There’s no question that broadly speaking, big brains are smart. Take humans, for instance: our brains weigh in at about 3 pounds on average, nearly four times the size of the brains of chimpanzees (whose brains weigh in at less than a pound apiece).
What’s less clear is why. There are a number of theories: maybe intelligence evolved to give us a competitive edge in foraging, or maybe it helped us keep track of increasingly complex social interactions. Ideally, we’d like a theory to explain the evolution of intelligence broadly, so researchers have tried to these hypotheses across multiple species (for instance, comparing relative brain size and social group size among hoofed mammals like horses and deer1).
But brain size alone – even when scaled as a proportion of overall body size – is not an ideal measure of intelligence. The trouble is that small animals often have considerably higher brain-to-body mass ratios – ant brains, for instance, can weigh nearly 15% of their total body mass (the equivalent of a 20 pound human head!), and mice have about the same brain-to-body mass ratio as we do. So how can we study brain evolution, when even primates span a 3000-fold difference in body size (comparing a gray mouse lemur and a gorilla)?
Enter the encephalization quotient, or EQ, a measure of brain size relative to what we would predict, given that there is a curved relationship between brain size and body size (allometry is the technical term for this). It’s the best yardstick we have for the evolution of intelligence. Until now, that is.
In a new study published online in the journal PNAS, researchers have taken a slightly different approach of comparing the rate of evolutionary change of brains with that of the body2. The idea is that, evolutionarily speaking, there are four different scenarios that could take place with EQ: encephalization could increase due to selection for a bigger brain (with body size remaining more or less constant), or it could increase because of selection for a smaller body (with brain size remaining constant, or even decreasing, just less so than body size). Conversely, encephalization could decrease because of selection for a smaller brain, or because of selection for a larger body. The key thing is that not all of these scenarios are necessarily linked to changing intelligence. Contrary to the assumption of most research on brain evolution, selection on body size alone can change EQ, without necessarily affecting the brain.
The authors of the new study compared rates of brain and body size evolution across three mammalian groups: primates, bats and carnivores. And they found that much of the evolutionary change in all three groups has been driven primarily by selection on body size, and not brains. In seals, for instance, EQ may have decreased as a result of selection for much larger bodies (check out sea elephants for an extreme example), without necessarily entailing a loss of intelligence.
It’s worth noting that this is not the first time evolutionary history has been considered in the context of EQ. For instance, in another recent paper, Susanne Shultz and Robin Dunbar used measurements of living and fossil specimens to look at encephalization in mammals. They still found evidence that social group size tends to be correlated with increasing EQ across evolutionary time in cats, dogs, primates and other groups, including aquatic mammals3.
Nevertheless, it’s obvious that changes in body size can have a dramatic effect on EQ without actually affecting intelligence. Just look at the bodies of other humans: bigger people aren’t (necessarily) any less smart. So allometry alone is not the whole story. If we want to understand how brains get big, we have to consider that brawniness evolves too.