Small animals have enormous brains for their size

byeukaryote 6mo13th Feb 20194 comments

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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:

  • Different species have moral worth in proportion to how many neurons they have
  • Different animal species have moral worth in proportion to how smart they are
  • Different species have moral worth in proportion to the amount of complex thought they can do
  • Different species have moral worth in proportion to how much they can learn**

…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.)

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