Cephalopods behave in ways that certainly suggest they’re highly intelligent. An octopus named Inky, for example, made a notorious escape recently from the National Aquarium of New Zealand, exiting his enclosure and slithering into a floor drain and, apparently, out to sea.

Cuttlefish can scare off predators by forming eyespots on their bodies in order to look like giant fish. But they only use this trick against predators that rely on vision to find prey. If a predator that depends on smell shows up, the cuttlefish are smart enough just to flee.

Octopuses show the same flexibility when scientists bring them into labs. In one study, researchers at Hebrew University presented octopuses with an L-shaped box with food inside. The animals figured out how to push and pull the morsel through a tiny hole in the wall of their tank.

Another feature that cephalopods share with other smart animals is a relatively big brain. But that’s where the similarities appear to end. Most of the neurons that do the computing, for example, are in the octopus’s arms.

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Most strikingly, cephalopods die young. Some may live as long as two years, while others only last a few months. Nor do cephalopods form social bonds.

They get together to mate, but males and females don’t stay together for long or care for their young. While chimpanzees and dolphins may live in societies of dozens of other animals, cephalopods seem to be loners.

Mr. Amodio and his colleagues think the evolutionary history of cephalopods may explain this intelligence paradox. About half a billion years ago, their snaillike ancestors evolved to use their shells as a buoyancy device. They could load chambers in the shell with gas to float up and down in the ocean.