Primates vs birds: Is one brain architecture better than the other?

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The bor­ing an­swer to that ques­tion is, “Yes, birds.” But that’s only be­cause birds can pack more neu­rons into a walnut-sized brain than a mon­key with a brain four times that size. So let’s for­get about brain vol­ume for a sec­ond and ask the re­ally in­ter­est­ing ques­tion: neu­ron per neu­ron, who’s com­ing out ahead?

You might won­der why I picked birds and pri­mates in­stead of, say, dogs and cats, or mice and elephants, or any other pair of dis­tinct an­i­mals. But check out this mouse brain:

[By
Ma­munur Rashid – Own work, CC BY 4.0]

See how, on the out­side of the lobe (the part closer to the up­per right­hand cor­ner), you can pick out a se­ries of stripes in a neat lit­tle row? Those stripes are the six lay­ers of the neo­cor­tex, a speci­fi­cally mam­malian in­ven­tion—all mam­mals have it, and no one else does. Peo­ple have been point­ing to this struc­ture to ex­plain why we’re so much bet­ter than fish since the scala nat­u­rae fell out of fa­vor.

And that would be a pretty con­ve­nient story if birds hadn’t come along and messed the whole pic­ture up. If you look at a similar cross sec­tion of a bird’s brain, it kind of just looks like a struc­ture­less blob. For a long time, com­par­a­tive neu­roanatomists thought birds must be at a more prim­i­tive stage of brain evolu­tion, with no cor­tex but huge basal gan­glia (the bit that we have sit­ting un­der our own fancy cor­tex). But we’ve since re­al­ized that this “lower” struc­ture is ac­tu­ally a to­tally differ­ent, in­de­pen­dently-evolved form of cor­tex, which seems to con­trol all the same ar­eas of be­hav­ior that mam­malian cor­tex does. In fact, birds have sub­stan­tially more of their brain neu­rons con­cen­trated in their cor­tices than we mam­mals have in ours.

Alright, so it’s not that sur­pris­ing that an­other form of cor­ti­cal tis­sue ex­ists in na­ture. But could it re­ally work as well as ours? Sur­pris­ingly, no one has re­ally tried to figure this out be­fore.

If, for in­stance, pri­mates were head and shoulders above birds, that might mean that in­tel­li­gent brains aren’t just en­er­get­i­cally ex­pen­sive (in terms of the en­ergy re­quired for de­vel­op­ing and op­er­at­ing neu­rons), they’re also ex­cep­tion­ally tricky to get right from a de­sign stand­point. Of course, if bird and pri­mate ar­chi­tec­tures worked equally well, that doesn’t mean brains are easy to get right–it would just mean that evolu­tion hap­pened to stum­ble into two in­de­pen­dent solu­tions around 100 mil­lion years ago. Still, that would im­ply sub­stan­tially more flex­i­bil­ity in neu­ral tis­sue ar­chi­tec­tures than the world in which one tis­sue ar­chi­tec­ture out­stripped all oth­ers.

An­swer­ing the ques­tion of birds vs. pri­mates con­clu­sively would be an enor­mous un­der­tak­ing (and to be hon­est, an­swer­ing it in­con­clu­sively was a pretty big pain already), so in­stead I fo­cused on a very small sam­ple of species in a nar­row range of brain sizes and tried to get a re­ally good sense of how smart those an­i­mals in par­tic­u­lar were, rel­a­tive to one an­other. I also got 80+ other peo­ple (non-ex­perts) to look at the be­hav­ioral reper­toire of these an­i­mals and rank how cog­ni­tively de­mand­ing they sounded.

With my method­ol­ogy of just dig­ging through all of the be­hav­ioral liter­a­ture I could find on these species, full and rep­re­sen­ta­tive cov­er­age of their en­tire be­hav­ioral reper­toire was a ma­jor challenge, and I think it fell well short of ad­e­quate in some cat­e­gories. This can be a big prob­lem if an an­i­mal only dis­plays its full cog­ni­tive ca­pac­i­ties in one or a few do­mains, and worse, you might not even know which those are. I think this wasn’t as big an is­sue with the species I stud­ied as it could have been, since we have pretty good pri­ors with re­spect to what se­lec­tive pres­sures drove cog­ni­tive de­vel­op­ment in the smartest an­i­mals (like pri­mates and par­rots). Plus, sci­en­tists are much more likely to study the most com­plex and in­ter­est­ing be­hav­iors, and those are very of­ten the ones that dis­play the most in­tel­li­gence.

One of the be­hav­iors sci­en­tists are re­ally keen on is tool use. Our sur­vey par­ti­ci­pants seemed to like it too, be­cause they rated its im­por­tance higher than any other cat­e­gory, and it ended up be­ing the most dis­crim­i­na­tory be­hav­ior, too–nei­ther the small-brained mon­key nor the small-brained par­rot had recorded ex­am­ples of tool use in the wild, while both of the larger-brained an­i­mals did.

In the end, peo­ple didn’t seem to think the two pri­mate species I in­cluded acted smarter than the two bird species or vice versa, but did think the larger-brained an­i­mals acted smarter than the smaller-brained an­i­mals. The fact that this sur­vey­ing method both con­firmed my in­tu­itions and didn’t seem to­tally over­whelmed by noise kind of im­pressed me, be­cause who knew you could just ask a bunch of ran­dom peo­ple to look at some an­i­mal be­hav­iors and have them kind of agree on what the smartest were? That said, we didn’t val­i­date it against any­thing, and even though we have rea­sons to sus­pect this method works as in­tended (see the full ar­ti­cle), how well and whether this was a good im­ple­men­ta­tion aren’t clear.

So this is all pretty cool, but even if we could prove defini­tively that macaws and squir­rel mon­keys are smarter than grey par­rots and owl mon­keys, it’s not a knock-down ar­gu­ment for ar­chi­tec­ture space be­ing chock full of fea­si­ble de­signs, or even for birds and pri­mates hav­ing iden­ti­cal per-neu­ron cog­ni­tive ca­pac­ity. It’s mostly just a demon­stra­tion that the old self-flat­ter­ing dogma of pri­mate ex­cep­tion­al­ism doesn’t re­ally hold wa­ter. But it also points to an in­ter­est­ing trend: in­stead of try­ing to tweak a bunch of pa­ram­e­ters in brains to squeeze the best pos­si­ble perfor­mance out of a given size, evolu­tion seems to have got­ten a lot of mileage out of just throw­ing more neu­rons at the prob­lem.

There’s a lot more dirt here, in the full anal­y­sis.