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In my world, 3-inch humans live on an Earth-like planet. My question, provided by a comment on a previous question, is: What would an anatomically correct 3-inch human look like?

How strong would he be? Would he have different physical abilities? Would heat, cold affect him differently? Would he have to eat a lower or higher percentage of his body weight to thrive? And so on.

If these humans have evolved on this planet, would evolution have acted differently on them? For example, I have already made an assumption that they would reproduce at a higher rate given that the world would be a much more dangerous place for them.

Background:

Earlier, I asked how to calculate the weight of a 3-inch on an Earth-like planet. Thanks to members with math skills, I got a quick answer. Thanks guys! Answer: about a fifth of an ounce.

In the course of answering my question, it was suggested that a 3-inch man might possibly by 24 times as strong:

"A three inch tall man would be 0.0416666 as tall as a 72 inch man. Thus his leg bones would have 0.0017361 times the cross section of a 72 inch man's, and his volume and weight would be 0.0000723 that of the 72 inch man. Thus his leg bones would bear only 0.041666 the weight per unit of area of cross section, and so would his muscles. He would have about 24 times as much strength for his weight than a 72 inch man would have." --- M. A. Golding

Golding also suggested that evolution would likely give such a man much more slender bones, as it has kangaroo mice.

void_ptr asks "whether the 'perfectly scaled' human would have any issues with thermal regulation, nutrients storage and such."

Here is a link to that question; How much would a 3" human male weigh?

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    $\begingroup$ Please remember that SE's model is one-specific-question/one-best-answer. You've asked a lot of questions, which makes the question too-broad. Consider as an example your first question, which specifically asks only about weight. $\endgroup$ – JBH Sep 21 '18 at 21:22
  • $\begingroup$ OK. I did worry afterward that adding evolutionary questions might make it too complex. I thought the rest were more like prompts. Anyway, I will heed your advice and recraft shortly. Thanks! $\endgroup$ – Adam Orth Sep 21 '18 at 23:00
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    $\begingroup$ Hi, I have slightly different advice for making this question less broad. Do NOT turn this into 6 or 7 questions. Rather take the ideas that have already been given, and do some worldbuilding on your own, make some decisions based on what you LIKE or find INTERESTING to write about, until you hit a specific question that is a block to the process. Only you can make decisions like "When does a 3" human have so many differences that he stops being a human and is just a made-up species?" 3in is one difference, but add a tail and fur and whiskers and..., TBH we can follow the idea only so far. $\endgroup$ – wetcircuit Sep 21 '18 at 23:08
  • $\begingroup$ Yep, thanks. I get it. I'm actually pretty far into my world, about 35,000 words. My questions have to do with checking on what I've written so far and on what a native population might look like. But, I won't pepper folks with questions, no worries. $\endgroup$ – Adam Orth Sep 21 '18 at 23:16
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    $\begingroup$ 35,000 words? . . . You're getting there. Pretty soon, you shall have scraped the surface! And thanks for editing your query, though you really don't need to reference or link to other (even related) queries. $\endgroup$ – elemtilas Sep 22 '18 at 2:54
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Basically, the overall size and dimensions of a three to four inch tall human (i.e., recognisably humanoid in shape, rather than three inches tall and four feet wide), would look and be shaped something like this:

[https://i.ytimg.com/vi/joDV-k8rYag/hqdefault.jpg

enter image description here

The Pygmy Marmoset is one of the smallest primate species. Also, this monkey looks a little more human-like than the the other contenders (a lemur and a tarsier).

They live in small troupes of six to eight adults with the children in a sort of communal care society, indicating that Tiny Humanz might evolve a similar structure. They can live up to about 20 years and are highly adaptable, curious creatures. They're about 4 to 6 inches tall, so just a bit bigger than your Fairy sized people.

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Probably Not Sentient

There Really isn't any way to fit enough brain matter into a body that small and end up with a creature that possesses the same psychological makeup that we have. They might be a bit more intelligent than mice or rats (rats are actually quite clever, just not sentient). They would probably not have complex language or be capable of abstract thought. They would really just be really clever rats with a high level of dexterity.

