As tiny as they are, bees are an incredible source of information for scientists, contributing to our knowledge in areas from agriculture to navigation. Just last week, we reported that bees use the electric fields of flowers to identify profitable plants, an ability that helps them forage more efficiently. A study in today’s issue of Science identifies another surprising cue that can play a significant role in bees' foraging behavior: caffeine.

Several plants, including species in the genuses Coffea and Citrus, produce nectar that contains caffeine; the concentration of caffeine in some of these flowers rivals that in a cup of instant coffee. We know what effects caffeine has on humans, but how does caffeinated nectar affect bees pollinating these plants?

To answer this question, the researchers taught bees to associate the odor of a particular chemical, called 1-hexanol, with a sugary reward. For some of the bees, the scientists added a small dose of caffeine to the sucrose reward solution in order to determine whether the consumption of caffeine affected how well the bees learned or remembered the association.

There was a small effect of caffeine on the rate of learning: bees that were given caffeine-laced nectar learned to associate the scent with the reward a bit faster than the bees that weren't given caffeine. Where the researchers saw the biggest difference, however, was in the bees’ long-term memory. When given caffeine, bees were three times more likely to recall the association 24 hours after being trained, and twice as likely to remember it 72 hours later.

The researchers were able to rule out the possibility that the caffeine made the bees more responsive to scent. Instead, the caffeine appeared to affect the bees’ cognition in some way.

Probing a bit deeper, the researchers then turned to the question of how, exactly, caffeine enhanced the insects’ memories. Although bee and human brains are quite different in terms of structure, they function in many similar ways. A group of neurons in the bee brain called “Kenyon cells” are somewhat homologous to our hippocampal neurons, which play a role in associative learning. The researchers recorded activity from bees' Kenyon cells, and found that caffeine increases these neurons’ excitability, making them more likely to fire in response to sensory stimuli. The more responsive these neurons are, the faster the bees’ brains can make the connection between the scent stimuli and the sweet reward.

But there’s a limit to the advantageous effects of caffeine; too large a dose can be toxic to insects. As a precaution, bees have developed the ability to detect the drug—and avoid it—in high concentrations. So caffeinated plants must toe the line, dosing their nectar with enough caffeine to help bees remember them, but not enough to drive the bees off. Plants that can successfully manipulate the bees’ behavior are rewarded with faithful pollinators that are “buzzed” on just enough caffeine to keep them coming back.

Science, 2013. DOI: 10.1126/science.1228806 (About DOIs).