How smart are octopuses really?

Very smart, by invertebrate standards the smartest of all: cephalopods, the family of octopuses, squid and cuttlefish, are the most intelligent animals without a backbone. An octopus has over 500 million neurons, about as many as a dog, and it uses them to unscrew jar lids, open latches and solve puzzles that require pushing or pulling. It even remembers the solutions, and can solve the same puzzle when it is rearranged.

What makes it truly strange is where those neurons are. Two-thirds of them sit not in the head but in the nerve cords of the arms. Each arm can carry out complex reflex actions without waiting for orders from the brain, a bit like a team whose hands half think for themselves.

Just ask the staff of the Sea Star Aquarium in Coburg, Germany. Their octopus, Otto, juggled hermit crabs, banged on the glass with a rock and, more than once, short-circuited the lamp above his tank by shooting a jet of water at it. Other captive octopuses have climbed out of their tanks, raided a neighbouring aquarium for a meal and slipped back home.

In the wild, veined octopuses collect discarded coconut shells, carry them some distance and reassemble them into a shelter. Octopuses also change the colour and texture of their skin to vanish against the background. That mix of problem-solving and flexibility is why some countries now legally treat cephalopods as at least as sentient as vertebrates.