A cup of coffee doesn't just provide a jolt for people in the morning. Bees may crave a buzz too. Scientists have found that some plants, like the coffee plant (Coffea), use caffeine to manipulate the memory of bees. The nectar in their flowers holds low levels of caffeine that pollinators find highly rewarding. (Read more about caffeine in National Geographic magazine)
Bitter-tasting caffeine primarily arose in plants as a toxic defense against herbivores like garden slugs. At high doses, caffeine can be toxic and repellent to pollinators.
However, at low concentrations, caffeine appears to have a secondary advantage, attracting honeybees and enhancing their long-term memory, said lead author Geraldine Wright, a neuroscientist at Newcastle University in England, whose study was published online March 7 in the journal Science.
"We show that caffeine—a compound whose ecological role is mainly to deter and poison herbivores—actually acts like a drug in an ecologically relevant context," Wright said. "The plant is secretly drugging the pollinator. It may help the bee, but the plant cares more about having a pollinator with high fidelity!"
Wright's team wanted to investigate what effect caffeine could have on the learning and memory of honeybees, so they measured the caffeine content in two types of plants, Citrus and Coffea. Both have elaborate flowers and strong scents that attract honeybees. The plants benefit from bee pollination by producing more fruits and seeds. (Read more about learning and memory in honeybees.)
To measure the "pharmacological effects" of the drug, the team trained individual honeybees using a classic conditioning procedure where the bee sticks out its tongue for odor and food rewards. (Watch of a video of a similar type of bee experiment.)
The researchers found that a memory association formed for the odor that came with the caffeine, a buzz-inducing reward.
The greatest effect was seen in the long-term memory experiment, with three times as many bees remembering the scent and sticking out their tongues for the caffeine reward 24 hours later, and twice as many recalling it 72 hours later.
Caffeine changes how neurons in the bee's brain respond to learning and memory tasks, Wright said. It causes cells to have a stronger reaction to sensory input, a change that also leads to long-term potentiation, a key mechanism underlying memory formation.
The effects of caffeine on learning and memory in people is not as clear. "But I think there is overwhelming evidence that we return again and again to consume caffeine because of the way we feel after drinking it," Wright said.