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Does size limit intelligence?

I may make a small, but very intelligent species for a journal thing that I am making.

So could a microorganism achieve human-like intelligence? Or a small fly? Or does the brain have to be a specific size for things to be considered intelligent?

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    $\begingroup$ The purpose of this site is to help you design and consistently use the rules and systems of a fictional world of your own creation. From that perspective, the answer is obviously "yes" as you control the rules. If you're asking about whether or not this could happen in Real Life, that's off-topic for this site and better asked someplace like Earth Science (expect the answer to be "insofar as we understand sapience, no"). Does that affect your Q, or should we ask the mods to migrate it? $\endgroup$ Sep 21, 2021 at 2:44
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    $\begingroup$ If I had a lab and some resources I could gather evidence that mice are generally smarter than many of my coworkers. $\endgroup$ Sep 21, 2021 at 4:24
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    $\begingroup$ My favourite chapters of James Blish's The Seedling Stars are the two with microscopic humans. Very much not hard science but worth a read for worldbuilding inspiration. $\endgroup$ Sep 21, 2021 at 10:36
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    $\begingroup$ in Greg Bear's Blood Music a scientist genetically engineers his own white blood cells to have human-levels of intelligence by giving them tiny organic DNA-based computation devices. Weirdness ensues, including burning snow and the downfall of standard physics. $\endgroup$
    – John O
    Sep 21, 2021 at 13:43
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    $\begingroup$ It doesn't limit sarcasm, at least, not in the MCU. "You're only a genius on earth, Stark." - Rocket Raccoon $\endgroup$ Sep 21, 2021 at 23:28

7 Answers 7

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Does size limit intelligence?

Yes, but the limits are not where many assume them to be.

Older work on the subject relying on brain size (weight), body to brain weight ratio or complexity (all those little wrinkles & folds) has been superseded by more recent work.

Intelligence appears to be limited by 'total processing power, seems it's the number of neurons & connections between neurons (so much as processing power & memory in computers, unsurprising then when you think about it) rather than it's physical size but there are physical limits to how small you can make a neuron, how closely you can pack them & how many connections you can cram in inevitably leading to limits on the physical size of a brain that has 'intelligence' of any specific level.


Some links, the elephant brain & birds forebrains, as you can see current thinking is that number of neurons & their connections in higher thought centres (not the whole brain) is where intelligence is.


So you can't just make a brain smaller & smaller & retain the same level of intelligence, eventually you reach a point where you have to begin shedding neurons (& intelligence) to get any smaller.

The smallest brains with the highest intelligence by weight (& the highest neuron count by weight) known in nature are birds (unsurprising as they need to keep weight down for flight, so any tricks there are to achieve more with less for brains will have been heavily selected for by evolutionary pressures), they have more neurons per gram of brain than in mammals.

It's not unlikely birds are pretty close to having got as close to as small as you can go for the brains' weight to intelligence ratio given the evolutionary pressures they're under to cut weight for flight & (very rough unscientific) extrapolations from a Ravens 15g brain suggest something in the region of a 45g bird brain 'might' plausible be as intelligent as a human, that's likely as small as you can go & have adult human intelligence (that of course is just the brain, you'll need a body around it too).

But it will also depend on how much of that 45g brain is given over to higher reasoning rather than other brain functions like processing sensory information & motor control of course.

"very rough unscientific" aka, 'mine'.


A 45g bird brain is speculative & there may be reasons birds techniques won't scale easily to larger ones, cooling requirements of large brains may mean you just can't pack neurons as tightly for one.

Some of the heaviest brains in birds are probably found in macaws, the hyacinth macaws is 24.7 g


Could a microorganism achieve human-like intelligence?

No, there is absolutely no way you can achieve that without magic, see above.

