Wednesday, January 18, 2012

Coffee, please, with extra insecticide

Everything that a living thing does, it does for a reason. Evolution doesn't tolerate uselessness for very long: an organism that spends precious resources growing a body part, or indulging in a behavior, that doesn't enhance its fitness will eventually be out-competed by others that don't. (Vestigial structures are a matter for another post.)

Considered that way, caffeine doesn't make much sense. Why on Earth would a plant make a chemical whose only apparent purpose is to give humans a nice little buzz? In the wild, that doesn't get coffee or tea plants anything, does it? Actually, it does, because caffeine does more than make us jittery. To the plant, it's something else entirely -- a valuable insecticide.

Caffeine belongs to a large class of chemicals called alkaloids, a great many of which are made by a huge variety of plants for a number of different reasons. Whether or not you realize it, you're already familiar with quite a few alkaloids and their effects on humans. There's caffeine, of course; morphine, a valuable painkiller but also one of the key components of heroin; cocaine, a dangerous street drug; capsaicin, the stuff that makes peppers taste hot; and theobromine, the active ingredient in chocolate. All five of those serve the same general purpose in the plants that make them -- they scare off herbivores. Whether by producing an unpleasant taste, by making the herbivore's mouth burn, or by being lethally toxic, a plant full of alkaloids makes a less than satisfying meal.

Caffeine in particular, while essentially harmless to humans, is lethal to many insects. They are much less capable of metabolizing the substance: the same dose of caffeine that acts in humans as a mild central-nervous-system stimulant will paralyze or kill many insects. Even when it doesn't kill the pests outright, it immobilizes them for long enough that their own predators might find an easy meal.

None of this is any reason to swear off coffee or tea. You are not an insect, and the amounts of caffeine in your favorite beverage are thoroughly harmless. If you already stick to decaf, though, you can offer your caffeinated friends a new explanation: "I just don't like the taste of insecticide in the morning!"

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