Is the ratio Brain Mass/Total Mass still considered a valid indicator of intelligence?

Question

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).

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(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.

Answer 1

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.

Answer 2

Short answer
It is mainly synaptic connectivity that determines intelligence, rather than brain size per se.

Background

• For a starters, Albert Einstein, considered to have been a reasonable intelligent person, had an average sized brain (Sukel, 2009).
• Secondly, there are not only neurons in a brain that determine its size. The brain contains a relative large proportion of glial cells, which perform mainly a supportive function and are not part of the main neural network in the brain. A brain with a larger proportion of glial cells can thus be bigger than another equally sized brain with a higher proportion of neurons (Sukel, 2009).
• Thirdly, even the number of neurons per se is not a valid indicator of intelligence. Of course, in the very extreme, an organism with a few hundred neurons like C. elegans (Jabr, 2012) has hard-wired cognitive limitations due to the low number of processing units available to it, as opposed to, e.g., the human brain with some 100 billion neurons. However, scientific studies across several animal species, including humans, have shown that brain size alone is not a valid measure of intelligence. For example, birds with tiny brains have shown to be capable of some remarkable clever stuff, including the use of tools to obtain food. Rather, scientists now argue, it is the number of connections in the brain that matter, i.e., the organization and molecular activity at the level of the synapses is the main determinant of intelligence (Sukel, 2009).
• Lastly, as mentioned by others, intelligence is notoriously hard to measure. Species evolved in different environments. Learning to use a tool to dig up ants by chimpanzees is quite a clever thing, but makes absolutely no sense for a humpback whale diving 3 kilometers below sea level in search of giant squid. Intelligence is a relative thing.

<sub>References
- Jabr, Sci Am (October, 2012)
- Sukel, Sci Am (October, 2009)</sub>

Answer 3

Considering that as humans had larger brains in prehistoric times, then no, brain size doesn't mean more intelligent. I aren't well versed into this subject, so I don't particularly know if they could learn better than us, but there are speculations of why their brains were larger than ours. The most primitive humans, had to deal with a lot of predators, and unnatural things in their environment. Mist of them contained a vast amount of knowledge of what is "bad" or "good", and that was a requirement if they wanted to survive in that period. As we settled down, and our natural predators became our food, such needs became less and less important. Maybe they were actually smarter than us.