Opinion – Suzana Herculano-Houzel: About garlic, cloves and dinosaurs

Last month I told you that I discovered how many neurons tyrannosaurs and other carnivorous bipedal dinosaurs had in their telencephalon: many, numbering in the billions — as many as we find today in parrots, monkeys and baboons. In other words, the animals had the biological capacity to be cognitively flexible and intelligent, to use objects as tools and to make their own, and even to build cultures. Nothing an asteroid couldn’t annihilate, of course (humans, learn, please).

How did I come to this conclusion if there are no dinosaur brains in my freezers?

By similarity and parsimonious inference. Today’s land birds arose at the time of the dinosaurs—in fact, to be correct, they are dinosaurs, theropod-like tyrannosaurs. Dinosaurs (including birds) are part of an even larger group: reptiles. This simplifies the question about the brains of dinosaurs: were they like the brains of other reptiles alive today, or were they already like the brains of land birds that survived the asteroid — or something in between?

Using data that a former collaborator generated on the numbers of neurons that form the telencephalon of terrestrial birds and what we usually call “reptiles”, I showed that the biggest distinction between the two groups is that the first, warm-blooded, have a much greater number of neurons in the same telencephalon volume. In other words, size does not matter even in two types of reptiles, with and without wings — either warm-blooded or cold-blooded.

I have a whole other story to tell about how the same “invention” that allowed the maintenance of a high body temperature would have made possible much greater numbers of neurons in the brain, as well as a bigger brain, but the fact is that warm-blooded flying reptiles (I mean birds) have much larger brains, and with many more neurons, than cold-blooded crawling reptiles. Whence my question: where did dinosaurs fit in?

The answer is “halfway,” which earned them the nickname “mesotherms” eight years ago, when researchers put various types of dinosaurs into the blender of mathematical analysis.

For me, it was the equivalent of putting apples and oranges in the processor and drawing conclusions, from the result, about the characteristics of a fruit that doesn’t exist. Looking directly at the fruits, I mean the data for different types of dinosaurs, it’s obvious that theropods, like the T. rex, had brains the size expected for warm-blooded birds, while most of the others had brains the size expected for cold-blooded reptiles. And if the rules that apply today for birds were already valid for theropods, then I can use your brain size to estimate your neuron numbers.

And, in addition, to show that dinosaurs were not mesotherms at all. Especially in times of big data, it is necessary to remember to separate eyes and ears: the mathematical “middle ground” can always be calculated, but it is a worthless biological aberration.