My colleagues have learned since they were little children that if larger animals have more neurons in their brains, it’s because they have to be that way. Why does it have to be like this? Because powering a larger object should require more neurons, they said, and also because going the extra mile to power a larger object, which requires more energy, should also require more neurons to create mental maps of the larger environment. Evolutionary Police in Service Sign below.
He’s fine. But what does the data say?
For a long time, they did not say anything: there was no data on the number of neurons in the brain. There have been studies measuring hippocampal size, which seem to confirm expectations that species that go further have larger hippocampi.
But now that we know how to turn brains into soup to calculate how many neurons they are composed of, and even have a way to estimate the number of neurons in the hippocampus, the moment of truth has arrived – thanks to two students in my lab, Greg Casalino, who began the study as an undergraduate intern, and Mia Elbon, who completed the study as part of her doctoral dissertation. Greg and Mia combed through the literature to create a huge spreadsheet that included body size, brain size, and population distribution of different species of mammals, how much energy each uses, what their diet is, and finally, how far each species walks, and how many neurons are in the cerebral cortex and hippocampus.
The larger the body, the more energy a species uses, which is pure physics – and Mia has shown that more individuals disperse into less dense groups, partly because there are fewer larger animals that can fit in the same area, but also because there is less food for each one. This is why diet is important: a savannah supports more herbivores than carnivores of the same size, because the latter’s food is scarcer. The more larger individuals dispersed into thinner groups to obtain their daily bread, the greater their number increased, in an inversely proportional manner among species.
But this has nothing to do with the number of neurons in the hippocampus or cerebral cortex.
Let’s look at primates, for example. They are medium-sized animals that explore relatively small areas of 1 km2 – But it contains the same number of neurons as terrestrial carnivores, which explore areas a thousand times larger, and aquatic carnivores, which swim in areas up to a hundred thousand times larger. As well as animals that explore the same area, for example 10 km2 They can contain anywhere from 5 to 200 million neurons in the hippocampus, depending on whether they are marsupials, carnivores, artiodactyls, or primates.
The lack of correlation is evidence that exploring different distances with the same number of neurons in the hippocampus is clearly possible – and also that the traditional explanation, that animals have larger brains with more neurons because they need to go further, is flawed.
The funny thing is that the few scientists who actually study how the hippocampus maps the world actually expected this: the same neurons in the hippocampus form a map whose size changes depending on the size of the environment in which they are located. The brain finds a way to do what it can with what it has.
reference
Elbon MC, Casalino G, Herculano-Houzel S. Home range is not restricted by hippocampal neuron numbers.
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