Opinion – Suzana Herculano-Houzel: Many associative neurons make crows smart

Crows, birds of the same family that includes the Brazilian rooks, are animals as intelligent as great primates – despite the tiny brain, which fits in the bird’s little head, about the size of a gorilla’s thumb. Like monkeys and chimpanzees, and much better than tamarins and even dogs, crows know how to identify themselves in the mirror; distinguish quantities; and not only use objects as tools, but also make their own, with beak and claws.

How is it possible for animals with such small brains to be capable of so much? When working in Brazil, my collaborators in the Czech Republic and I, who had easy access to birds of various species for research, found that songbirds, including parrots and crows, have numbers of neurons in their cerebral cortex comparable to the numbers found in monkeys.

The finding is equivalent to discovering that a spoon and a bowl of soup contain similar numbers of seeds — which is only possible if the seeds in the spoon, like the neurons in birds, are much smaller than the seeds in the dish, like the neurons in mammals. The more neurons, the greater the ability of the cortex to process signals and information, we think.

But not all cortical neurons are the same: there are those that process sensations and movements, and others that put things together, which allows the brain to create associations, find patterns and invent rules. These are the associative neurons—and perhaps, in terms of cognitive flexibility, which is my definition of intelligence, that’s what matters, far more than the total number of neurons.

To test this possibility, my group and two colleagues in Germany compared three species of crow with pigeon, chicken and ostrich, the largest of birds with the largest brain. Once again, we have seen that size does not matter: even if crows lose the ostrich in terms of numbers of sensory neurons, it is crows that gain, and by far, in numbers of associative neurons, which we estimate to be as many as in the cortex of the chimpanzee. The study has just been published in the Journal of Comparative Neurology.

My next question now is how much energy does it cost to operate so many neurons in brains of such different sizes: an amount proportional to the number of neurons, the size of the brain, or something else? That is, what determines how much energy a brain uses? Are birds more efficient than we primates in this regard?

I tried to approach the subject when I was still working in Brazil, in a project in collaboration with the same researchers in Germany. They received eleven times the amount from the German government that I could even request from Faperj, and I never received it, because the state of RJ went bankrupt. Luckily for my research, I was able to move to another country, and how much energy a brain now costs is a question I can answer. Wait for me!