Caffeinated plants provide an unforgettable experience.Honeybees rewarded with caffeine remember the smell of specific flowers longer than bees given only sucrose, according to a study published in Science. “By using a drug to enhance memories of reward,” the study says, “plants secure pollinator fidelity and improve reproductive success.”
Many drugs used by humans come from plants. But what role do the drugs play for the plants themselves? Frequently, they play the role of toxic avenger, providing a chemical defense against attacks by herbivores. But in smaller doses, they often have pharmacological effects on mammals. The researchers looked at two genera of caffeine-producing plants—Coffea and Citrus. “If caffeine confers a selective advantage when these pants interact with pollinators,” the investigators reasoned, “we might expect it to be commonly encountered in nectar.” And it was. Caffeine at very low doses was measured in the nectar of several of the caffeine-producing plant species, including several Coffea species, as well as some citrus nectars—grapefruit, lemons, and oranges among them.
Next, the researchers wanted to find out if the caffeine-laced nectar could affect learning and memory in pollinating bees. They trained individual honeybees to associate various floral scents with sucrose containing various concentrations of caffeine. This pairing of odor and reward, with high-concentration sucrose as the control, demonstrated that low doses of caffeine had almost no effect on the rate of honeybee learning—but a profound effect on long-term memory. Three times as many caffeinated bees remembered the conditioned floral scent 24 hours later, “and responded as if it predicted reward.” Twice as many bees remembered the scent at the 72-hour mark.
What’s the trick? Caffeine’s ability to influence mammalian behavior is due to its action as an adenosine receptor antagonist. “In the hippocampal region,” the authors write, “inhibition of adenosine receptors by caffeine induces long-term potentiation, a key mechanism of memory formation." The Kenyon cells in mushroom bodies of the insect brain, which showed “increased excitability” under the influence of caffeine, are similar in function to hippocampal neurons, they write. “Remembering floral traits is difficult for bees to perform at a fast pace as they fly from flower to flower and we have found that caffeine helps the bee remember where the flowers are,” said Geraldine Wright of the UK’s Newcastle University, who was lead author on the study. “So, caffeine in nectar is likely to improve the bee’s foraging prowess while providing the plant with a more faithful pollinator.”
It is an interesting balancing act by nature: Too much caffeine makes the nectar toxic and repellent to honeybees. Too little, and there is no behavioral effect on bee memory. “This implies that pollinators drive selection toward concentrations of caffeine that are not repellent but still pharmacologically active,” says the report. Humans have selected for a not-too-much, not-too-little dose of caffeine in the form of soda drinks and coffee. Is it possible that the humble coffee bean is pharmacologically manipulating us into taking good care of it? And do we drink it when we read or study because, for one thing, it enhances long-term memory? And speaking of memory, people often forget where they tucked the oregano, but they usually have little difficulty remembering where they stashed the coffee.
More pragmatically, honeybees on caffeine may lead researchers toward a better understanding of the foraging strategies of pollinator insects, and allow for improved management of crops and landscapes.
Wright G.A., Baker D.D., Palmer M.J., Stabler D., Mustard J.A., Power E.F., Borland A.M. & Stevenson P.C. (2013). Caffeine in Floral Nectar Enhances a Pollinator's Memory of Reward, Science, 339 (6124) 1202-1204. DOI: 10.1126/science.1228806
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