Humans have large brains compared to most species around them. Also, humans consider themselves smarter than the species around them. This might raise the question: Is intelligence a direct consequence of having a large brain?

One who has a big body, needs a bigger brain…
First thing to note is that we almost never see a species with a large body size and a very small brain size. One who has a big body, needs a big brain to normally function. A bigger body needs more neurons because it possesses more sensory tissue, as it has bigger sensory organs like eyes, ears, skin etc. which are connected to the brain via nerves. A bigger body doesn’t only need more, but also bigger neurons due to basic biophysical constraints. Bigger brains also contain more copies of neuronal circuits for better memory storage, detail in perception, precision in sensory processes and parallel processing.
These three factors cause a big animal to have a relatively big brain independent of the fact whether the animal is intelligent or not.

Still research reveals that, comparing all kinds of species to each other, the species with higher absolute brain size are on average more intelligent than the small-brained.

However this theory was seriously criticized when examining the intelligence of a specific bird species: The great tit.

What is considered to be intelligence?
Before going in depth of this research we should know how scientists define intelligence and how they test it in animals. Pulses in sensory organs determine an animals automatic response in a certain situation. But when the animal wants to reach a certain goal, it can use its intelligence to overwrite/inhibit the automatic response in order to achieve this goal, a process called self-control. Next to this, an animal also needs to have a certain understanding of the world and combine this with its knowledge and experience, in order to be able to act towards a goal.

Testing intelligence in the great tit
Only this year, 2018, the great tit was tested on its intelligence using the most commonly used test, the cylinder task. The task was, simply enough, to get some food (reward) out of a long cylinder laying down on the floor. First they trained the birds in this task with an opaque cylinder, needing the birds to walk around the cylinder and going in with their heads in order to get the food. Then in the test situation, the birds were presented with a transparent cylinder so they could see the food immediately. Their automatic response would be to peck towards the food, but they’d fail due to the transparent wall. The succeeding response would be to inhibit this automatic response, memorize what it has learned, combine this information and make a plan to go around the cylinder to get the food out and achieve the goal of getting the food.

Birds are incredibly intelligent
Unexpectedly, 80% of the great tits that were trained and tested in this research, succeeded the first time. This is a very special finding considering the fact that other birds having a similar brain size scored 53% or below on the same test, making the great tit a very intelligent being for its little brain size.

On top of that, the same experiment has also been done with primates (humans closest relatives) which scored, not surprisingly, somewhat higher, 90-100%. Very surprisingly however was the success of corvid species (ravens, jackdaws and New Caledonian crows) which scored also 90-100%, with jackdaws and ravens scoring even better than gorillas and bonobos. The absolute brain size of a jackdaw is roughly 94 times smaller than that of a gorilla. On top of that, the brain size of the great tit is only 8% of that of a jackdaw. This made the researchers conclude that absolute brain size can’t be a good predictor of intelligence when comparing birds to primates.

The composition of the bird brain is totally different than that of primates…
It has just been discovered that the density of neurons in the bird brain is way higher than in the primate brain. The article mentions that the number of neurons in intelligence related regions of the brain is the same or even higher in certain birds. Since the composition of the brain is totally different between these species, it makes no sense to relate brain size to intelligence, however the actual number of neurons in intelligence related brain regions might be a good predictor for intelligence. This is in line with a number of scientists arguing that we should look at the absolute number of neurons, their size, connectivity and the available energy that affects information processing all together to predict the intelligence of a species. Unfortunately, experiments on all of these factors haven’t been carried out just yet.

Although, when looking at species that are more phylogenetically related (like different kinds of birds among each other) we see that absolute brain size and also relative brain size (having a relatively large brain for the body size) are good predictors for intelligence.

So are bigger brains better?
We can safely say bigger brains aren’t necessarily better. The cylinder task shows that small-brained corvids are in a way more intelligent than some of the big-brained primates. Also the great tit scores way better than expected for its small brain size. To conclude, there’s a lot of ongoing debate about what would be the best predictor for intelligence across species, but what is certain is that more experiments examining intelligence should be designed and more animals should be tested in those, in order to get a better view on this problem.

Sadly enough, it seems to be a recurring problem in today’s science: The more we learn, the more questions we get. The brain: will we ever uncover our own great mystery?