# Can a human skeleton be made of cartilage?

I'm working on this new story in which many people have developed "special qualities" or gifts - call it extreme DNA mutations, if you will. There's this character who has several rows of teeth (maybe they also grow back easily?), and because of it, he's really interested in sharks! So I wanted to give him a bit more than that, and therefore, I was wondering how crazy these two traits would be for a human being to have:

• Being able to dislocate their lower jaw

• Having a cartilage skeleton

Seriously, I have no idea what consequences this would result in. Is cartilage resilient enough to carry a human body? Is its elasticity impairing in some way? Or would it help him resist punches and so? Would he have any trouble carrying heavy things or am I going bonkers?

• This is what life for "cartilage people" would be like: Osteomalacia – Alexander Oct 19 '17 at 0:37
• So, there is no way for him to lead a normal life if he had a cartilage skeleton? I'm kind of sad about that, but maybe I could try to give him some different distinctive traits, apart from the teeth. – Elsie Oct 19 '17 at 0:39
• It has to be a human-like creature, not human. With enough handwaving you can make them almost indistinguishable. – Alexander Oct 19 '17 at 0:46
• Many members of my extended family have ended up in medical journals. Because they're just odd. One grew back a kidney. That's not supposed to happen. Several relatives of mine had a 3rd set of teeth come in rather than 2.They also get cancer a lot. Mainly they survive it, but there's a high rate in my family--not breast cancer, but everything else. And most of them seem to age slowly. I have a theory that my mother's side has an increased turn over rate of cellular regeneration, which explains most of this, including the cancer. – Erin Thursby Oct 19 '17 at 17:27
• If you're looking for an alternative shark attribute, electroreception is pretty cool. en.wikipedia.org/wiki/Electroreception – RFox Oct 20 '17 at 9:20

Cartilage is no where near as strong as bone.

Pros

• Amazing flexibility

• More capable of absorbing low-velocity and narrow-focus impacts (bones are brittle, this addresses the issue of breakability).

Cons

• Susceptible to crushing blows (high-velocity or broad-focus)

• Easily bruised (high-velocity or broad-focus)

• VERY susceptible to nerve damage

• Couldn't lever almost anything (all actions performed by "brute strength")

That last point is incredibly important. Without the rigid strength of bone, your only method of (among other things) lifting things is brute strength. New musculature would be required to compensate for the loss of rigidity. In the end, the cartilage person would never be as strong as a normal person, but would be much more agile and flexible.

• That was fast and really well explained! So, for example, in a martial arts competition setting, he would have trouble facing broad-focus hits such as a knock-down, but he'd be agile and flexible. Do you think he could strengthen his musculature so he can be a bit more resilient? – Elsie Oct 19 '17 at 1:06
• Honestly, the musculature would need to be different. Bone-based musculature is designed to use bones as leverage points. New musculature would be needed to compensate for the lack of leverage. I'm afraid that just changing the bones and strengthening the muscles would not be enough. – JBH Oct 19 '17 at 2:39
• Is it possible you meant rigid rather than ridged in the last block? +1 btw. – Joe Bloggs Oct 19 '17 at 7:58
• @JoeBloggs, :-) I certainly did... I balme the kyebraod. – JBH Oct 19 '17 at 16:33
• Extra muscle would make the problem worse. Muscle is always pulling on the bones - the body is held together by the tension of the muscles. If you have move muscle, you have more tension pulling those bones. The lever actions required for lifting depend entirely on the strength of the bone. So, unless you have a muscle on the opposing end of the lever that becomes rigid AND longer (which is actually what cartilage exists to do), you won't get any more strength just by having a stronger muscle attached to weak bone. At best, you could have cartilage and muscle intertwined. – user3685427 Oct 22 '17 at 19:40

Sharks have skeletons largely made out of cartilage (as well as multiple rows of teeth) and have done this for over 240 million years, so they are doing something right.

That something is becoming the apex predators in the various ecosystems they have evolved to live in. Being seagoing creatures they have evolved strong, flexible, hydrodynamic bodies with lots of teeth to enable them to hunt down prey in the most efficient way possible.

If your characters happened to live in the ocean and hunt and eat schools of fish, seals or tuna while swimming around, then a cartilage skeleton could be justifiable. However, Dolphins and Ichthyosaurs have very similar lifestyles but have skeletons made of bone.

