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William H. Stoddard

The issue for brain size may not be, or not be only, absolute mass. It appears that mammal brains, at least, tend to have a mass that's proportional to body surface area. That kind of makes sense, if you figure that a substantial part of the neocortex is elaborate maps of the body surface, which accounts for two dimensions of brain volume, and that cortical columns in all mammals have more or less the same thickness, which accounts for a third dimension. So brain mass ought to go as somewhere around the 2/3 or 3/4 power of body mass. If you divide human body mass by 100, you might divide human brain mass by around 30, going to somewhere around 40 ml, I think.

Now, human brains are huge in proportion to body surface area. Other hominids such as chimpanzees have only about half as much; other primates, and I believe songbirds, have half as much as THAT; and generic mammals have half as much as that---at least according to the book I keep on my shelves, though its results aren't recent.

I suppose there could also be a requirement for absolute brain mass to support some more complex cognitive functions. I haven't seen any studies of that issue.

Jim Cambias

That's a really interesting idea I had not encountered. Most discussions of "cephalization" stick to brain vs. body mass -- presumably because it's easier than figuring out surface area.

I wonder if whales and elephants get an IQ boost by virtue of having thick hide -- their large surface area is relatively under-supplied with sensory nerves thereby giving them an effectively higher brain to body surface area ratio.

Thanks for the info!

William H. Stoddard

Some of that applies to humans as well. If you look at homunculus diagrams (showing which parts of the cortex represent tactile and motor neurons for which parts of the body), you'll see that the human hands are enormous, taking up as much space as the rest of the body below the neck, and the same is true of the human face (with the tongue having a disproportionate share). Tactile neurons on your legs and your back are pretty far apart. Of course, a huge share of our sapience involves things we do with our hands and our speech organs.

I once saw a homunculus for a platypus, and it had huge overrepresentation for the bill, where the electrosensory organs are located. And I expect that an elephant's brain map would have a huge trunk.

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