Small animals have enormous brains for their size

post by eukaryote · 2019-02-13T05:12:22.794Z · score: 55 (27 votes) · EA · GW · 4 comments

This is a link post for https://eukaryotewritesblog.com/2019/02/12/small-animals-have-enormous-brains-for-their-size/

One thing that surprised me when working on How Many Neurons Are There was the number of neurons in the brains of very small animals.

Let’s look a classic measurement, the brain-mass:body-mass ratio.* Smarter animals generally have larger brain sizes for their body mass, compared to animals of similar size. Among large animals, humans have famously enormous brains for our size – the highest of any large animal, it seems. But as we look at smaller animals, that ratio goes up again. A mouse has a comparable brain:body-mass ratio to a human. Getting even smaller, insects have higher brain:body-mass ratios than any vertebrate we know of: more like 1 in 6.

But brain mass isn’t quite what we want – brains are mostly water, and there are a lot of non-neuron cells in brains. Conveniently, I also have a ton of numbers put together on number of neurons. (Synapse counts might be better, but those are hard to come by for different species. Ethology would also be interesting.)

And the trend is also roughly true for neuron-county:body-mass. Humans do have unusually high numbers of neurons per kilogram than other animals, but far, far fewer than, for instance, a small fish or an ant.

If you believe some variation on one of the following:

…then this explanation is an indication that insects and other small animals have much more moral worth than their small size suggests.

How much more?

Imagine, if you will, a standard 5-gallon plastic bucket.

Now imagine that bucket contains 300,000 ants – about two pounds.*** Or a kilogram, if you prefer.

Imagine the bucket. Imagine the equivalent of a couple large apples inside it.

A bucket. Two pounds of ants.

Those ants, collectively, have as many neurons as you do.

You may notice that an adult human brain actually weighs more than two pounds. What’s going on? Simply, insect brains are marvels of miniaturization. Their brains have a panoply of space-saving tricks, and the physical cells are much smaller.


*Aren’t the cool kids using cephalization quotients rather than brain-mass:body-mass ratios? Yes, when it comes to measurements of higher cognition in vertebrates, cephalization is (as far as I’m aware) thought of as better. But there’s debate about that too. Referring to abilities directly probably makes sense for assessing abilities. I don’t know much about this and it’s not the focus of this piece, anyway.

**Yes, I know that only the first question is directly relevant to this piece, and that all of the others are different. I’m just saying it’s evidence. We don’t have a lot of behavioral data on small animals anyways, but I think we can agree there’s probably a correlation between brain size and cognitive capacity.

***Do two pounds of “normal-sized” ants actually fit in a five-gallon bucket? Yes. I couldn’t find a number for “ant-packing density” in the literature, but thanks to the valiant efforts of David Manheim and Rio Lumapas, it seems to be between 0.3 gallons (5 cups) and 5.5 gallons. It depends on size and whether ants pack more like spheres or more like blocks.


Suggested readings: Brian Tomasik on judging the moral importance of small minds (link is to the most relevant part but the whole essay is good) and on “clock speeds” in smaller animal brains, Suzana Herculano-Houzel on neuron count and intelligence in elephants versus humansHow many neurons are there. (The last piece also contains most of the citations for this week. Ask if you want specific ones.)

4 comments

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comment by Denis Drescher (Telofy) · 2019-02-22T12:07:06.020Z · score: 5 (2 votes) · EA · GW

This is fascinating! I’ve heard (though it may well be bunk) that intelligence in humans is somewhat correlated with brain size but that the brain size is limited by the size of the birth canal. (Which made me think that c-section should lead to smarter people in the long run.) But if there’s still so much room for optimization left without changing the brain size, does that merely indicate that the changes would take too many mutations to be likely to happen (sort of why we still have our weird eye architecture when other animals have straightforward eyes) or that a lot of human thinking happens at a lower abstraction level than that of the neuron so that, e.g., whole brain emulation at a neuronal level would be destined to fail?

comment by Carl_Shulman · 2019-02-27T22:56:30.276Z · score: 21 (7 votes) · EA · GW

Recent large sample within-family data does seem to establish causal effects of brain size on intelligence and educational attainment. The genetic correlation is ~0.4, so most of the genetic variance isn't working through overall brain size.

Some kinds of features that could contribute to genetic variance in humans, but not scale for arbitrary differences across species:

  • Mutation load (the rate at which this is trimmed back, and thus the equilibrium load, depends on the strength of selection for cognitive abilities)
  • Motivation: attention to learning, play, imitation, and language comes at the expense of attention to other things
  • Pleiotropy with other selection combined with evolutionary limits (selection for lower aggression also causes white patches in fur via changes in neural crests, and retention of a variety of juvenile features), e.g. selection for disease resistance changing pathways so as to accidentally impair brain function (with the change surviving because of its benefits)
  • Alleles that provide resistance to disease (genetic variance is maintained in a Red Queen's Race) that damages the brain would be a source of genetic variance, likewise variants affecting nutrition or other environmental influences
comment by aarongertler · 2019-02-15T20:39:28.507Z · score: 4 (4 votes) · EA · GW

Upvoted! Thanks for the clear argument, descriptive title, and amusing cartoon. Ideally, good Forum posts will also be memorable, so that people remember them in future discussions, and images are a good way to achieve this.

comment by rafa_fanboy · 2019-02-13T20:57:08.048Z · score: -22 (8 votes) · EA · GW

this is why chickens are so important man