Historically taste aversion learning arose as a problem in evolutionary biology. The English naturalist Charles Darwin was puzzled by an incongruity: Some tender caterpillars were brightly colored and exposed themselves so that they caught the eye of every passing bird. Such behavior appeared maladaptive. Years later, the English anthropologist and naturalist Alfred Russell Wallace suggested that brightly colored butterfly larva probably tasted bitter and might be poisonous; therefore the colors served to deter birds and other predators. Subsequent research supported Wallace's hypothesis. Consumption of the colorful insects causes gastric nausea and emesis, and after one or two trials birds and other predators learn to avoid them. As larva, these insects feed on plants that evolved the bitter toxins as a defense against herbivores; the insects turned that defense to their own advantage.

Taste aversion learning proved...