In laboratory experiments, honeybees were roughly three times as likely to remember a floral scent a day later if the nectar had a minuscule amount of the stimulant than if it was caffeine free. They were also twice as likely to remember a caffeine-laced scent a full three days later. Pretty impressive for an insect that lives only a few weeks.
The findings, published in Friday's edition of the journal Science, hints that flowers may have been capitalizing on the bitter compound long before humans built the first Keurig machine.
"It's the first evidence that I know of where a plant is actually using a drug to manipulate the behavior of an animal to its own benefit," said study leader Geraldine Wright, a neuroethologist at Newcastle University in England.
Bees and other insects are crucial players in a plant's sex life. Plant pollen sticks to their legs while they feed on a flower's nectar and fertilizes the next bloom they visit. So flowers have developed a formidable arsenal of tools -- color, design and scent -- to attract them.
Plants can also use bitter and even toxic compounds to deter animals from munching on their vital tissues. Some of those compounds, like caffeine and nicotine, have powerful effects on mammalian brains as well.
Wright perked up when she first heard that certain citrus plants had caffeine in their nectar. Why would a citrus plant produce caffeine -- a compound that's quite costly for a plant to make -- in the nectar it's using to attract bees, which eschew the bitter stuff at high levels?
Wright gathered flower nectar from three species of coffee plants and four types of citrus. All of them, she found, contained very low doses of caffeine -- well below the threshold that might cause a bee to sour on a flower.
Next, the researchers saddled more than 900 bees in tiny harnesses and puffed a whiff of fragrance in their faces just before feeding them a sugar solution laced with different doses of caffeine. For the scent, the team used hexanol, the primary contributor to the distinctive smell of cut grass and a common ingredient in floral cocktails.
After six rounds, the bees were trained to unfurl their straw-like feeding tubes when they smelled the hexanol, rather like Pavlov's dogs were trained to associate the sound of a bell with an imminent meal and begin salivating.
Ten minutes after the training ended, the scientists put the bees through their first memory test. The bees were slightly more likely to stick out their snouts when they smelled the hexanol if they had been fed caffeine-laced sugar, but not by much. Remarkably, the effect grew stronger as time passed: Signs of improved memory lasted for up to three days. The researchers surmised it was a testament to the power of a shot of insect espresso.
Wright's team also removed bee brains from their bodies, deposited them into saline solution, hooked them up to electrodes and flooded them with caffeine. The researchers found that the caffeine affected neurons involved in sensory integration and long-term memory -- and its effect looked similar to that seen in mammalian brains.
Lynn Adler, an evolutionary ecologist at the University of Massachusetts Amherst who was not involved in the study, said she had never before considered that caffeine contributed to learning and behavior in insects by affecting their brain chemistry. "It's really exciting," she said.
Adler, who studies the role of such compounds in plants, said it would be premature to conclude that plants were in effect drugging the bees so they would return later. But, she added, there are countless strange compounds in nectar besides sugar, and many of them could have ulterior motive.
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