Brain size versus body size and the roots of intelligence

Behavior that we consider intelligent is oddly prevalent in the animal kingdom. Animals with completely different brains than ours – a type of octopus and different birds – deal with tools, to give just one example. It seems self-evident that the brain needs a certain level of size and sophistication to enable intelligence. But figuring out why some species seem to have intelligence while closely related ones don’t is difficult — so difficult that we don’t really understand it.

One of the simplest ideas was that size is everything: you have a big enough brain, and at least you have the ability to be smart. But many birds seem quite intelligent despite the small brains – perhaps because of that crawling more neurons at a certain size than other species. Some researchers prefer the idea that intelligence comes from having a brain large for the size of your body, but the evidence is there a little mixed.

This week a team of researchers published a paper in which they argue that the answer is a little of both: relative and absolute size matter when it comes to the brain. They argue that a specific approach to brain development helps empower it.

What makes intelligence?

To study what makes intelligence, you need to define the word. This can be a slippery thing to destroy. We all know (and/or) people who are brilliant in some circumstances but stupid in others. Likewise, the animal may engage in the use of tools but is unable to know how to find its way around a simple obstacle. So defining intelligence in different ways may produce different answers as to whether a particular type qualifies or not.

For the present work, the focus has been on the mental facilities of birds. Researchers have defined intelligence as an innovation or tendency to exhibit new behaviors. (Owls had to be excluded from the study because their behaviors are difficult to observe.) The number of papers reporting inventive behaviors was normalized by dividing it by the total number of papers describing any behavior in a species to adjust for the fact that some were simply better studied than others.

The researchers then compared this to the brain’s features with three questions in mind. One was whether intelligence was linked to specific regions of the brain — specifically a region called the pallium in birds, which appears to handle many of the same functions as the neocortex in humans. This area, among others, is where the brain integrates sensory information and plans activities.

By taking advantage of a system that allows them to count the number of neurons present in different regions of the brain, researchers can test whether intelligence correlates with brain size as a whole, with pallium specifically, or with the ratio of brain size to body size. The research team could also look at the evolutionary history of the brain in intelligent species and try to understand how any associations they discovered came about.

¿Por qué no los dos?

In general, bigger brains mean more complex behaviour. The authors concluded that “the number of neurons in the whole brain is positively correlated with behavioral innovation propensity, especially technical innovations that supposedly require more advanced cognition.” But controlling for body size showed that the relative size of the brain still mattered. If a species has more neurons than you would expect based on its body size, it is more likely to be involved in complex behaviors.

The researchers suggest that we tend to view this as an either/or situation — it should be either the total brain volume or the brain-to-body ratio. By preparing our analyzes to compare the two, we constrained our ability to determine that both correlates appear to be correct simultaneously. 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 general conclusions, the number of neurons in the pallium increased with both absolute brain size and brain volume relative to body size. Cerebellar neurons were significantly elevated as a function of absolute brain size. There was no clear pattern in the number of neurons in the brainstem.

Corvids and parrots are noted for having some of the most complex behaviors in the bird world. Analyzing them separately, the researchers showed that the number of neurons is rapidly proportional to body size — much faster than other groups of birds. How do these types end up with an unusually large number of neurons? They tend to have a longer growth period after they hatch, this time being used to mobilize more neurons into the pallium. Parrots tend to continue generating neurons for a longer time, and neurons do not mature as quickly as other neurons.

Obviously we would want to do a similar analysis with groups other than birds to see if this is a general rule or how birds have produced species with diverse intelligence. But, even if this result is a general indication of the “how,” it really doesn’t help us answer the “why?” Researchers suggest that parrots tend to be the largest, long-lived birds. Therefore, the reward time for owning well-developed mental devices is longer, even if it takes longer to develop these devices.

Which seems pretty counterintuitive until you start thinking about exceptions. Crows such as crows and gypsies are only about seven years old, yet some are still capable of it very sophisticated behavior. Jays are not particularly large birds. And many large, long-lived birds did not end up with any behaviors that indicated intelligence. So even if this goes on, there’s a lot we don’t know about why some animals turn intelligent.

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