Would Need A Warm (but not Hot) Climate

Assuming these creatures are really just 1 to 1 miniaturized replicas of humans and carry no extra adaptations like thick fur or the ability to hibernate they would not do very well in cold weather. Humans only do well in cold weather because they have things like abstract thought that enables them to do things like start a fire or make themselves warm clothing. Your humans have approximately the brain capacity of a large rat, so they probably aren't going to be doing anything impressive like discovering fire or planning months ahead of time for events that haven't happened yet.

Thermo-regulation Would Be difficult Anyways

Humans are able to keep cool because we have so much surface area secreting sweat. Smaller surface area, smaller ability to keep cool. Additionally we keep warm (when naked and away from a heat source) because our body sacrifices blood flow to our extremities in an attempt to keep as much heat in our core. Again, smaller mass means less insulation and heat radiates away faster as well. They would get cold easier, they would overheat easier.

This Means Fast Metabolism

An inordinate amount of their metabolic process would go toward cooling off and warming up. Rodents have developed several strategies to avoid this problem like hyper-fast metabolisms, fur, hibernation, near constant activity, or long periods of torpidity. This means that with none of these adaptations (again, assuming a 1 to 1 mini replica) they would need to be eating a lot of high calorie foods and have excellent access to drinking water.

This Mean's Shorter Lifespans

Having a metabolism that is primarily occupied with just not overheating or freezing to death means a lot of stress on the heart and circulatory system. While an exact number for the lifespan is really impossible to pinpoint, they would probably have lifespans that are much shorter than a normal human being. Probably between 3 to 10 years max based on animals of a similar size and metabolism.

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  • $\begingroup$ I was just thinking about brain size! Here is an interesting article from Scientific American: scientificamerican.com/article/… $\endgroup$ – Adam Orth Sep 22 '18 at 2:12
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    $\begingroup$ IMHO, I think brain matter has nothing to do with being sentient or not, the same way that processing power/frequency has nothing to do with the physical size of the actual processor. One is measured in bits (or Hertz), the other in meters/inches, and there's no relation between them (check 1980 computers!) (if there were, elephants would be waaay smarter than we are). And we simply do not know enough about "brain-software" to make such a claim. That's my opinion anyway.. but.. if you can prove me wrong.. go for it. $\endgroup$ – Physicist137 Sep 22 '18 at 15:35
  • $\begingroup$ "sentient" means "aware of ones own momentary existence". "Able to display own initative" That's certainly true for all mammals and birds, and probably most fishes. $\endgroup$ – Karl Sep 22 '18 at 19:20
  • $\begingroup$ We can nitpick at semantics all day, but the end result is that the brain will comprise of only a fraction of its original mass and capacity. They will not posses conscious human psychology as we know it. $\endgroup$ – TCAT117 Sep 22 '18 at 20:54
  • $\begingroup$ @TCAT117 Despite what Karl said, I wasn't arguing semantics: when I said what I did was actually referring to 'conscious human psychology', as you put it. You can't just make such a claim. Sure, a fraction of original mass, true. But 'capacity' is not measured in Kg, so you can't say it is a fraction. You can't simply know. I know the OP didn't ask for hard-science, but still, what you said is IMO a very bold claim. For all we know, they might be waay smarter, or way dumber, as a processor can be waay faster or waay slower (for a fixed mass). Brain size and intelligence are not correlated. $\endgroup$ – Physicist137 Sep 24 '18 at 20:32
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That depends.

  1. He could be much thinner than we due to the lesser importance of gravity on the smaller scales. If there will be only gravity. But it won't. The surface tension is MUCH more important for such body. He can walk on the water if greases his feet. And can be caught by a morning dew drop if not.

  2. BUT: if he is warm-blooded, his warmth waste is terrible. So, he needs to have excellent fur or be rounded and thick and fat.

More simply, look at a hamster and a locust. They are examples of warm and cold body of that size.

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