I tried to understand why bigger brains do not simply mean smarter animals
I recently went down a rabbit hole on brain size, neuron counts, and animal intelligence, and it changed how I think about “smart” animals.
The first thing that surprised me is how expensive a large brain is. The human brain is only about 2% of body mass, but it uses roughly 20% of resting energy. So a species cannot just evolve a bigger brain for free. A large brain comes with tradeoffs: high energy demand, long development, slow maturation, and fewer offspring.
At first, I thought the Encephalization Quotient made sense as a way to compare intelligence. EQ compares actual brain size to what would be expected for an animal of that body size. But from what I understand, EQ becomes misleading if we treat it as a general intelligence ranking. A small animal can score highly by EQ without having the absolute neural machinery of a larger-brained animal.
Then neuron counts made the picture even more complicated. Suzana Herculano-Houzel’s work showed that humans have about 86 billion neurons, not the often-repeated 100 billion. But even total neuron count is not enough, because distribution matters.
The elephant example is what made this click for me. African elephants have around 257 billion neurons, far more than humans. But most of those neurons are in the cerebellum. That seems to reflect the huge sensorimotor demands of controlling a massive body and a complex trunk. Their cerebral cortex has far fewer neurons than the human cortex.
So “more neurons” does not automatically mean “more human-like intelligence.” The important question seems to be where the neurons are, how densely they are packed, how they are organized, and what ecological problems the animal evolved to solve.
Birds are another interesting case in the opposite direction. Some corvids and parrots show complex cognition with very small brains, probably because their neurons are packed very densely and organized differently from mammalian brains. That makes gross brain size look like a very poor shortcut.
The human case also seems less like a magical exception and more like a specific primate trajectory. We have a dense, metabolically expensive brain. Cooking may have helped make that sustainable by increasing usable calories, but I assume that is only one part of the story, alongside sociality, tool use, development, culture, and ecology.
For people with more background in evolutionary biology, neuroscience, or comparative cognition: is this a fair summary, or am I flattening something important?