Nothing kicks the brain in gear like a jolt of caffeine. For bees, that is.
And they don't need to stand in line for a triple soy latte. A new study shows that the naturally caffeine-laced nectar of some plants enhances the learning process for bees so that they are more likely to return to those flowers.
"The plant is using this as a drug to change a pollinator's behavior for its own benefit," said Geraldine Wright, a honeybee brain specialist at Newcastle University in England who, with her colleagues, reported those findings in Science on Thursday.
The research, other scientists said, not only casts a new light on the ancient evolutionary interaction between plants and pollinators but also is an intriguing confirmation of deep similarities in brain chemistry across the animal kingdom.
Plants are known to go to great lengths to attract pollinators. They produce all sorts of chemicals that affect animal behavior: sugar in nectar, memorable fragrances, even substances in fruit than can act like laxatives in the service of quick seed dispersal.
Lars Chittka, who studies bee behavior at Queen Mary, University of London, and wrote a commentary on the research in the same issue of Science, said that in the marketplace of plants seeking pollinators, the plants "want their customers to remain faithful," thus the sugary nectar and distinctive scents.
"The trick here," said Chittka, who was not involved in the research, "is actually to influence the memorability of the signal using a psychoactive drug. And that's a new trick in the book for plants."
Robert Raguso, who studies the interactions of plants and pollinators at Cornell University and was not part of the study, called the paper impressive, adding in an email, "It makes the reader think twice about where natural products that have economic importance to humans actually came from before we 'discovered' and co-opted their biology."
Wright did not set out to investigate the evolutionary stratagems of plants. Rather, her goal was to use the honeybee as a model to study drugs that are commonly abused.
About eight or nine years ago, she said, "I ran across this paper on caffeine in floral nectar." And then, she said, she thought, "This could be quite interesting because there might be some ecological interaction between the plants and the pollinator. That's how it started."
Several varieties of coffee and citrus plants have toxic concentrations of caffeine in leaves and other tissues but low concentrations, similar to that in weak coffee, in the nectar itself. The toxic concentrations help plants to fend off predators.
But Raguso pointed to a well-known axiom that "The dose makes the poison," a principle that Wright and her colleagues followed in lab experiments.
She conducted learning experiments in the lab with bees to see whether they associated a reward with an odor, the reward being either sugar water or a combination of sugar water and caffeine in the same concentrations found in the nectar of coffee and citrus plants.
The effect of caffeine was not obvious at first, but as Wright refined her experiments, it became more clear that the chemical had a profound effect on memory.
"If you put a low dose of caffeine in the reward when you teach them this task, and the amount is similar to what we drink when we have weak coffee, they just don't forget that the odor is associated with the reward," she said.
After 24 hours, three times as many bees remembered the connection between odor and reward if the reward contained caffeine. After 72 hours, twice as many remembered.
They then tested the effect of caffeine on neurons in the bee brain and found that its action could lead to more sensitivity in neurons called Kenyon cells, which are involved in learning and memory. Wright said this was one plausible route for enhancing memory but not definitive.
Insect and human brains are vastly different, and although caffeine has many effects in people, such as increasing alertness, whether it improves memory is unclear.