Some like it hot--especially chili pepper plants

By Gary Raham
Nature Writer and Illustrator

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Ancient Bolivians have been adding chilies to their food for at least 8,000 years, as evidenced by traces of these spicy fruits on their milling stones and in their crockery.

A chili's kick to the palate (not to mention a kick to the adrenal glands and a spike in heart rate) may be enough to explain their popularity, although chilies also help keep food from spoiling by killing certain microbes. The active ingredient in chilies--capsaicin--also is used as a topical salve to reduce the pain of arthritis (by temporarily overloading pain receptors) and may have been used by the Mayans to treat infected wounds, gastrointestinal problems and earaches.

But biologists have long wondered what plants get out of manufacturing capsaicin--especially when certain varieties of the same species don't make the effort. Domesticated bell peppers, for example, with virtually no capsaicin, were bred from varieties of quite spicy cayenne peppers.

Joshua Tewksbury, a scientist working at the University of Washington, appears well on his way to answering this question, although it has taken some strenuous detective work in the back country of Bolivia--most likely the original chili homeland. Like lots of elegant biological studies, Tewksbury's work demonstrates that nature usually balances costs and benefits in complex ways that reflect the intricate tangle of relationships within living communities.

A chili plant's main job, of course, is to make more chili plants. Sometimes plants make noxious chemicals to ward off pesky animals intent on eating them, but usually these chemicals appear in leaves, stems and roots as well as the seed-bearing fruits. Chili peppers make capsaicin in just their fruits and the amount goes up as the fruit ripens.

Tewksbury and a colleague, Gary Nabhan, suspected that capsaicin protects chilies from rodents. To test this idea he found mild and spicy varieties of the same chili and offered them to laboratory pack rats and cactus mice. Sure enough, the rodents ate the mild chilies and avoided the hot ones.

But chilies also need the help of animals to spread their seeds and soften them up for germination. Rodents crush seeds and destroy them, but another group of animals, the birds, serve as great seed dispersers. Birds don't mind capsaicin in their food--which is why some birders spike their birdseed with chili powder to discourage squirrels. The capsaicin does slow bird digestion, however, and this may help soften up the seed coats for later germination.

Tewksbury might have been content with capsaicin's role in discouraging mammals and facilitating bird digestion, but he suspected something else was going on--especially since the main reason for failure of chili seeds to germinate in the wilds of Bolivia was fungal rot. Pink mold in the genus Fusarium specializes in attacking chili peppers.

Again, using both mild and spicy varieties of the same peppers, Tewksbury and another colleague, Noelle Machnicki, found that the amount of fungal infection was directly related to the amount of capsaicin present. The spicy compound warded off fungal attack.

So why does a species have mild varieties at all? The fungus becomes more of a problem in humid environments. Spicier plants grow in humid areas; milder varieties in drier areas can forego the metabolic expense of manufacturing capsaicin.

More fieldwork has shown that this may not be the complete story either. Several species of insects spend their time on chili fruits. The insects dig pits in the fruits that seem to serve as conduits for fungal infection. Do the insects carry the fungus from plant to plant or do their excavations in the fruit just make it easier for the fungus to grow toward the seeds? Does capsaicin have any effect on the insects? These are questions Tewksbury is eager to address.

In the April 2009 issue of Smithsonian magazine Tewksbury is quoted as saying, "I hope to be working on this system for another 10 to 20 years. I can't see myself running out of questions in less than that."

Tewksbury is a good example of the fact that for scientists the questions are the spicy part of an intellectual challenge. Answers, while they can be satisfying, are about as bland as a crisp bell pepper.