Spiciness is a chili pepper's best defense against seed-attacking microbes. But not all chilies are hot. Because producing that heat comes at a price. So says a study in the journal Proceedings of the Royal Society B. [David C. Haak et al., "Why are not all chilies hot? A trade-off limits pungency"]

Researchers looked at wild chili populations along a 185-mile stretch of southern Bolivia, as the climate goes from wet to dry. At each site, they taste-tested chilies for heat. Later they analyzed samples for concentrations of capsaicinoids—the spicy stuff. Turns out only a fifth of the chili plants in dry regions packed heat. But in wet areas, all the chilies were picante. Because that's where fungi attack the peppers, and the chilies need chemical weapons to protect their seeds.

But there is a cost for that defense. The spicy chili plants in this study use water far less efficiently—and during periods of drought, they produce only half as many seeds as their mild cousins.

As for home gardeners hoping to grow spicier chilies? Don't figure on simply watering your garden more. The authors say that to select for spicy genes you’ll have to tend the pepper patch for some 20,000 generations.

—Christopher Intagliata

[The above text is a transcript of this podcast.]