Signs of Fall 11: Caffeine

Photo by J. Scortzman, Flickr

I know that “Caffeine” is a funny title for an ecology blog, but I will, eventually, get to my ecological point about it. Caffeine is, of course, the active chemical (OK, let’s be honest here, the “drug”) that makes tea and coffee such an essential part of most of our mornings (and for some of us an essential part of our afternoons and evenings, too!). Naturally caffeinated beverages like coffee and tea are so compellingly attractive (if not outright addictive!) that the manufacturers of synthetic beverages (companies that make sodas and energy drinks) have also included sometimes staggeringly high levels of caffeine in their concoctions. A great data base that lists some of these natural and synthetic beverages and their caffeine levels is out on the website http://www.caffeineinformer.com/  Everyone should be aware of the sources of caffeine that they are ingesting every day.

There are also many other sources of caffeine. Decaffeinated coffee and tea still contain and deliver surprisingly high amounts of caffeine. Non-cola sodas are frequently large sources of caffeine. Chocolate (or anything with chocolate in it) is also a caffeine source, as are many over the counter pain relieving medications (Excedrin Extra Strength, for example, contains 130 mg of caffeine in a two tablet dose. That’s more caffeine than you get from a Starbuck’s Double Shot!).

The Mayo Clinic describes the effects of caffeine on the human body from the positive impact of increased alertness to the negative impacts of headaches, insomnia, elevated anxiety levels, increased production of stomach acids, elevated blood pressure, accelerated osteoporosis and fibrocystic disease, muscle twitches, and, in pregnant women, slowed fetal growth and the increased chance of miscarriage.

Caffeine, then, is a serious drug to which MANY of us are addicted!

So what is caffeine and why do plants make it?

Caffeine, drawing by NEUROtiker, Public Domain

Caffeine is fairly simple molecule (a picture of its molecular model is to the left) that plants can make with a minimum of metabolic energy expense. It also can readily be synthesized synthetically in great quantities. In plants, caffeine is classified as a “secondary chemical.” Secondary chemicals do not participate in the metabolism of a plant. They have no direct physiological functions, but they are not the accidental products of the breakdown of some larger molecule. They are specifically synthesized from smaller precursors often, although this is not the case of caffeine, with a lavish expenditure of the plant’s metabolic energy.
Secondary chemicals are used by the plant to secure its place in its ecosystem. Many secondary chemicals act as insect repellants and insecticides. They can also act as alleopathic chemicals that prevent competing plants from growing too near the secondary chemical producer. In past blogs I have talked about black walnut trees and eucalyptus trees and the batteries of secondary chemicals that they produce that can influence the soils in which they grow for many years! Secondary chemicals may also kill or at least discourage parasites, and they may prevent herbivory by large or small plant consumers. A few weeks ago I talked about the secondary chemicals made by ferns and their historical impacts on dinosaurs and their present day impacts on fern grazing horses.

Coffee tree, Photo by T. Brooks, Flickr

Lots of plants make small amounts of caffeine and a few (like coffee and cocoa trees) make a lot of it. So it must play one of these expected secondary chemical roles in the ecology of the particular plant, right?

Maybe not, and this is why I started writing this blog in the first place.

In the October 15, 2015 issue of Current Biology, Margaret Couvillon and nine of her students and project volunteers at the University of Sussex (United Kingdom) found that honey bees foraging from sucrose nectar feeders to which a small amount of caffeine had been added returned to that feeder more often than bees foraging from an identical, but non-caffeinated, sucrose feeder, and that the bees continued to return to the caffeinated feeder much more frequently and for a much longer period of time (four to five days longer!) after the feeder was empty! The caffeine ingesting bees also recruited a larger number of their hive mates (via very vigorous waggle dancing) to fly out to and forage from the caffeinated nectar feeder than did the bees feeding on the un-caffeinated nectar.

Photo by I.Tsukuba, Flickr

In an interview with the New York Times, Couvillon stated that “caffeine caused the bees to overestimate the quality of the resource.” The bees feeding on the caffeinated nectar interpreted the food source as much larger and much richer than it actually was (hence their persistence in returning to the feeder and the “all hands on deck” approach to harvesting the food source. This observation may explain why so many plants have low levels of caffeine in their tissues: it is part of an evolutionary contest between the plants (which need the bees to pollinate them) and the bees (which need the plants to make the flower nectar on which they feed). The sucrose in the nectar is energetically expensive to make. If the plants put more and more of their metabolic energy into the manufacturing of nectar sucrose (which they must do to attract sufficient numbers of bees to accomplish their pollination), then they will have less energy for their own growth and homeostatic needs. But, if a plant can spike its sucrose-rich nectar with caffeine (a chemical that it can make for less metabolic energy than the amount of sucrose needed to pull in the bees), then it can attract large and very persistent swarms of bees for a much lower metabolic cost! Any plant that can add caffeine to its flower nectar, then, has an energetic advantage over any other plant that must rely on large volumes of sucrose-rich nectar to attract its pollinators.

A great deal for the plants! The bees, though, get shorted a bit on the sucrose they need to sustain their activities and their hives. Less food from the flower nectar but, and sorry about this pun, they do get a great buzz!