Other Minds: The Octopus, the Sea, and the Deep Origins of Consciousness

by Peter Godfrey-Smith

Zebrafish were tested first to see which of two environments they preferred. They were then injected with a chemical suspected to cause pain, and in some cases, the less preferred environment had a painkiller dissolved in it. The fish now preferred this environment, but only when it contained dissolved painkiller. They made a choice they’d not normally make, and they made it in a situation where the idea of a more painful or less painful environment would be quite novel to them: evolution could not have set them up with a reflexive reaction to this situation. Similarly, in a study in chickens, birds with damaged legs chose a food that would usually be less preferred, provided that it contained painkillers. Robert Elwood has done experiments of a similar kind in hermit crabs, the small crabs who live in shells made by various mollusks. Hermit crabs are arthropods, relatives of insects. Elwood gave the crabs small electric shocks, and found they could be induced to leave their shell by a shock. But not always: they were more reluctant to leave a higher-quality shell than a low-quality one—they had to be shocked more.