Have you ever wondered why (some) plants make caffeine? They certainly have not anticipated our desire for alluring chocolate or stimulating coffee. Caffeine is part of the plants defence mechanisms.
Plants can’t run away, so to ward off herbivores they have evolved a range of chemical defences to discourage animals from eating them. This elaborate chemical warfare system is part of the reason that plants such as the common cress have more genes than we humans do.
Ironically, most of humans recreational drugs of choice are these chemical warfare compounds. Nicotine in tobacco, opioids in opium poppies, tetrahydrocannabinol in marijuana are all anti-herbivore defence molecules, either stupefying or killing the animal that tries to munch the plant.
So it is with caffeine, a bitter alkaloid that deters the biter.
But, and you know I’m going to say this yet again “it’s the dose that makes the poison”. All of these compounds at lower doses have effects that humans like (even if they are not actually beneficial). In the case of bees, low doses of caffeine may improve their memory.
Some plants, coffee plants and citrus plants, secrete caffeine in the nectar of their flowers. At a lower level than found in, for example, the tissue around coffee seeds (roughly 1/500th or even less), some some of the concentrations approach that you would find in a cup of instant coffee. If the bitter caffeine is driving off herbivores, what are low levels doing in nectar, which plants use to attract pollinators? Isn’t this counter productive?
A recent paper sheds light on this conundrum.
In humans consumption of low levels of caffeine improves our ability to memorise things (at least short term memory). Could this be happening in bees? The researchers set out to test this idea by training bees to associate a particular scent with a sugary reward. They mixed different concentrations of caffeine in some of the sugar solutions, while others had no caffeine.
Low dose caffeine had a small effect on how fast the bees learnt, but it had a dramatic effect on the bees long term memory. While most control bees forgot the association of sent with sugar after 24 hours, when bees were on caffeine they remembered the association 24 hours later, and there was some memory left after 72 hours.
They did a number of experiments to show that this was not due to having a better sense of smell due to caffeine, and they could show that caffeine affected the section of the bee brain that is involved in memory.
Importantly, the levels of caffeine that increased memory retention in bees is in the range found in the nectar of coffee and citrus flowers, while the level of caffeine seen in the seed tissues of coffee plants is repellent, or even lethal, to them.
So plants can use caffeine to both repel predators and attract pollinators. A rather dramatic demonstration of the dose making the poison (and that evolution is smarter than we are).
If you are out hiding Easter eggs for the kids Easter egg hunt, maybe you should have a good cup of espresso first, so you can remember where you hid them the next morning.