The point being, evolutionary features are created and conserved based on their usefulness in the environment the creature lives in. A giraffe does not evolve to live in the Great Plains or Russian Steppes, because the long neck and legs would not provide much, if any, benefit in that environment. Similarly a cartilage skeleton provides no benefits to creatures living on land, and many other answers have gone into greater depth as to why.

So when you are contemplating creating a creature or modifying an existing one for a story, it is wise to consider the environment and ask yourself how would such a feature or behaviour help the creature in that environment. If the answer is not at all, then perhaps a rethink is in order.

No it really would not work.

Humans start with skeletons made of mostly cartilage, the soft spot in a child's skull is cartilage that has not ossified into bone yet, so you can get an idea of how strong it is. Cartilage is rather weak, sharks do not have very powerful bites, especially not compared to their size, because their jaw just cannot withstand the forces.

A cartilage skeleton would never support the weight of an adult human, your people could not even stand. They might not even be able to breath properly as mammalian breathing relies on a stiff rib cage to generate negative pressure.

• Oh :'( I thought it would be impossible, but I would like to find a way to make it work - or even figure some alternative that makes it similar to a cartilage structure... – Elsie Oct 19 '17 at 1:21
• sharks do not have very powerful bites Nope. White sharks have the most powerful bite among living animals of our time. Also their jaws are the only part of their skeletons that is bony (this is true for all sharks). – Renan Oct 19 '17 at 5:30
• Yeah I thought that claim seemed a little...fishy! – The Architect Oct 19 '17 at 5:45
• Saltwater crocodiles have 10 times the bite force of a white shark. – demonkoryu Oct 19 '17 at 10:01
• A great white weighs 5000 lbs, and bites with around 625 PSI, while a 400lb lion and can bite with 650 PSI. shark bite forces are pathetic compared to their mass. Also tesserae are not exactly the same thing as bone. – John Oct 19 '17 at 10:44

Maybe a combination of human skeleton and cartilage may work for your character. If the mutation is that your character's human skeleton is completely coated with a layer of cartilage, then the bones may be stronger and more flexible. The cartilage in between joints continue to grow and fuse over all of the human bones in the embryonic state through birth. Afterwards the cartilage layer grows with the bones as your character ages.

• Yes! This is really helpful, since it maintains the character's concept while making it more realistic than my "100% cartilage" idea. Just as I said: maybe the cartilage plates around his bones keeps regenerating non-stop, instead of vanishing and calcifying. Thank you! – Elsie Oct 20 '17 at 14:37

It might be possible for it to be semi-cartilage or made of a flexible yet strong material but 100 percent cartilage is impossible

• Welcome to WorldBuilding Shoto! It would be nice if you could edit your answer to expand it by explaining why it would be impossible. One-liners are often not weel received as they don't really help the OP understand why something is possible or impossible. If you have a moment please take the tour and visit the help center to learn more about the site. Have fun! – Sec SE - clear Monica's name Oct 19 '17 at 21:03
• I've been thinking of an alternative, thanks to the help of a friend, who happens to be a biologist. And what I've come up with is kind of what you're suggesting in this comment: having a percentage of cartilage in his skeleton, the same way kids do have a epiphyseal plate or physis made of cartilage that keeps decreasing over time, and finally vanishes at an approximate age of twenty. Maybe, for some reason, my character's physis keeps regenerating instead of just decreasing over time, since our bones' cells keep evolving and reconstructing the components of our structure. – Elsie Oct 20 '17 at 14:33
• The whole skeleton could be made of multiple disks of bone connected by a cartilage similar to our spines – Katsumi Shimizu Oct 22 '17 at 17:51

Concerning sharks: While they do have a skeleton made out of cartilage, it's important to note that they do not use it as an anchor for muscle movement. Shark muscles actually pull against their skin to move - basically, they have an exoskeleton. A soft one.

A shark's body is very flexible, but it's also "floppy" - they are able to move quickly in the water by "whipping" it back and forth, but this wouldn't work on land for anything that wants to support itself on four legs, let alone two.

A cartilage skeleton also offers very little protection from impacts, so not advisable for a creature whose brain takes up a good portion of their head.