From Orion Magazine:
MEASURING THE MINDS OF OTHER creatures is a perplexing problem. One yardstick scientists use is brain size, since humans have big brains. But size doesn’t always match smarts. As is well known in electronics, anything can be miniaturized. Small brain size was the evidence once used to argue that birds were stupid—before some birds were proven intelligent enough to compose music, invent dance steps, ask questions, and do math. 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. “It is as if each arm has a mind of its own,” says Peter Godfrey-Smith, a diver, professor of philosophy at the Graduate Center of the City University of New York, and an admirer of octopuses. For example, researchers who cut off an octopus’s arm (which the octopus can regrow) discovered that not only does the arm crawl away on its own, but if the arm meets a food item, it seizes it—and tries to pass it to where the mouth would be if the arm were still connected to its body.