We humans love us some caffeine. The mild stimulant have saved many a student, parent, and hard working adult from nodding over their desks. And it's a natural product of plants like the coffee plant and the tea bush. But the question is, why do these plants have it in the first place?
It turns out that there are two answers to that question. First, caffeine is a natural pesticide, which can paralyze and kill insects that want to chomp on the leaves, berries, or other parts of the plant. It's good for keeping a bug off your back.
But these plants also produce flowers, and these flowers need bees. So it's somewhat surprising to realize that the coffee plant, as well as plants from the Citrus genus (yup, that means oranges), have caffeine in their nectar. After all, if caffeine is a poison to some bugs, you don't want to be poisoning your pollinators!
But it turns out that bees aren't like other bugs, and may enjoy themselves a jolt like humans do! Whether they enjoy it or not, they certainly remember it!
Wright et al. "Caffeine in Floral Nectar Enhances a Pollinator’s Memory of Reward" Science, 2013.
(Oh, yeah, need a jolt of the good stuff. Source)
The authors started out by examining exactly HOW much caffeine was in the nectar of various coffee and citrus plants. And the concentrations of caffeine in the nectar could get up to that of one cup of coffee (though, obviously, in a much smaller volume total). I'm starting to wonder if there's a "honeyed nectar" energy drink in the future.
But the question is, what does it do to the bees? To look at this, the authors of the paper took some bees and trained them to pair the scent of flowers (in this case, they used 1-hexanol, which is more like the scent of grass, but whatever). They then added either nothing, or one of 7 different doses of caffeine to the mix.
While the caffeine only really had a weak effect on the bees' ability to learn the task, it had a much more interesting effect on the bees' longer term response (up to 72 hours), you can see that the bees received caffeine remembered the association for LONGER than they did with just sucrose.
But how is it working, how are the bees' memories being affected by the caffeine? While bees certainly do have brains, they aren't really similar to ours in terms of organzation. So while we have a hippocampus that has a great deal to do with memory, bees don't. Instead they have something called Kenyon cells, which help organize signals during learning in a similar way to the hippocampus. And these Kenyon cells also have adenosine receptors, the receptors that caffeine acts on to produce its effects.
The scientists hypothesized that caffeine might be acting at the Kenyon cells to produce the effects they saw on bee memory. When they applied caffeine and recorded from the Kenyon cells, they saw that caffeine acted at adenosine receptors to cause a small depolarization of the Kenyon cells, making them more likely to fire.
But of course, caffeine IS toxic at high doses, in humans as well as bees. The caffeine may help the bees at low doses, but at high doses it can kill them. But the authors found that the bees have areas on their mouthparts that can sense caffeine, and easily avoid high caffeine concentrations...but not low ones.
So caffeine can help bees remember, and return to, flowers which contain caffeine. A pretty trick for the flower! But what I would have liked to see here as well is something called place preference, which can be done in bees. Place preference associates a specific place with something that may or may not be rewarding. In mice, for example, you use a two-chamber apparatus where you pair one side with a drug (say, caffeine!) and the other side with saline. After a few pairings, you put the mouse in between the two chambers, and see which chamber he spends more time in. If the mouse likes the drug, he'll spend more time in that side of the chamber. This is place preference, and is usually associated with the rewarding properties of a drug.
I think it would be possible to do this with bees, they have excellent spatial navigational abilities, and obviously they also have the long term memory to go with it. We know that they remember the caffeinated flowers...but do they PREFER them? The answer would be a nice definitive one, caffeine keeps bees coming!
But the current findings are also plenty interesting on their own. This means that caffeine is not just a plant pesticide, it also could play a role in keeping the bees bumbling back! If the bee remembers the location of the flowers better, it's more likely to return. This means the plants may get something good out of giving bees a little buzz.
Wright, G., Baker, D., Palmer, M., Stabler, D., Mustard, J., Power, E., Borland, A., & Stevenson, P. (2013). Caffeine in Floral Nectar Enhances a Pollinator's Memory of Reward Science, 339 (6124), 1202-1204 DOI: 10.1126/science.1228806