We think of caffeine as being a plant defence mechanism - a chemical compound produced to deter possible insect predators. And as we sip our morning cup of coffee, feeling our brains become more alert and our memory better, most of us assume it's just a lucky chance that causes caffeine to have such beneficial effects on us humans.
So why, a team of plant scientists asked, do flowers produce caffeine in their nectar? Surely deterring pollinating insects from nectar would be an evolutionary dead-end?
What's more, since the human relationship with caffeine is relatively recent, its impact on our brains is most likely to be by-product of its true ecological role.
The team investigated, and found that caffeine can enhance bees' memory, making them more effective pollinators of those particular plants. By adding just enough caffeine to their nectar - not enough to make it bitter, but enough to have a pharmacological effect - the plants were more likely to be pollinated and to set seed, out-competing their neighbours
In a paper published in Science, 8 March 2013, the team concluded:
"Plant defense compounds occur in floral nectar, but their ecological role is not well understood. We provide evidence that plant compounds pharmacologically alter pollinator behavior by enhancing their memory of reward. Honeybees rewarded with caffeine, which occurs naturally in nectar of Coffea and Citrus species, were three times as likely to remember a learned floral scent as were honeybees rewarded with sucrose alone. Caffeine potentiated responses of mushroom body neurons involved in olfactory learning and memory by acting as an adenosine receptor antagonist. Caffeine concentrations in nectar did not exceed the bees' bitter taste threshold, implying that pollinators impose selection for nectar that is pharmacologically active but not repellent. By using a drug to enhance memories of reward, plants secure pollinator fidelity and improve reproductive success."