Humans aren't the only organisms that experience addiction. A little creature called the horn worm caterpillar has been discovered to succumb to addiction too, a food addiction. These critters usually eat whatever plant they hatch on. Often what they hatch on are plants of the nightshade family, such as potatoes, tomatoes, and eggplant. However, if they hatch on stuff like wheat or alfalfa, they'll eat that and do just fine.
The interesting thing is that if the caterpillar is reared on something like wheat, then given potato to munch on, the caterpillar will willingly eat the potato; but if reared on a nightshade plant, then given wheat, the critter will starve to death before changing its diet. Researchers have found that there is a chemical in nightshade plants that turn the horn worm caterpillars into addicts. If they never encounter a nightshade plant they'll eat almost anything, but once they taste a nightshade plant, there's no going back to anything else.
The root of the addiction lies not just in this chemical, but in the chemical receptors located in the whiskers just outside of the caterpillar's mouth. The receptors are something like taste buds. These receptors become extra sensitive to the addictive chemical, to the point that the caterpillar will die before eating anything lacking this chemical. If researchers remove the whiskers, though, the caterpillars will once again eat pretty much anything. However, the chemical in nightshade plants doesn't seem to be bad for the caterpillar. In fact, it inspires the caterpillar to eat more and grow rapidly.