Brain size vs body size and the roots of intelligence

Image of a crow on a fence.

Behavior that we would consider intelligent is oddly common in the animal kingdom. Animals with very different brains than ours—a species of squid and various birds—deal with tools, to name just one example. It seems intuitive that a brain needs a certain size and sophistication to enable intelligence. But figuring out why some species appear to have intelligence but closely related ones don’t has proven difficult – so difficult that we don’t really understand it.

One of the simplest ideas was that size is everything: have a big enough brain and you at least have the potential to be smart. But many birds, despite having small brains, appear to be quite intelligent — possibly because they are stuff more neurons in a certain volume than other species. Some researchers prefer the idea that intelligence comes from having a large brain relative to your body size, but there is evidence to support this a bit mixed.

This week, a team of researchers published a paper arguing that the answer is a little of both: relative and absolute size matter when it comes to the brain. And they argue that a specific approach to brain development helps make this possible.

What makes smart?

To examine what constitutes intelligence, you need to define the word. And that can be a slippery thing to pin down. We all know (and/or are) people who are brilliant in some circumstances and idiots in others. Similarly, an animal can use tools but not be able to bypass a simple barrier. The different definition of intelligence can therefore lead to different answers as to whether a certain species is suitable.

For the current work, the focus was on the mental abilities of birds. Researchers defined intelligence as innovation, or the tendency to exhibit novel behaviors. (Owls had to be excluded from the study because their behavior is difficult to observe.) The number of articles reporting innovative behaviors was normalized by dividing by the total number of articles describing any behavior of the species to to offset the fact that some are simply better studied than others.

The researchers then compared this to brain characteristics with three questions in mind. One was whether intelligence correlates with specific brain regions — specifically an area called the pallium in birds, which appears to serve many of the same functions as the neocortex in humans. In this area, the brain integrates sensory information and plans activities, among other things.

Using a system that allows them to count the number of neurons present in different areas of the brain, the researchers were able to test whether intelligence correlated with the size of the brain as a whole, with the pallium in particular, or with the ratio of brain size to height. The research team could also look at the evolutionary history of the brains of intelligent species and try to understand how the discovered correlations come about.

Why not both?

In general, larger brains meant more complicated behavior. “The number of neurons throughout the brain is positively associated with the propensity for behavioral innovation,” the authors conclude, “particularly with technological innovations thought to require more advanced cognition.” But controlling for body size showed that the relative brain size still played a role. If a species had more neurons than would be expected based on its body size, then it was more likely to engage in complex behaviors.

The researchers suggest that we’ve looked at this more as an either/or situation — it has to be either the overall brain size or the brain-to-body ratio. By setting up our analyzes to compare the two, we have limited our ability to determine that both correlations appear to be true at the same time. When specific brain regions were analyzed independently, the pallium was the most significant region implicated in complicated avian behavior; the cerebellum also contributed, but to a lesser extent.

Consistent with the general conclusions, the number of neurons in the pallium increased with both absolute brain size and brain size relative to body size. Neurons in the cerebellum increased largely as a function of absolute brain size. And there was no clear pattern in the number of neurons in the brainstem.

Corvids and parrots are considered to have some of the most complex behaviors in the bird world. By analyzing them separately, the researchers show that the number of neurons scales rapidly with body size – much faster than in other groups of birds. How do these species come to have an unusually large number of neurons? They tend to have a longer development time after hatching, and this time is used to pack more neurons into the pallium. Parrots tend to create neurons longer, and the neurons don’t mature as quickly as others.

Of course, we’d like to do a similar analysis with groups other than birds to see if this is a general rule or how birds produced species with different intelligences. But even if this finding is a general indication of the “how,” it doesn’t really help us answer the “why.” The researchers suspect that parrots tend to be larger, long-lived birds. So the payback time for sophisticated mental hardware is longer, even if that hardware takes longer to develop.

Which seems pretty intuitive until you start thinking about the exceptions. Corvids such as crows and jays only have a lifespan of around seven years, but are still capable of a few very challenging behavior. Jays aren’t even particularly large birds. And many large, long-lived birds lack behaviors that indicate intelligence. So, even if this is true, there’s a lot we don’t know about why some animals become intelligent.

natural ecology and evolution2022. DOI: 10.1038/s41559-022-01815-x (About DOIs).

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