Octopuses have the largest brains of any invertebrate. Athena’s is the size of a walnut—as big as the brain of the famous African gray parrot, Alex, who learned to use more than one hundred spoken words meaningfully. That’s proportionally bigger than the brains of most of the largest dinosaurs.
Another measure of intelligence: you can count neurons. The common octopus has about 130 million of them in its brain. A human has 100 billion. But this is where things get weird. Three-fifths of an octopus’s neurons are not in the brain; they’re in its arms.