Data from: 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.