Brain size versus body size and the roots of intelligence

The behavior we think of as intelligent is strangely spread across the animal kingdom. Animals with brains very different from ours – one species of octopus and various birds – attach to tools, to give just one example. It seems intuitive that the brain requires a certain level of size and sophistication to enable intelligence. But figuring out why some species seem to have intelligence while closely related ones haven’t has proven difficult—so hard that we don’t really understand it.

One of the simplest ideas is that size is everything: have a big enough brain, and you at least have the potential to be smart. But many birds seem to be quite intelligent, despite small brains—presumably because they cram a certain amount of more neurons than other species. Some researchers support the idea that intelligence stems from having a large brain relative to your body size, but the evidence is 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 he argues that a specific approach to brain development helps enable this.

What Makes Smart?

To study what makes up for intelligence, you need to define the term. And it can be a slippery thing to nail. We all know (and/or have been) people who are talented in some situations but stupid in others. Similarly, an animal may engage in tool use, but may be unable to figure out how to find its way around a simple obstacle. So defining intelligence in different ways can lead to different answers as to whether a given species qualifies.

For the present work the focus was on the mental features of birds. Researchers defined intelligence as the tendency to innovate or exhibit novel behavior. (Owls had to be excluded from the study because their behavior is difficult to observe.) The total number of papers describing any behavior in the species was to be adjusted for the number of papers reporting novel behaviors. Some are simply better studied than others.

The researchers then compared this to the characteristics of the brain, asking three questions. One was whether intelligence is related to specific brain regions—specifically a region called the pallium in birds, which appears to handle functions similar to the neocortex in humans. This area, among other things, is 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, researchers can test whether intelligence is related to the size of the whole brain, specifically along the pallium, or With proportions from brain size to body size. The research team can also look at the evolutionary history of brains in intelligent species and try to understand how any connections they discovered came about.

why not both?

In general, bigger brains meant more complex behavior. “The number of neurons throughout the brain is positively associated with behavioral innovation tendencies,” the authors conclude, “especially technological innovations that require more advanced cognition.” But controlling for body size showed that the relative size of the brain still mattered. If a species had more neurons than you would expect based on body size, they were more likely to engage in complex behavior.

The researchers suggest that we have seen this as an either/or condition—it must be either total brain size or brain-to-body ratio. By setting our analyzes to compare the two, we have limited our ability to recognize that the two correlations together seem to be true. When specific brain regions were analyzed independently, the pallium was the most important region associated with complex bird behavior; The cerebellum also contributed, but to a lesser extent.

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

Corvids and parrots are noted as having some of the most complex behaviors in the bird world. Analyzing them separately, the researchers show that the number of neurons increases rapidly with body size – far more rapidly than in other groups of birds. How do these species end up with an unusually large number of neurons? Their growth period after hatch is longer, and this time is used to pack more neurons into the pallium. Parrots continue to generate neurons for a long time, and neurons do not mature as quickly as others.

Obviously, we would like to do a similar analysis with groups other than birds to find out whether this is a general rule or how birds have produced species with different intelligences. But, even though this finding is a general indication of “how”, it is actually a sign of “why?” does not help us to answer. Researchers suggest that parrots are large, long-lived birds. Therefore, the payoff time for sophisticated mental hardware is longer, even though the said hardware takes longer to develop.

Which seems pretty intuitive until you start thinking about exceptions. Corvids like crows and jays only have a lifespan of seven years, yet they are capable of some very sophisticated behaviors. Jays are also not particularly large birds. And many large, long-lived birds haven’t ended up with any of the behaviors that indicate intelligence. So, even if it holds, we don’t know a lot about why some animals are intelligent.

Nature Ecology and Evolution2022. DOI: 10.1038/s41559-022-01815-x (About DOI).

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