[Eli5] what’s the meaning of the ratio brain size/body mass? Why a bigger brain isn’t alone a sign of bigger intelligence? What has body mass to do with it?
In: Biology
The bigger the brain the more “computing power” it has sure but shape, composition and.. smoothness… Has a lot to do with it too.
Also body mass is in there too because the higher the body mass the more background processes the brain needs to stay on top of, thus lowering the amount of what we’d call intelligence
A large portion of the brain is devoted to keeping the body alive and doing background tasks, not just intelligence and conscious thought. A larger body needs a larger brain: larger muscles farther away due to body size need stronger signals to activate, higher amounts of hormones need to be released since there’s a lot more blood in the body to dilute them, bigger sensory organs need more processing power (bigger eyes, for example, have more photoreceptors and provide more data to the brain), etc.
Intelligence is a function of the size of a specific area of the brain particularly the cerebral cortex.
So you can have an animal like an elephant whose brain size is three times the size of humans but the cerebral cortex is much smaller than humans.
A lot of the brain’s computing power is consumed on more mundane tasks like motor function and organ monitoring and sensory input processing.
Bigger animals have more sheer mass to them and more cells that need to communicate with the brain, so their brains are generally larger by simple necessity. A sperm whale has a brain several times larger than a human but they’re not out there doing calculus – most of that space is consumed with the functions of their gigantic body and very little is reserved for higher-order processing.
Humans have a big brain and a relatively compact and spindly body, we’ve dedicated a relatively large amount of physical brain space to thinking about stuff.
Thanks to everyone! I grasped it!
Imagine that you took a human and made them half as tall. Their brain is also going to be half as big (technically it might be less, since square/cube law), but the ratio between the size of their brain and the size of their body would remain the same. And, indeed, we see that people with dwarfism who have smaller skulls and smaller brains are just as intelligent as everyone else.
This leads to a quandary: why are humans intelligent? Obviously a whale is going to have a bigger brain than a human. A blue whale’s brain might weigh more than an entire human body! But the whale is not dozens of times more intelligent than a human (that we have observed, but we can’t be totally sure). So being intelligent isn’t just a function of how big your brain is, but instead might be about the size of your brain relative to the rest of your body. Or, in other words, you don’t just want a big brain in absolute terms, but a big brain to body mass ratio.
Neuroscience is not currently advanced enough to know this. People can give you answers somewhat, but it will just leave you with more questions.
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