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Okay, I have an humanoid giant that’s 50 feet, or 15.2 m, tall. If we were to scale an 1.7-m tall, 60-kg individual up to that size, they would have a mass of 43.2 tonnes. The proportional strength of humans allows them to be able to move, even under a gravity of up to 4.5 g. The giant would need to be 1.99 times stronger per mass than a human, so we could have feet that are 1.5 times wider and longer than a human’s, in terms of body proportions, as well as a body that tapers, so the head and hands are .9 times thinner proportionally.

That should be more than enough to allow the giant to move under Earthlike gravity. Next, there’s the problem of temperature. We want an internal temperature of about 310 K, and we need to find what the actual mass of the giant would be, with them tapering to be more structurally sound. As the volume will be 1.47 times higher than if proportions were consistent, so will the mass, putting the mass at an estimated 63.5 tonnes.

From that, we can use scaling laws to calculate that the giant will need 293 thousand kilocalories per day, which translates to 14.2 kW. As cellular respiration converts 64% of that energy into heat, that equates to 9.09 kW of heat. That’s about 114 times more energy than a resting human does. As we’re working with an truncated pyramid of the model of how size increases, that means that the surface area would be 108 times larger than an human’s.

If we assume that the skin increases in thickness linearly, then it’s 8.96 times thicker, which means that heat loss per mass will be 10.6% of that of a human’s. If we make it human—level thickness, then it’s at 95.1% per mass, so we will need extra-long fingers and toes to increase area enough to reach an thermal equilibrium. Finally, structures will need to be more complex to effectively deliver nutrients and oxygen, while eliminating waste. There will be a need for a lot more blood vessels, instead of supersizing existing ones.

An with the lungs, they need more alveoli, instead of scaling up the existing ones. About 720 times as much, with the lungs proportionally larger to make more space and to slightly reduce density. Now I ask, would this giant be able to survive on Earth? Why or why not?

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  • $\begingroup$ What do they normally breath and what's the usual atmospheric pressure they're accustomed to on their world? Do you want them to be able to survive on Earth? $\endgroup$ Apr 21, 2021 at 17:06
  • $\begingroup$ Related but not a duplicate. $\endgroup$ Apr 21, 2021 at 17:38
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    $\begingroup$ Usually it's about scaling the heart itself. As long as you only want an outwardly humanoid appearance, and are willing for the internal organs to be completely different, it might be possible. But a proportionally-sized human heart's just going to be a non-starter. $\endgroup$
    – John O
    Apr 21, 2021 at 18:07
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    $\begingroup$ Related: How to make a realistic 'giant' $\endgroup$
    – Alexander
    Apr 21, 2021 at 18:12
  • $\begingroup$ Your skin needs to be much thicker, the blood pressure in the legs is huge. giraffe need super thick and tough skin to prevent fluid build up on the legs. You also need lungs MUCH larger proportionally. More alveoli doesn't help much and you can't get alveolar density anywhere near what you suggest. alveoli work of f a branching system the more you have the more and larger tubing you need to supply them. 720 times as many alveoli means lungs 720 times as large, 65 tons is as much as the largest dinosaurs. $\endgroup$
    – John
    Apr 21, 2021 at 19:29

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Since your giant is a bit shorter than the tallest dinosaurs (Sauroposeidon) and lighter than the heaviest dinosaurs (Argentinosaurus), I will assume that it could in theory be feasible with the right adaptations. You seem to already be considering several of them but there are a few more really important ones you should consider:

Distribution of Bone Density: If your giants have pneumatized bones in its upper body, and more dense bones in its legs, then the upper body would weight less to compress down on the lower body, and the lower body would be able to endure more compression. This is thought to be the main reason that sauropods were able to evolve to be so big.

Reinforce the Spine: As John points out in comments there are some things you will want to do to make the spine better specifically. Sauropods used ball and socket spinal columns, but this is because they needed to resist the tension of the neck and tail's leverage. That said, A creature that stands vertically like a person does not care about tension so much as compression. Human like intervertebral discs are already more optimal for this. Instead of changing the joint type, you just need to make sure to include a larger cartilage contact area. This can be done with proportionally bigger spinal joints, or even by expanding the cartilage out between broad floating ribs.