They'd have to organise themselves into some sort of colony or modular organism to achieve the necessary neuron count but then you won't really have a microorganism any more.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch
    Sep 22, 2021 at 17:07
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Not Really

There are a couple of creatures that are small and rather clever. I remember stories about octopuses for example escaping from their tanks, and one the says the octopus snuck into another thing to eat some of the fish and then snuck back into its own tank. I even remember the story about a guy who was training a raven to use a keyboard only for that raven to sneak out of it enclosure, destroy the keyboard, and sneak back into its enclosure in order to play innocent. While it is true that bigger size might allow more room for a bigger brain, never underestimate the cleverness of the little guy.

Microorganisms might have to work together though

I think for a microscopic organism you’re going to have to have a lot of them working together as a colony in order to become sentient. There’s an interesting species from a lovely Stargate episode called Message in a Bottle to give you an idea of what I’m talking about. https://stargate.fandom.com/wiki/A%27t%27trr

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    $\begingroup$ "Microorganisms might have to work together" & then you have, 'an organism' :) which defeats the purpose. $\endgroup$
    – Pelinore
    Sep 21, 2021 at 8:33
  • $\begingroup$ That some creature does something clever doesn't mean it's intelligent. These small critters can't hold a candle to human intelligence. They have a few intelligent behaviours, while humans have thousands. The complexities of culture, language, understanding, technology and how to use it are simply incredibly vast even for our low educated people. The ability to open a jar and the understanding that there can be consequences if they are caught in some bad thing are tiny facets compared to what we do. $\endgroup$
    – Trioxidane
    Sep 21, 2021 at 9:38
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    $\begingroup$ I'm confused with your raven example. A guy tries to train a raven to use a keyboard. Rather than using the keyboard, the raven instead destroys the keyboard. And that's evidence that ravens are highly intelligent? $\endgroup$
    – Stef
    Sep 21, 2021 at 15:14
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    $\begingroup$ @Stef really. The raven might just be an anti-vaxxer. $\endgroup$ Sep 21, 2021 at 23:30
  • $\begingroup$ @Stef it's evidence that the raven had developed a theory of human cognition sophisticated enough to realize that if he (the raven) destroyed the keyboard, the human might be unhappy about it, and devise a way to avoid taking the blame. Most animals wouldn't think to do that. $\endgroup$ Sep 23, 2021 at 3:55
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Hive Minds

As others have written, size matters.

However, it doesn't have to be the size of the individual that counts.

Single ants or bees are stupid. Anthills and beehives are much smarter. Nowhere near human smart, but perhaps in another world they could be.

On Earth, hive minds "think" using pheromones. This is slow. Slooow. You probably want find a faster way for hive mind members to communicate.

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    $\begingroup$ In Vernor Vinge's A fire upon the deep, there is a species of aliens where each individual is composed of a pack of 3 to 7 dog-like bodies, who communicate together with sound. Sound is pretty slow, but still much faster than pheromones. As a consequence, when two individuals come too close to each other (which happens, for instance, during sex), they become very confused, because of the two packs' intra-communications being mixed together. $\endgroup$
    – Stef
    Sep 21, 2021 at 15:20
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    $\begingroup$ Rather than a hive mind, you could imagine small organisms being able to "tap" into some larger form of computing power. Imagine if a race of small imps live on trees; and those trees have lots of neurons (they're big trees). When an imp is standing on a tree, they are linked to the tree's neural network. The imp's own brain is used mainly for their personality and conscience; the tree's neural network is used for smart thinking. When an imp wanders away from trees, it retains its personality, but becomes pretty dumb and can only focus on one thing at a time. $\endgroup$
    – Stef
    Sep 21, 2021 at 15:23
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We don't know, but an intelligent insect or microorganism is dubious.

The size of the brain scales with the size of the species. It is the brain-to-body-size ratio that matters, more than the absolute size of the brain. This is thought to be because a larger body requires more neurons for sensory and motor control, so only the neurons beyond that minimum count towards abstract "intelligence." So a small species could be intelligent if it had a large brain relative to its size.

