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In many fantasy worlds, giants are as intelligent as human beings and others races that they either avoid stepping on or step on regularly. However, due to the square cube law, giants have a limited size where they can exist; but due to again the square cube law, the larger you are, the harder it is to keep a powerful brain up and running. Thus, my question in:

How big can a giant get before getting to big to support a complex brain?

The giants them selves will be confined to the laws of physics, and thus no magic is allowed.

Also, these giants will be confined to the land and be adapted to such an environment (Large feet, stocky build, etc.) So no water buoyancy to help them.

Edit. Due to the need for clarification as to why giants would loose human intelligence as thy get bigger, I will put a brief explanation on the issue of large, bipedal intelligence. The issue, per usual, is the Square Cube Law. I.e, the bigger something gets, its surface area goes up by square but the volume goes up by cube. This leads, to large bipedal any way, a need for increased muscle mass and bone structure. However, that new mass needs support to. This results in a viscous cycle where the organism would eventually get crushed under its own weight. Thus, weight savings need to be made, and for giants, the most likely weight saving measure that s first to go is intelligence. Here is a link for a video that explains it with more detail here

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    $\begingroup$ Interesting question, I don't know enough about neurology to comment but can the processing part of the brain stay the same size whilst additional volume can be for other aspects like an Orca having a larger segment for emotions? $\endgroup$
    – user69935
    May 24 '20 at 19:55
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    $\begingroup$ The relationship of brain size to body mass and intelligence isn't well understood, especially given that there's only one life form on Earth with human level intelligence. I'm not sure if this is possible to answer with hard-science. $\endgroup$
    – Halfthawed
    May 24 '20 at 20:19
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    $\begingroup$ "The larger you are, the harder it is to keep a powerful brain up and running": citation needed. Why would a giant have so much trouble housing a human-sized brain in his gigantic skull? $\endgroup$
    – AlexP
    May 24 '20 at 20:54
  • $\begingroup$ Why not make them as big as you like then give them brains that function in a way different from ours? Like how an animal smaller than a human can actually be stronger. So I see no reason to try to bother with keeping them limited to certain sizes. Unless it's very important to you. $\endgroup$
    – Seallussus
    May 25 '20 at 0:03
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    $\begingroup$ The bigger the body, the greater the energy needs. Thus, the more room has to be devoted to the digestive system and the lungs. Also, brains are limited in size by the ability of the body to supply their energy needs. $\endgroup$ May 25 '20 at 2:12
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Actually, the larger the animals, the easier it is for it to have a brain with human level intelligence.

There is a minimum size of brain necessary for human level intelligence.

Nobody knows what that minimum size is.

There are approximately 6,500 known species of mammals at the present.

https://www.sciencedaily.com/releases/2018/02/180206090658.htm1

The vast majority of them have tiny bodies with less mass than a human brain. The mass of an average human brain is about

The adult human brain weighs on average about 1.5 kg (3.3 lb).

https://en.wikipedia.org/wiki/Brain_size2

And many species of mammals have brains much smaller than that since their entire bodies are not that massive.

But there are hundreds of large mammal species ranging in size from small humans to larger than humans. Mammals species with such large bodies can possibly support large brains. and some mammal species with such large bodies actually do support large brains.

And there are about ninety or a hundred species of mammals, primates, proboscideans, and cetaceans, with brains roughly in the range of human size, and in some cases far larger.

So it is possible than between one and about a hundred species of mammals on this planet are approximately as intelligent as humans and thus count as persons.

Some person claim that the absolute size of the brain doesn't matter as much as the ratio of brain to body size. If an animal has a larger body it may need a larger brain to senses sensations from its larger body and to control its larger body. Thus it is possible that even the largest brained nonhuman mammal does not have a brain sufficiently large for human level intelligence.

Some people might think that the size of brain necessary to control the body would scale with the dimensions of the animal. An animal twice as large would need twice as large a brain to control its body, in addition to the parts of the brain that give intelligence.

Some people might think that the size of brain necessary to control the body would scale with the square of the dimensions of the animal, and thus with the surface area of the body. An animal twice as large would need four times as large a brain to control its body, in addition to the parts of the brain that give intelligence.

Some people might think that the size of brain necessary to control the body would scale with the cube of the dimensions of the animal, and thus with the volume and mass of the body. An animal twice as large would need eight as large a brain to control its body, in addition to the parts of the brain that give intelligence.

Note that having the brain scale linearly requires much less brain than having it scale as the square of dimensions, and having the brain scale with the square of the dimensions requires much less brain than having the brain scale with the cube of the dimensions.

I also note that some body plans are much more complex than others and might need much larger brains to control than others.

Brain-to-body mass ratio, also known as the brain-to-body weight ratio, is the ratio of brain mass to body mass, which is hypothesized to be a rough estimate of the intelligence of an animal, although fairly inaccurate in many cases. A more complex measurement, encephalization quotient, takes into account allometric effects of widely divergent body sizes across several taxa.1 The raw brain-to-body mass ratio is however simpler to come by, and is still a useful tool for comparing encephalization within species or between fairly closely related species.

https://en.wikipedia.org/wiki/Brain-to-body_mass_ratio3

Encephalization quotient (EQ), encephalization level (EL) or just encephalization is a relative brain size measure that is defined as the ratio between observed to predicted brain mass for an animal of a given size, based on nonlinear regression on a range of reference species.[12][13] It has been used as a proxy for intelligence and thus as a possible way of comparing the intelligences of different species. For this purpose it is a more refined measurement than the raw brain-to-body mass ratio, as it takes into account allometric effects. Expressed as a formula, the relationship has been developed for mammals and may not yield relevant results when applied outside this group.[14]

https://en.wikipedia.org/wiki/Encephalization_quotient4

Encephalization quotient was developed in an attempt to provide a way of correlating an animal's physical characteristics with perceived intelligence. It improved on the previous attempt, brain-to-body mass ratio, so it has persisted. Subsequent work, notably Roth,1 found EQ to be flawed and suggested brain size was a better predictor, but that has problems as well.[unbalanced opinion?]

