Brain size and EQ (Encephalization Quotient)

Some EQ comparisons and what not to do with them

The first set of EQs I'm using on this page are from a single source and will vary somewhat from other figures that may be found in other sources (as you'll see in the sets of EQs I present later on this page). For instance, in this first source human EQ is listed at 5.07, while other sources may use figures up to 8 plus for humans. Besides different methods of calculating EQ there may be some variation in the specimens used as source material -- most, if not all, animals vary from individual to individual in both body and brain size, and this can make a difference depending on what source specimens are used for comparison. We tend to have a fairly large number of specimens for animals like humans, and others like rats, because we've studied those animals the most -- this should make it possible to get a better idea of the range of EQ within those species compared to less commonly studied species. So remember to be very careful about comparing EQs obtained from different sources; different articles, papers, or lists may have EQs calculated in different ways. You'll see that the individual numbers may vary from different sources, but the general outlines of comparison, the relative numbers (of, say, humans compared to monkeys) are similar.

Also, as Schoenemann, in a 2004 article (P. Thomas Schoenemann "Brain Size Scaling and Body Composition in Mammals", Brain Behavior Evolution 2004;63:4760), points out that: "They [These results] also suggest that some measure of lean body mass is a more appropriate scaling parameter for comparing brain size across species than is overall body weight." (pg. 47)

This of course makes sense since fat varies according to food availability; it's a food source that's stored when times are good and used up when times are bad. However, Schoenemann later points out that although using Fat Free Weight would be best, it's difficult to do and isn't all that much more accurate in practice, even though it's potentially better:

"However, because FFWT [Fat Free Weight] is much more difficult to estimate than WT [Weight], and because the differences in EQs are likely to be small between the two methods, this is impractical even if theoretically more appropriate. There might nevertheless be subtle but important differences in EQ estimates, which could be significant in some cases given the vast range of FFWT and WT in mammals, and this should be kept in mind when comparing brain sizes across species." (pg. 58)

The effectiveness of EQ comparisons breaks down with very large (extremely large) animals -- very large whales fall into this category. For instance, the Sperm whale (EQ .28), Blue whale (EQ .15), and Humpback whale (EQ .18) are not thought to be exceptionally unintelligent, although they have very low EQs. Until you get into the larger whale-sized creatures (and after all these whales are 6 or more times larger than an African elephant) EQ works quite nicely.

Now take a look at these EQs and I'll talk about at what you shouldn't do with information like this:

Humans 5.07
Proboscis monkey 1.11
Red colobus 1.50
Patas monkey 1.93
Capuchin 2.52
various non-human primates ranging from 1.04-4.04 (not including lemurs, which tend to have EQs lower than 1 in his set; both lemurs and langurs bring the primate average down)

Bottlenose dolphin 3.60
various dolphins ranging from 2.43-4.45
Ringed seal 1.37

Manatees .32
Caribou .78
Wildebeest .68
Warthog .40

Hippopotamus .27
African elephant .63

Armadillo .26
Opossum .39

First off, too bad about the armadillo; good thing they're cute and can't bite. Then let's look at these numbers from an environmentally deterministic view, like the AAT/H typically uses. Remember, it's not a view that should be used, but it's instructive.

Look at the numbers for hippo and elephant; the semi-aquatic mammal has a much lower EQ. Look at the grouping of herbivores which includes the manatee; the aquatic manatee has a dramatically lower EQ. Look at the monkeys; the one which uses the environment that AAT/H proponents say may be similar to the one we used (the proboscis monkey) has, by a large amount, the lowest EQ. The savanna-dwelling patas monkey has a much higher EQ. Now if you were using the environmental determinism of the AAT/H, you would be forced to conclude that aquatic environments are horribly unlikely places to find higher EQs and that savanna environments are great places to find them.

But I've already mentioned that using this sort of environmental determinism is a bad idea; it just doesn't make sense (and these numbers should help anyone, even AAT/H supporters, understand why it doesn't make sense -- maybe they should consider not doing it anymore). It's not that you can't find some correlations at times -- the problem is just the reverse; there are always a great many correlations all over the place. But finding a correlation is not finding a cause.

This is well known in science, so well known that there's a maxim about it: correlation is not causation. As an example, let's say you look at the reading abilities of 5 year-olds and 10 year-olds, and you measure their height too. Guess what, they correlate extremely well -- so does increased height cause increased reading ability? Does increased reading ability cause increased height? Or are both due to some other factor or factors? Anytime you see a correlation you have to look past the correlation if you looking for the cause -- the correlation is not necessarily the cause. It might be in some cases; it often isn't. And be sure to check to see if the correlation is actually accurate too.

