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 .)
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.
>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.
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".
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.
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.
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].
