Brain size versus body size and the roots of intelligence


Behavior that we would call intelligent is strangely prevalent in the animal kingdom. Animals with brains very different from ours – a species of octopus and various birds – engage with tools, to give just one example. It seems intuitive that a brain needs a certain level of size and sophistication to enable intelligence. But understanding why some species seem to have intelligence while other 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 matters: get a big enough brain and you at least have the potential to be smart. But many birds seem quite smart despite their small brains, perhaps because they will cram more neurons into a given volume than other species. Some researchers favor the idea that intelligence stems from a large brain relative to your body size, but the evidence is a bit mixed.

This week, a team of researchers published a paper claiming that the answer is a bit 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 it possible.

What makes intelligence?

To study what makes intelligence, you must define the word. And it can be a slippery thing to nail. We all know (and/or are) brilliant people in some circumstances and dumb in others. Similarly, an animal may engage in tool use, but be unable to find its way around a simple fence. Thus, defining intelligence in different ways can produce different answers about whether a given species is eligible.

For current work, the focus has been on the mental abilities of birds. Researchers have defined intelligence as innovation or the tendency to adopt new behaviors. (Owls had to be excluded from the study because their behaviors are difficult to observe.) The number of papers reporting innovative behaviors was normalized by dividing by the total number of papers describing any behavior in the species to account for 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 correlated with specific brain regions, specifically an area called the pallium in birds, which appears to handle many of the same functions as the neocortex in humans. This area is, among other things, where the brain integrates sensory information and plans activities.

Taking advantage of 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 was correlated to the size of the brain as a whole, to the pallium in particular, or to the ratio of brain size to body size. The research team could also examine the history of brain development in intelligent species and try to understand how the discovered correlations occurred.

¿Por que no los dos?

In general, bigger brains meant more complicated behavior. “The number of neurons in the whole brain is positively associated with the propensity for behavioral innovation,” the authors conclude, “especially technical innovations that are thought to require more advanced cognition.” But controlling for body size showed that relative brain size still mattered. If a species had more neurons than you would expect based on its body size, then it was more likely to engage in complex behaviors.

The researchers suggest we’ve tended to think of this as an either/or situation — it has to be either total brain size or brain-to-body ratio. By configuring our analyzes to compare the two, we have limited our ability to identify that both correlations appear to be true simultaneously. When specific brain regions were analyzed independently, the pallium was the most important region associated with complicated bird behavior; the cerebellum also contributed, but to a lesser extent.

Consistent with general findings, the number of neurons in the pallium increased with both absolute brain size and brain size relative to body size. Cerebellar neurons grew largely in line with absolute brain size. And there was no clear pattern in the number of neurons in the brainstem.

Corvids and parrots are known to have some of the most complex behaviors in the bird world. By analyzing them separately, the researchers show that the number of neurons changes rapidly with body size, much faster than other groups of birds. How do these species end up with an abnormally high number of neurons? They tend to have a longer development period after they hatch, and this time is used to pack more neurons into the pallium. Parrots tend to keep generating neurons for longer, and neurons don’t mature as quickly as others.

Obviously, we will want to do a similar analysis with groups other than birds to find out if this is a general rule or how birds have produced species with varied intelligence. But, even if this discovery is a general indication of the “how”, it does not really help us to answer the “why?” The researchers suggest that parrots tend to be larger, long-lived birds. Thus, the return time to having sophisticated mental material is longer, even if the development of said material takes longer.

Which seems pretty intuitive until you start thinking about exceptions. Corvids like crows and jays only have a lifespan of about seven years, but are still capable of very sophisticated behavior. Jays aren’t even particularly large birds. And many large, long-lived birds did not end up with behaviors indicative of intelligence. So even if that holds, there’s a lot we don’t know about why some animals end up being intelligent.

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


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