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I was reading this(1) and it led me back to ask a very basic question (I'm not a neuroscientist). All the way back to undergrad anthropology and neuroscience courses I remember being taught the general rule of relative intelligence was that one looked at the ratio of the brain mass over the total mass of the animal (or the estimations therein from let's say fossils).

I know a lot of the interesting neuroscience research going on these days does looks into bird brains, particularly within Corvidae. It would seem that birds are often much more efficient in the abilities they seem to show with considerably less brain mass. I do realize that birds often weigh very little as well, so perhaps the ratio is preserved?

I also realize that within birds, they see better ratios in more intelligent birds. But what about the comparison from mammals to birds?

Dinosaurs are often predicted to not be intelligent because of the enormous body size and small cavities for a brain. Now I realize that some dinosaurs were actually quite tiny, but this is just an example.

Given birds' close genetic link to dinosaurs, could it simply be that they were just doing more with less? The mammalian brain is a huge caloric burden, so perhaps this would show an efficiency that could be selected for?

Thus as the title suggests, my main question:

Is the ratio of brain mass to body mass still considered to be a valid indication of intelligence of a species in modern (current) neuroscience? Certainly there are exceptions, but is it still considered the rule of thumb?

EDIT: I also wanted to point out the first article(2) I started to read after formulating the question. It made me question the usefulness of encephalization in addressing this issue at all, but I don't/didn't feel adequately trained to evaluate the conclusions of the meta study.

Again, this still leaves us comparing within primates, which still leaves me feeling that Corvidae have some really impressive efficiency going on with their overall brain mass. Which then leads to hope that some dinosaurs could be at least equally intelligent, if not more so (noted that this is complete conjecture).


(1) Front Hum Neurosci. 2013 Jun 6;7:245. doi:10.3389/fnhum.2013.00245. Print 2013.

(2) Brain Behav Evol. 2007;70(2):115-24. Epub 2007 May 18.

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up vote 7 down vote accepted

I don't know about brain mass to body size, but ratio of neocortex to brain volume is correlated with primate evolution (Figure 3, here). If you subscribe to the view that humans and more closely related primates are more intelligent than more distantly related primates, then that is an important correlate.

I think the issue with this question is that we're still having trouble defining intelligence.

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+1 for the last sentence. –  nico Jul 18 '13 at 6:23
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Exactly; the whole brain mass / total mass thing might be a rather anthropocentric test. This will always be the problem in dealing with tests of intelligence, self-awareness, and the like. –  Brandon Invergo Jul 18 '13 at 12:04
    
+1 I certainly agree that we have problems with defining intelligence, and that even getting theory of mind levels poses problems. I also enjoyed the reference. Is the issue then, that across higher taxa (say class) we can't rigorously compare intelligence. –  Atl LED Jul 19 '13 at 18:43
    
@AtlLED: I would say, yes. Even if you want to talk about temperament, there's a lot to debate. Here's a review: homepage.psy.utexas.edu/homepage/faculty/Gosling/reprints/… –  dd3 Jul 19 '13 at 20:06
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