[alexpotato]

Because, IIRC, a lot of neurons are dedicated to motion/sensing.

Bigger animals may require more neurons to handle moving larger and/or more complicated muscle groups.

Interesting related point there is the encephalization quotient which is related to the predicted ratio of brain size to body mass. On the wikipedia page [0] they list the EQ for various animals. Humans are the highest but dolphins and ravens are not far behind.

[molticrystal]

To further emphasize that having neural material focused on the appropriate functions is more important vs how much you have, here is a story about a guy whose brain is mostly hollow and filled with fluid, it probably did cause his IQ to be 75 and causes him weakness in his legs, but otherwise he lives a normal adult life more or less.

https://www.newscientist.com/article/dn12301-man-with-tiny-b...

[gwern]

Volume != neurons. In any case, 75 is awful and is usually considered borderline retarded. (If you're tempted to respond with other cases of higher IQ, note that they are often retracted or unconfirmed and likely fraudulent in some way; see https://www.gwern.net/Hydrocephalus .)

[willmw101]

>Volume != neurons

Exactly. Most of the newer research on this topic suggests that it's neural connection complexity, and specifically frontal lobe volume, rather than overall brain size that determines intelligence or brain power.

https://neuroscience.stanford.edu/news/ask-neuroscientist-do...

>Luckily, there is much more to a brain when you look at it under a microscope, and most neuroscientists now believe that the complexity of cellular and molecular organization of neural connections, or synapses, is what truly determines a brain’s computational capacity. This view is supported by findings that intelligence is more correlated with frontal lobe volume and volume of gray matter, which is dense in neural cell bodies and synapses, than sheer brain size. Other research comparing proteins at synapses between different species suggests that what makes up synapses at the molecular level has had a huge impact on intelligence throughout evolutionary history. So, although having a big brain is somewhat predictive of having big smarts, intelligence probably depends much more on how efficiently different parts of your brain communicate with each other.

[mattkrause]

As a counterpoint, rats without a cortex can do...basically everything normal rats can do--except trim their toenails. The classic reference for this is Whitslaw's 1990 chapter "The decorticate rat".

This thread has links to a copy, plus a bunch of related studies in humans and animals. https://twitter.com/markdhumphries/status/107105276276554137...

[heavenlyblue]

Have you read the PDF given?

The whole nail part is basically a single sentence in the paper.

For example, decorticate rats are unable to escape narrow alleyways because they can not turn around due to their tonsils touching the walls and them being unable to ignore that feeling.

Another example is that they take a few seconds vs (!) 5 minutes to groom themselves on average.

[mattkrause]

Yes! I got interested in this when my colleague worked with a person who had an entire hemisphere resected as a teenage.

FWIW, the nail thing is a bit of a neuroscience meme. I heard--and stole--this quip from multiple people in several different situations. There's also a really striking figure in that chapter (p. 7 or 8).

No argument that the rats' behaviors are affected. I suppose whether you find the slowness of their grooming expected (because of brain damage) or impressive (because it happens at all) is a matter of taste. Glass^W Skull half-empty or half-full, if you will.

[heavenlyblue]

> No argument that the rats' behaviors are affected. I suppose whether you find the slowness of their grooming expected

They are not slow, they just stop grooming themselves well enough.

[gwern]

Stick that rat outside in a wild rat colony, and we'll see how well "everything normal rats can do except trim their toenails" works out.

[mattkrause]

The cool thing about Whitslaw's work is that it focused on natural behaviors (rather than like...a rotorod test).

I don't think he released them into the wild (would be a tough experiment with 80s tech), but there are a bunch of studies of their interactions with conspecifics. They can mate[0], though less successfully than controls, but playfight a bit better than they do[1].

[0] https://psycnet.apa.org/record/1983-29790-001 [1] https://doi.org/10.1159/000114124

[acchow]

Doesn't this demonstrate the opposite of what you were claiming?

[lacksconfidence]

I feel like the quotes agree with parent:

> “If something happens very slowly over quite some time, maybe over decades, the different parts of the brain take up functions that would normally be done by the part that is pushed to the side,” adds Muenke, who was not involved in the case.

[Ajedi32]

Did you see the scans? The dude's head is practically empty (brain 55-75% smaller than normal) and nobody even noticed until he was 44 years old and got an MRI.

[divbzero]

I think it’s that a priori you would expect a hollow brain to have a far more drastic effect and not allow for a mostly normal adult life.

[pishpash]

Why would you expect that, when a tiny insect can do pretty intelligent things? What "unexpected" things humans can do are probably all in the >75 IQ range.