RE: Elephants use a large number of neurons to control their trunk and ears
This is interesting.
Kaufmann hypothesized that the dorsal and lateral subnuclei in elephants may be responsible for the dorsal and ventral trunk muscles.
I've read before that the human body wouldn't benefit from more fingers because our brains our already max'ed out controlling ten of them (ask a piano player how much concentration it takes to perform at high levels), so it definitely makes sense to me that dedicated neurons would be needed to control an elephant's trunk and ears.
As far as complexity and raw numbers of neurons go, though, I would think that controlling ten independent fingers is more complex than controlling 1 trunk and two ears, so I'm not sure why elephants would need a larger number of neurons than humans (after adjusting for volume).
Edited to add: It's also interesting that the log graph you mentioned puts dolphins even higher than elephants, and they don't have fingers, trunks, or large and movable ears.
I'm not sure about us having problems with more digits. I remember an experiment some years ago where a person was given a robotic sixth finger. It took him little time to get used to it and eventually when it was taken away the brain missed the extra digit.
I'll look for it.
Here it is:
https://www.ucl.ac.uk/news/2021/may/robotic-third-thumb-use-can-alter-brain-representation-hand
I remember reading even older, tho. Perhaps I'm mixing memories.
I'll never remember where or when I read that. It was some time ago. But this would suggest that maybe it was wrong, anyway.