Small animals have enormous brains for their size

Link post

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 hu­mans, How many neu­rons are there. (The last piece also con­tains most of the cita­tions for this week. Ask if you want spe­cific ones.)