Currently the best predictor for intelligence across all animals is forebrain neuron count.[citation needed] This was not seen earlier because neuron counts were previously inaccurate for most animals. For example, human brain neuron count was given as 100 billion for decades before Herculano-Houzel[15][16] found a more reliable method of counting brain cells.

It could have been anticipated that EQ might be superseded because of both the number of exceptions and the growing complexity of the formulae it used. (See the rest of this article.)[unbalanced opinion?] The simplicity of counting neurons has replaced it.[citation needed] The concept in EQ of comparing the brain capacity exceeding that required for body sense and motor activity may yet live on to provide an even better prediction of intelligence, but that work has not been done yet.[citation needed][unbalanced opinion?]

https://en.wikipedia.org/wiki/Encephalization_quotient#Perspective_on_intelligence_measures5

Clearly there is still a lot of research to be done to predict how much brain is needed to control a body of specified size, and how much more brain is needed for intelligence.

It seems to me that if a humanoid giant has a body that is ten times as massive as a human body, it might possibly need a brain tens times as massive as a human brain to control its body, and if has a body 100 times as massive as a human body it might possibly need a brain 100 times as massive as a human brain to control its body. But if the extra part of the brain needed for intelligence didn't have to be any larger than the extra part of the brain needed for intelligence in a human sized body, the giants could have total brain mass and energy consumption that was proportionally less than that of a human.

And it is possible that the brain mass needed to control a body actually increases a bit less than with the cube of the body's dimensions, which would mean that your giant's brain size could be a bit smaller compared to its body mass and it could still be intelligent.

So unless some expert in brain size relative to intelligence says that the best current estimates and calculations show a specific upper body size for an intelligent being, I would assume that a giant humanoid could be just as intelligent as a human with a somewhat smaller relative brain size.

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Towards an Answer...

The question of gigantic humanoids has come up in Worldbuilding a couple times in the past, and we do have some data on relative maximums in size for Homo sapiens.

Historically, the tallest human ever was just under nine feet tall; and the heaviest human ever was 1,400 pounds (100 stone). Both men were at least of average intelligence: Brower was married, so must have been intelligent enough to apply for a license & answer during the ceremony; Wadlow graduated high school and entered college with the intention to study law (he would have made one imposing prosecutor!)

Within Worldbuilding, the question of greatest height and maximum possible dimensions for a human have arisen over the years, and I think may be helpful to you. The first query goes into some good detail of scaling and some of the pitfalls a human-like giant would face. The second query gives a rather larger maximum overall size, but also hints that various physiological factors will have to be sorted (heart size, heat transfer, blood pressure, etc). The raw range seems to be somewhere between 8 and 24 feet.

The maximum height for a human-like giant that doesn't suffer from too many physiological issues and health problems seems to be in the nine to ten foot range.

Given Halfthewed's comment about brain size to body mass & intelligence, I think it would be fair to offer the following answer:

A human-like fantasy giant of 9 to 12 feet in height should possess perfectly normal human-like intelligence. And also: a human-like fantasy giant of 12 to 24 feet in height may very well possess perfectly normal human-like intelligence.

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    $\begingroup$ Can't believe that guy got two children.... Guess I have a chances too lol $\endgroup$
    – user75689
    May 24 '20 at 22:22
  • $\begingroup$ "Must have been intelligent enough to apply for a license and answer during the ceremony" - that's not a high bar :D $\endgroup$
    – bytepusher
    May 24 '20 at 23:42
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Like many organs, the vertebrate brain actually evolved because of our large size--the larger you are, the more you need a dedicated system to communicate quickly between cells. I don't see any reason being large would make anyone less intelligent. Elephants and whales are both pretty smart animals!

However, because it takes longer for messages to travel the body, the larger you are, the slower your reflexes get, as well as your ability to think, period. If you've ever watched a fly move, you'll notice that its legs move faster than you can see. That's why!

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Mammals have grown quite large without intelligence decreasing. When one compares whales to their smaller relatives: porpoise, orca; one doesn't find a marked decrease in intelligence. Whales have a fairly sophisticated vocabulary that is indicative of higher intellectual processes, more complex than most land mammals. And it is in the aquatic mammals that we find the greatest size variance.

Intellectual capability does not necessarily decrease with larger size, at least when comparing large mammals to their smaller close relatives, which provides an intelligence comparison with the least number of extraneous variables.

However, what you may also want to consider is: how large can a hominid get before the body layout cannot support the large size? This is assuming you don't want to alter physics as we know it.

Upright creatures in the 30 foot tall range have existed as dinosaurs: T-Rex and Spinosauras. However, the very large bipedal dinosaurs all had a massive tail to counterbalance the body. As near as we can tell, they didn't walk upright, but in a position we might describe as hunched over. And that large size appears to be so they can take down larger prey - something humans do with intellect rather than size.

In the real world, humans have evolved to their current size, because that's the size that has proven to be optimal for the conditions.

It would be interesting (and realistic) to come up with a very large mammalian creature of the intellect and capability of a human, with a body optimized for the larger size.

And the conditions under which such a size increase would be beneficial.

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