We humans are a swollen-headed lot. We like to think we’re smarter than all other creatures, and perhaps uniquely blessed by some benevolent deity. Nevertheless, we need to be wary of our comfortable sense that we are at the top of the earthly hierarchy, since it provides a too-easy justification of the way we treat other animals. We eat them, kill them for sport, drink their milk, wear their skins, ride on their backs, ridicule them, house them in zoos and breed them to our own specifications.
How then do we justify our self-proclaimed superiority? One way is to appeal to our very swollen-headedness, looking to the brain itself as proof of our status on the planet. This strategy, though, has proven unexpectedly elusive. In terms of sheer brain size, for example, we have to defer to the elephant and the whale, whose brains are more than four times as big as our own. We cannot therefore claim to be the brainiest of creatures. Perhaps, though, the absolute size of the brain is not really a good measure of intelligence. Large animals need large brains simply to control those big bodies, and deal with all of the information that arrives from their large surfaces. So maybe we should consider not the absolute size of the brain, but rather the ratio of brain size to body size.
Here we come out rather better. Our brains weigh about 2.1 per cent of our body weights, while those of our closest cousins, the chimpanzee and bonobo, are about 0.61 per cent and 0.69 per cent, respectively. The figures for the Asian elephant and killer whale are 0.15 per cent and 0.094 per cent, respectively. So far so good—we can dismiss those lofty elephants and voluminous whales as large but fairly dumb. Sadly, though, the mouse comes out better than we do, with 3.2 per cent, and in small birds the ratio may be as high as 8 per cent.
One approach to this problem is to retreat into mathematics, and hope to bamboozle our creature cousins with equations. Other things being equal, smaller animals have larger ratios than bigger animals do. The psychologist Harry Jerison plotted log brain size against log body size across a wide range of species, and then used a technique called linear regression to compute the slope of the line relating one measure to the other. The slope of this line was ⅔, meaning that body size mattered less the larger the animal. This led to calculation of the expected brain size based on body size, and dividing this into the brain weight leads to what Jerison called the encephalization quotient.*
Comparison of brain sizes of various mammals
This quotient turned out to be 7.44 in humans, followed by dolphins at 5.31 and chimpanzees at 2.49. The elephant weighs in, as it were, with a quotient of 1.87 and rats have a miserly quotient of 0.4. The mouse is blessedly reduced to a quotient of about 0.5, so we can stop worrying about that. This quotient also pretty well gets rid of any serious challenge from birds, although comparison is difficult because the slope of the line is rather different for birds. We should be wary of the dolphin, though, which may be closer to us than we’d like to think, and of course there may be other creatures busily working on formulae to prove that they, after all, are top dogs.
Another approach is to examine the neocortex, the outer layer of the brain which houses higher-order functions such as language, thinking and memory. Focusing on the neocortex has the decided advantage of getting rid of birds altogether, since they possess no neocortex at all (other brain areas perform some of the same functions, but let’s not go there). Taking the ratio of neocortex to the rest of the brain places us above other primates with 4.1, closely followed by the chimpanzee at 3.2, gorilla at 2.65, orangutan at 2.99 and gibbon at 2.08.
Group size seems to increase with the ratio. Estimates based on the ratio suggest that gibbons should hang out in groups of about fifteen, orangutans in groups of about fifty, and chimpanzees in groups of about sixty five. These values are pretty close to what has been observed of the animals in the field, except for the solitary orangutan, which seems to prefer its own company. Based on what we know from the brain sizes of our hominin forebears, group size should have been roughly constant at around sixty in the australopithecines, but increased steadily with the emergence of the genus Homo from around 2.5 million years ago, culminating in Homo sapiens. According to the formula, humans should belong in groups of about 148, which is roughly the typical size of Neolithic villages. Of course, modern cities contain vastly more than that, but if you add up the people you are actually acquainted with it may be not too far off the mark. The relation of neocortical size to group size might be taken to mean that neocortical evolution, and perhaps intelligence itself, was driven by the pressures of social interaction.
If the path toward demonstrating our superior brain power has seemed tortuous, we can perhaps at least gain comfort from the thought that we are the only species working on the problem—or so it seems.