Gigantothermy: Your heat equations all assume that your giant has a human like metabolism, but that is not necessary. Warm blooded animals produce almost 10 times as much body heat as our cold blooded counterparts. Going back to dinosaurs as an example, it is believed that many dinosaurs had semi-cold-blooded metabolisms, but were able to stay warm and active because their large size allowed them to retain heat so much better than a smaller animal. So, instead of scaling up the idea of a human and trying to cool it down, try scaling down a human metabolism until you reach your desired temperature at your given size. Not only does this make designing your giant easier, but it also will reduce your calorie requirement.

The biggest problem with your design is evolution, not mechanical limitations. The humanoid body plan exists to be able to make use of tools. This means our bodies need to be able to hold up not just their own weight, but the weight of the stuff we plan to use our arms to carry. A giant made at the very upper limits of what is biologically possible will not be able to lift tools or other things. This makes giant humanoids selectively unfit. Moreover, one of the key reasons for tool use to matter anyway is so that we can build structures and weapons to protect ourselves from dangerous predators. Gigantism is a different strategy for dealing with the same problem, and this is a case where two adaptations seem to be mutually exclusive.

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  • $\begingroup$ @ARogueAnt. Sauroposeidon was 17m tall... or if you would rather go by weight as the limiting factor Argentinosaurus was 60-100 tons. $\endgroup$
    – Nosajimiki
    Apr 21, 2021 at 18:50
  • $\begingroup$ That's pretty amazing, I totally take it back. +1 $\endgroup$ Apr 21, 2021 at 19:10
  • $\begingroup$ Another problem with a human body plan is the spine, humans are supporting a larger portion of their weight on the worst structure in the body for it. Sauropods that rea get around it with ball and socket spinal column and having ribs all the way down the spine. $\endgroup$
    – John
    Apr 21, 2021 at 19:41
  • $\begingroup$ The giant is able to support 2.78 times its weight under Earthlike conditions, so making the bones in the lower body denser would work, and invertebral discs sound good. And yeah, a slower metabolism would work out, and might allow for thicker skin for more protection. $\endgroup$ Apr 21, 2021 at 21:49
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Unfortunately not realistic

You note already that more blood veins are needed as well as bigger lungs, but sidestep a bigger problem. You still need to push it all around.

Lungs need to become practically exponentially bigger with size. That means that the amount of air that needs to be refreshed is also much higher. In turn that means you need much stronger muscles to make that all happen. Currently a sneeze can bruise your ribs. The ribs of your giant will relatively be weaker...

But the bigger problem is the heart. To get the blood through the veins, you need more pressure. To get more pressure, you need a much bigger heart. To get the veins to withstand the pressure...

It's all a vicious circle. You need stronger this, so you need to reinforce that. That requires more whatever, which requires more of the first.

Mind you, it's not impossible to have larger animals. We see them regularly everywhere. But the problem is in scaling up existing body plans. You can't keep it all the same. Some things need to give, making your giant relatively weaker, less mobile or something else. A way to still pump all the blood is to have a secondary heart, or reinforce every vein and just supersize it. But that requires more space, so what will you give up? Would you make the giant just with less Metabolism, so it'll be slower but easier to still power?

All in all a 15m giant is just unfeasible, unless you dramatically alter it from an humanoid-like body plan.

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  • $\begingroup$ To add human brains need a massive amount of blood and thus very high blood pressure, , the largest sauropods had small brains that needed far far less blood and thus don't need blood pressure anywhere near as high. Some think they may even have had secondary gas exchange in the nasal cavity. $\endgroup$
    – John
    Apr 21, 2021 at 19:45
  • $\begingroup$ A slower metabolism and a second heart both work out quite well. And to deal with that, a thick ribcage that expands out, creating more room. $\endgroup$ Apr 21, 2021 at 21:51
  • $\begingroup$ Move the brain down like in Pierson's puppeteers. Just a thought. @TysonDennis $\endgroup$ Apr 21, 2021 at 22:05

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