Crows are fairly intelligent for animals, while being much smaller than dogs or dolphins who exhibit possibly comparable intelligence. A crow-sized animal with an unusually large brain and human intelligence could be plausible.

Below a certain size, you're going to have to invent new computation mechanisms other than neurons. Fruit flies only have around 100,000 neurons; this isn't enough to remember very much. But if your fictional creature encodes information in molecules like RNA, then it could remember a lot more. I'd still be skeptical about a microorganism having intelligence. Still, I'd be willing to suspend disbelief. It's been done before.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch
    Sep 22, 2021 at 3:22
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A nuclear brain

This book provides good physical arguments for the minimum size for an intelligent system based on chemistry. If we rely on molecules for information processing and storage, one bit cannot be smaller than a single atom (0.1 to 0.3 nanometers). From this, the book derives a rough minimum size for living cells based on the amount of information they must store (0.1 to 1 micrometers). This gives the minimum size of a neuron and the minimum size for a complex brain.

Hence, information processing based on chemical compounds is limited by the size of atoms and likely cannot create a complex brain at the microorganism scale. However, there is a regime of physics where the structures are 100,000 times smaller and the timescales are at least that much faster: nuclear physics. We can't really build complex structures from nuclear matter at standard conditions. Just like the organic molecules that make up life on Earth are not stable on the surface of the sun, our world is much too cold for complex nuclear "molecules" (if such things can exist). But maybe in the core of a neutron star, you could build something like a neuron out of nuclear matter that is $10^5$ times smaller. In that case, you could construct something with the complexity of a human brain at the scale of about 1 µm, similar to the size of microorganism. It could also operate much faster. Of course the rules in nuclear matter are much different than in chemistry and we know very little about them. Certainly quantum effects will be more important for the nuclear neurons than for chemical neurons.

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    $\begingroup$ These magnetar hive minds have been desperately trying to make contact for the past 10 billion years. I mean how much broader could a hint be than waving with recurring gamma ray bursts!? The whole hive is slaving to create the necessary electric currents for days each time. And yet no answer! They are pretty exasperated. $\endgroup$ Sep 22, 2021 at 8:43
  • $\begingroup$ I would like to mention the Book "Dragon's Egg" has similarly described organisms living on the surface of a Neutron Star: en.wikipedia.org/wiki/Dragon's_Egg $\endgroup$
    – stux
    Sep 23, 2021 at 12:56
  • $\begingroup$ @stux I thought this was an original idea, but it seems that Forward had the idea before I was even born! $\endgroup$ Sep 23, 2021 at 14:01
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Frame challenge: Instead on focusing on the possibilities for "intelligence" (where, after all, "intelligence" is a description of behaviour), perhaps you can focus on what behaviours you want your creatures to have, and how those creatures could achieve them.

For example, emergent behaviours can be (or appear to us) complex while while the components appear simple. As another example, evolution by natural selection has "developed" very complex machinery inside cells, in virus, bacteria, immune systems, and lots more. While not generally considered intelligence, it is complex. Maybe your desired behaviours can be achieved by evolution in appropriate conditions.

Of course, if "human-like intelligence" means "do everything humans do" including talking in English, then you probably need [idiosyncratic] humans for that...

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Let's take a cell 0.1 mm big. So it is about 10^-6 of a gram. But it is still more than 10^16 of atoms!

So if we want to emulate 100 billions of neurons in human brain - 10^11, and brain will take about 1/10 - 1/100th of the cell that is still 1-10 thousand of atoms per neuron. And we can fit transistor in a hundred of atoms.

Neuron is more complicated than single transistor, but ape/bird comparision shows that our brain architecture is highly redundant. And physics allows to lower energy consumption to reasonable values (google "minimal work cost of information processing").

So I would say we can fit human brain into a cell, maybe a bit bigger than 0.1 mm.

But for such efficient and powerful computing to be a result of evolution... That would be pretty much impossible. Unless you use completly different nuclear "chemistry", like inhabitants of neutron star from "Dragon's Egg" by Robert L. Forward.

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