Understanding the caffeine paradox: caffeinated nectar increases pollinators’ perceived value of flowers

Caffeine, nicotine and cocaine are toxic plant defence compounds that are used to deter herbivory, yet they are rewarding, compulsively consumed and addictive. This paradox remains poorly understood. The rewarding effects of plant-derived psychostimulants are thought to be an evolutionary side effect of their toxicity. However, caffeine and nicotine are found at low, nontoxic levels in plant nectars. Here, attention was directed towards caffeine, and whether caffeine changed pollinators’ valuation of flowers was examined. Bumble bees were trained to forage from two differently coloured flowers, with equal or different sugar concentrations separately on consecutive days or simultaneously. One flower type was caffeinated. Bees that experienced equal sugar concentrations from the two flower types showed no immediate preference, but they preferred the caffeinated flower after 24 h. In contrast, bees that experienced unequal sugar concentrations preferred the sweeter, noncaffeinated flower during training, but their preference for the caffeinated flower increased after 24 h compared with controls. These results indicate that caffeine influences bumble bee preferences by altering the perceived comparative value, rather than enhancing memorability. This finding provides a novel explanation for the paradox of plant-derived psychostimulants. That is, plants evolved to use compounds that target animal reward systems, thereby enhancing their perceived value to pollinators in comparison with other neighbouring flowers. • Memory enhancement alone cannot explain caffeine's floral preference effect. • Caffeine alters a bumble bee's long-term comparative value of flowers. • Plants may benefit by making caffeinated nectar comparatively attractive.