flexible.

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  • $\begingroup$ Some corals have a hexagonal frame structure. Perhaps a weird symbiosis? $\endgroup$ Oct 12, 2021 at 16:30
  • $\begingroup$ @JohnDvorak I suppose it might be possible but then you start to wonder where the creature's own ribs are and if it doesn't have them has it lost them at some point and if so why or did it even have ribs to begin with and so the list of questions go on. $\endgroup$
    – Hearsay
    Oct 13, 2021 at 6:49
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A rib cage has to protect the lungs AND allow them to expand and contract during respiration.

As you state, hexagonal cells make for a sturdy structure, which seems the wrong choice for something which has to regularly yield to the movements of the underlying structures.

The only way I see this making any biological sense is when the lungs do not operate in a alternated flow scenario (breath in - breath out) but under a continuous flow, like they do in fish gills, for example. In that case a sturdy structure doesn't prevent breathing and would actually save weight, giving same resistance for less mass. So it would make sense for a flying creature.

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    $\begingroup$ @John, which is exactly what I stated in the second half of the answer $\endgroup$
    – L.Dutch
    Oct 11, 2021 at 16:03
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    $\begingroup$ @L.Dutch it wasn't clear since you went right to gills as an example, birds for instance have a rigid ribcage because they use air sacs. $\endgroup$
    – John
    Oct 11, 2021 at 17:34
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    $\begingroup$ Fish and other non-mammals use ribs for a wriggling motion, which later evolved to be specialized for breathing. Either way, ribs are fundamentally about movement rather than protection - in the wild, a sharp injury to the liver or intestines is not very survivable, so why aren't there bones covering everything? $\endgroup$ Oct 11, 2021 at 21:30
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    $\begingroup$ Or the lungs are elsewhere in the body. As a spot for all the other less-contracting organs this setup sounds perfect $\endgroup$
    – Hobbamok
    Oct 12, 2021 at 10:40
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    $\begingroup$ Following up on this : we have two nostrils, two lungs, but only one trachea. If we evolved separated trachea, we could learn to breathe into one lung while breathing out of the other, with only one central muscle wall shrinking one lung while expanding the other. More efficient, and constant volume so we could strengthen the ribcage for better protection. $\endgroup$ Oct 12, 2021 at 12:46
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Just a different evolutionary history. ribs evolved in fish that were dorsally segmented had to flex side to side, so they are separate.

For an alien with a different evolutionary history a rigid chest might be fine, and thus hexagonal is workable.

Just give them something like bird or turtle breathing mechanism and the ribcage can be completely rigid.

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Advantages:

  • sturdier under continuous load, by distributing it more evenly
  • less mass
  • more opportunities for muscle insertion points that can distribute the load even better or allow faster reactions to changing the direction of movement (stronger support for jerky accelerations)

Disadvantages:

  • no flexibility - place the lungs elsewhere or operate them only along the cage axis (diaphragmatic breathing - not a bad thing by itself, but it will reduce the maximal lung capacity.
  • somehow brittle - if you crack one or two of the segments, the load imbalance is going to create a cascade failure
  • that muscles criss-cross and thoracic cage? It spells nightmare for a surgeon trying to close after an open heart surgery.

Use whenever the mobility and reaction time of torso and arms movement is a premium. May be useful for that sport featuring scrummage too.

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It's Foundational for Protective Armor

A turtle's shell is its rib cage. The bones have changed their shape to become a shell.

But what if the shell experiences significant wear? Re-growing bone is biologically expensive.

Then you could make the shell out of something like keratin (of finger nails, rhino horn, etc.) or of a bone-cartilage material like antlers. These materials are tough, but not as expensive for the body as normal bone.

In a world where creatures need to be armored, and that armor needs to wear significantly, those creatures would want strong mount points for their armor. Deer and rhino use their skulls, which are seriously strong bones.

So your creatures use the strong, hexagonal, ribs to support their heavy keratin shells.

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  • $\begingroup$ I'm imagining something descended from turtles that lost the need for a solid shell; the middle of each plate thinned and developed a hole. $\endgroup$ Oct 14, 2021 at 3:04
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It's your world, justify it however you please ;)

If you want a semblance of 'reality' about it, bear in mind that evolution doesn't try to solve problems; it tests to see how well a random mutation works. If said mutation doesn't kill you, it might get passed on to the next generation (it can get passed on even it it does kill you, but only after you've procreated).

So, as long as you can explain how any given set of mutations haven't lead to the death of the species you can do whatever you like.

Having said that, here's a thought on how you could get your desired rib structure:

It is an ancient bit of body plan from when the proto worms were sessile sea dwellers (think sea cucumber with tubular armour). Over time things get attached to this rigid tubular trunk, like fins, legs, tentacles and eyes. It does need to flex for breathing since the beastie has adapted its gill to continuously suck air though the hole in its head and 'blow it out its ass' ;)

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    $\begingroup$ “ It's your world, justify it however you please” We don’t do that here. $\endgroup$ Oct 12, 2021 at 16:37

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