Fish also behave in ways that indicate they consciously experience pain. In one study, researchers dropped clusters of brightly colored Lego blocks into tanks containing rainbow trout. Trout typically avoid an unfamiliar object suddenly introduced to their environment in case it’s dangerous. But when scientists gave the rainbow trout a painful injection of acetic acid, they were much less likely to exhibit these defensive behaviors, presumably because they were distracted by their own suffering. In contrast, fish injected with both acid and morphine maintained their usual caution. Like all analgesics, morphine dulls the experience of pain, but does nothing to remove the source of pain itself, suggesting that the fish’s behavior reflected their mental state, not mere physiology. If the fish were reflexively responding to the presence of caustic acid, as opposed to consciously experiencing pain, then the morphine should not have made a difference.

In another study, rainbow trout that received injections of acetic acid in their lips began to breathe more quickly, rocked back and forth on the bottom of the tank, rubbed their lips against the gravel and the side of the tank, and took more than twice as long to resume feeding as fish injected with benign saline. Fish injected with both acid and morphine also showed some of these unusual behaviors, but to a much lesser extent, whereas fish injected with saline never behaved oddly.