A very interesting question has been posed by colleague Mark Changizi (see his fascinating blogs at www.science20.com) related to brain size as it relates to body size. He presents good information that shows the rather constant relationship of brain size to body size. He asks the question, essentially, if bigger brains do not make for more intelligent animals, why are there bigger brains?
One particularly intriguing statement refers to the fact that neuronal tissue is a rather expensive form of tissue, so if the neuronal tissue is not really needed, why is it there? This is a good question, and perhaps the most parsimonious answer is that the neuronal tissue may be needed after all. For example, if we were to remove neuronal tissue from a cow, would the cow be affected? The most precise answer would involve determining exactly how much tissue was removed as well as the particular location(s) of the removal. However, in general, it would probably be the case that if we removed a non-insignificant amount (whatever that might be) of neuronal tissue from Ms. Bessie, Ms. Bessie would experience some non-insignificant change. This suggests to me that all of Ms. Bessie’s neuronal tissue (more-or-less) is needed. To argue further, we could then attempt to remove a proportionately similar amount of neuronal tissue from a rat. In so doing, would the essence of Mr. Rat change proportionately? Again, the most precise answer would involve determining exactly the right proportion as well as the functionally similar location(s) of the removal of neuronal tissue. Nonetheless I think it is plausible that if one were to take a non-significant amount of neuronal tissue from Mr. Rat, Mr. Rat would experience some non-insignificant change, much in the way we reasoned about the demise of poor Ms. Bessie.
Another particularly intriguing statement made by Mark (and I have seen similar statements made by others) refers to the assertion that a rat is roughly equivalent in intelligence to a cow. I wonder about that. Cows can do things that rats cannot (besides jumping over the Moon). Does that make a cow smarter or dumber? Or is it that we just do not have very good definitions of intelligence even for humans, let alone non-humans? In fact, I would argue that the best we can hope for is to compare humans against humans (or ducks against ducks, etc.) when it comes to intelligence, and as Francis Galton and other students of intelligence (yours truly included) have found, it is not as simple or straightforward a process as it looks. When one tries to determine the intelligence of another species…well, good luck. If one tries to compare the intelligence between species, one is engaged in a very speculative process indeed. To be more species-centric I will assert that I am a reasonably bright human being, at least in my culture at this particular period in time. (However, if you use any kind of language-based assessment tools, they must be in English—I do not want to look bad.) I will also confess freely that am a profoundly lousy and quite dumb duck. Even with my larger human brain, I really do not excel at duck things. However, the mallards outside of my door seem to fairly adept at swimming in the swift river current despite the cold temperature of the water.
My point is that, at least with mammals, the size of the brain is limited to the size of mom’s birth canal (more-or-less), and the brain tissue involved is probably the more-or-less minimum amount needed to be a reasonably good [insert your favorite mammal here]. I would find it really challenging to stand in a field and chew food all day long, but my bovine brethren seem to have no problem with this. Then again, they cannot type this blog. Who is smarter? I give up.
My, my! What a big brain!