I've already mentioned (on the BBC page) what actual correlations can reasonably be seen as causes for these EQ differences: predator species generally have higher EQs than prey species, prey species which use active predator-avoidance strategies generally have higher EQs than prey species which use passive predator-avoidance strategies, and animals which live socially, especially in relatively small groups with a lot of individual interaction, tend to have higher EQs than other species.

This is because, due to selection pressure during evolution, the more a species needs to think, the higher their EQ. But larger brains are not necessarily better, because the larger the brain, the more energy it takes to develop it and feed it. So animals tend to develop the size brain they need and that's about it. They are not, as Attenborough suggests, just waiting for the right diet so they can grow a bigger brain and outdo their competition.

By the way, you may have noticed that the AAT/H is largely made up of correlations, many of them inaccurate or imagined. (As seen in common questions put forth by AAT/H proponents such as "where do we find animals which are (fill in the blank)" -- where the "blank" is fat. or hairlessness, crying emotional tears, sweating, having breath control, etc.) But even if the correlations were real, they still aren't the evidence AAT/H proponents seem to think, just as the very real aquatic vs. savanna correlations I showed above are not evidence for the "savannas are better than aquatic" conclusion I put forward (with tongue in cheek). Likewise, Crawford's "aquatic is better" conclusion is not supported by the correlation he mentions, even if the correlation were accurate and not an inaccurate "apples to oranges" (predator to herbivore) comparison. His conclusion is damaged even more because his correlation isn't even accurate.


Let me show you one more example of why AAT/H proponents can't (and really shouldn't try) use brain size as evidence because of it's supposed correlation with the habitats and lifestyles they say the "aquatic ape" used. Note that these EQs are from a different source than those above and shouldn't be directly compared with those EQs, since they were calculated somewhat differently -- humans, for example, are listed at 6.28 or 8.07 rather than 5.07 as in the above set of figures -- always be careful about comparing EQs from different sources.

In the following sets of EQs the species AAT/H proponents most often point to as occupying a habitat and living a lifestyle like that of our supposed aquatic ancestors are bonobos and proboscis monkeys. The species which occupy savannas (besides humans) are baboons and patas monkeys. Note that bonobos, although they are demonstrably quite smart animals, have lower EQs than their close relatives the chimpanzees and humans. If one was to use the environmental determinism regarding brain size that Crawford, Cunnane, and other AAT/H proponents say we should use (and keep in mind that I do not suggest this is a good idea; just the opposite) we would be forced to conclude that a semi-aquatic lifestyle to the degree of bonobos is bad for brain growth. Likewise, when we look at the proboscis monkey and compare it to other monkeys, such as the guenon, mangabey, macaque, patas monkey, or baboon.


Species

EQ -- from Martin 1984

EQ -- from Jerison 1973
Homo sapiens 6.28 8.07
gibbon 2.40 2.60
common chimpanzee 2.38 3.01
bonobo 1.80 2.36
macaque (Macaca) 1.78 1.95
mangabey (Cercocebus) 2.09 2.29
guenon (Cercopithecus) 1.96 2.05
proboscis monkey (Nasalis) 1.07 1.24
Colobus 1.27 1.41
baboon (Papio) 1.74 2.05
patas monkey (Erythrocebus) 1.99 2.19
capuchin monkey (Cebus) 3.25 3.25

Now these conclusions do not hold, of course, because this sort of environmental determinism, as practiced by AAT/H proponents, is BS, to be blunt. But since they use it, one might wonder why they don't mention these uncomfortable conclusions which inevitably follow from it.

By the way, check out the forest-dwelling (and tool using) capuchin -- it just throws the final monkey wrench, so to speak, into the AAT/H-style environmental/dietary deterministic strategy for explaining brain growth. Just don't use that strategy; it doesn't fit the facts.


Jerison, HJ, 1973 Evolution of the brain and intelligence. Academic Press, New York.

Martin, RD, 1984 "Body size, brain size and feeding strategies", in Food acquisition and processing in primates. Chivers, D; Wood, B; Bilsborough, A, eds. Plenum Press, New York.

Schoenemann, P. Thomas, 2004 "Brain Size Scaling and Body Composition in Mammals", Brain Behavior Evolution 63:4760