A conversation I had the other day on the news of a new hominid species, Homo naledi, led to a comment that the species had small brains. Can brain size tell us anything about a species? Or can our fondness for our favorite organ mislead us into thinking that its size matters?
It’s tempting to think that a bigger brain is better. I think of brains sort of like I think of wallets. The bigger they are, the more valuable material I can shove in. But in actuality it never works that way: bigger wallets don’t have more money, they just end up with more receipt paper, candy wrappers and membership cards.
Similarly, bigger brains don’t seem to correlate with intelligence. Neanderthals had bigger brains than modern humans, suggesting that we evolved from cavemen to current intelligence levels despite shrinking brain size. If we compare across species, humans also have smaller brains than other members of the animal kingdom, like whales and elephants (we have almost 200,000,000 less brain cells than an average elephant). If we scale brain size by unit body mass, modern humans still get beat by ants, the tree shrew and small birds, having the same brain-to-body-mass ratio as a mouse. Even within our species, there’s inconsistent evidence on whether bigger brained people are any smarter than smaller brained people.
How do we explain differences between human intelligence and the intelligence of other animals if we can’t find a physical feature that puts us at the top of the list in brain measurements? By using Jensen’s encephalization quotient. The encephalization quotient, or EQ, compares brain size to expected brain size for similarly sized species. Here humans win out but I find the justification for this metric confusing. Why should brain size increase with body size? The metric also seems to depend on which species are we using to fit a line between body size and brain size. Using animals with more massive bodies, which could be reflecting adaptations for movement rather than adaptations for behavioral strategies/intelligence, changes the regression line. Depending on which species are selected for the line fit, we could reach a different outcome in the species with the highest EQ. For example, if we used only primates or only flightless animals with higher body masses would our brain size residual be more similar to other animals? The EQ also seems meaningless when we compare between species: are pigs less intelligent than horses? Intelligence is a hard thing to measure, and seems too complex to compare across species.
So if brain size has little to do with intelligence, what can brain size tell us?
Brain size correlates well with body size, head circumference and height. Big brains = big heads.