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I'm currently working on a semi-aquatic humanoid race of aliens, the Soliil. The majority of the race has proportions, anatomy and heights close to that of humans, however, I am working on a much younger subspecies that grows significantly larger than the average Soliil while retaining almost the same proportions, sometimes growing tall enough that they can no longer stand unsupported. Unfortunately I haven't been able to find a consistent answer with the few I found varying between 10ft and 20ft.

They still need to be capable of surviving on land, even if loosing some mobility, and able to swim. Preferably, I'd like them to remain close to the same proportions of their smaller cousins, albeit towering above them, but any minor internal changes that could push their maximum height further would be interesting.

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There are two issues: the capacity to bear weight and the ability to stand upright.

  • WEIGHT

How tall did the two-legged dinosaurs get? Wiki records the estimated heaviest theropod is the Spinosaurus aegyptiacus: up to 20.9 ton. Estimated sizes compared to a human:

enter image description here

Obviously the skeletal structure is diffent, but the fact remains that all that size rests on two ankles. Redistribute the weight on a stand-tall frame (of equivalent bone density and size) rather than a bent-over frame and you could have 40-50 feet (based on guestimating from the pretty picture).

What's good for the Theropod is good for the hairless monkey.

  • LEVERAGE

Theropods have a low center of gravity while a 50-foot human wouldn't. What a strong gust of wind would do to a creature that tall on two feet isn't pretty, and there's only so far you can lean over before your center of gravity shifts so much you can't keep your balance.

  • CONCLUSION

My pull-it-out-of-my-left-ear guess is this: the practical maximum height for a human (true bipedal) might only be 9 feet lest the winds common to Earth make standing any taller a royal pain in the rumpus.

What are the winds like on your world?

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  • $\begingroup$ Most of their settlements are based on the coast so strong sea winds are the standard for the most part. $\endgroup$ – RaptorBricks Nov 12 '17 at 12:16
  • $\begingroup$ 9 feet seems kind of low considering that there are already 8 foot tall humans walking around who don't get blown over by wind. Their main concern is musculoskeletal instability caused by evolution not optimizing bodies for this height. If they were optimized then I would expect them to at least be able reach the height of a male giraffe at 20 feet without being blown over. $\endgroup$ – Benjamin Aug 17 at 16:29
  • $\begingroup$ @Benjamin did you watch the video? I wasn't talking about a light breeze. A wind that blocks a 5'5" human from walking upright would bowl over an 8' human. Keep in mind that a 20' giraffe has most of its weight below the 10' point (50%) and has 4 legs! Humans have most of their weight above the 50% point and only have two. Huge difference. However, I do appreciate your restating what I already said about having trouble distributing the weight on just two feet. $\endgroup$ – JBH Aug 17 at 18:23
  • $\begingroup$ That level of wind is extremely unusual. For these rare instances, I suggest they could just sit down and wait for the storm to pass. $\endgroup$ – Benjamin Aug 17 at 18:28
  • $\begingroup$ @Benjamin you appear to have missed the point of both the question and my answer. If you disagree, please post your own answer so the community may judge it. $\endgroup$ – JBH Aug 17 at 18:52
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Humans have proven to be able to reach about 9 feet high, but that's pretty much the limit due to blood pressure issues. The human heart simply can't provide enough pressure to get much beyond that, simultaneously having too low pressure toward the head and too much pressure in the lower legs. However, if you postulated internal differences, multiple heart-like structures say, you could theoretically get greater height before you necessarily start getting into issues with bone strength due to the square-cube law. Giraffes, for example, are much taller than elephants but have thinner bones due to lowered mass. Humans, proportionally, would likely split the difference in terms of limb thickness.

So I don't think it's unreasonable to think you could have a humanoid, with similar proportions to a normal human, at 4.5 meters/15 feet. I would suspect that much beyond that you'd start needing a different body type.

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Meh. It depends on the elemental makeup. Is this a species with the same C:N:O:H:S:P ratios that humans are made of? Or is silicon allowed? Or could metals be incorporated into the bones? Can the heart be stronger because it is bigger? Etc.

What is the gravity on your world?

How do they get nutrition?

How often are they in water, and how long do they need to be on land?

Lastly, how tall do you want them to be? There are probably ways to get you there, playing with the ideas you are getting here.

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    $\begingroup$ This looks like comments for the OP. I do not see an answer or a series of answers based on contingencies you assume. $\endgroup$ – Willk Nov 12 '17 at 0:46
  • $\begingroup$ The answer is 'it depends and you haven't provided enough detail." I am surprised the question is still open. $\endgroup$ – DPT Nov 12 '17 at 1:15
  • $\begingroup$ - They're made up of pretty much the same stuff as humans, so can't really go for silicon. - Given their technological level, metal reinforcements would defiantly be an option for some and an increased heart size would likely be a good idea. - Gravity is near enough identical to earth. - How long they spend in water varies but will usually be in the range of 2-4 hours per day As for idea height, I don't have a concrete height down but the numbers I'm currently using are ~10ft-15ft, ideally I'd quite like them to be very close to the limit that their legs could support before failing. $\endgroup$ – RaptorBricks Nov 12 '17 at 2:00

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