SPOKANE, WASHINGTON--From red-hot chicken wings to kimchi, many spicy foods get their sinus-clearing zing from hot peppers. For some ecologists, chili peppers have posed a burning question: Why are they hot? The fruit owes its spiciness to a chemical called capsaicin, and plants generally put energy into producing such toxins for a reason, such as to deter enemies. At the annual meeting of the Ecological Society of America held here earlier this week, a researcher offered intriguing evidence suggesting that chilies wield their sting with the precision of a stiletto: to target seed-chewing mammals while leaving birds unscathed. Birds swallow the seeds but don't digest them, essentially acting as vessels for carrying chilies to new turf.
The story begins in South America, where chilies and their relatives originated. With the exception of stoic humans, mammals don't care for capsaicin, which stimulates neurons that sense pain. That's why the chemical is a key ingredient in pepper spray and a growing number of other concoctions for warding off belligerent backwood bears and backyard rodent pests. Birds, by contrast, seem to be impervious to capsaicin, apparently because they lack the right shape of a receptor--an ion channel--on their mucous membranes. Capsaicin binds to the ion channel and causes it to open, allowing ions to flow in and trigger nerve impulses that the brain interprets as pain.
Gary Nabhan, a plant ecologist at the Arizona-Sonora Desert Museum in Tucson, and grad student Joshua Tewksbury of the University of Montana, Missoula, studied over 150 hours of videotape collected last summer of around 25 chili bushes in southern Arizona--the northern edge of the plant's natural range--where the U.S. Forest Service last spring established a protected chili preserve. They found the pea-sized fruits were eaten only by birds, mainly a gray-brown bird called the curve-billed thrasher. The chilies were ignored by desert packrats and cactus mice foraging in adjacent bushes for hackberries, another red fruit that isn't spicy.
Put into cages, five packrats and five mice turned up their noses at chilies, although they readily ate an altered version that wasn't spicy--but after showing up in the feces, the seeds failed to germinate. The same kind of nonpungent seeds fed to birds germinated just as well after being expelled as those planted by hand. And spicy seeds eaten by birds germinated three times more often, for reasons that aren't yet clear, Tewksbury says. Capsaicin also acts as a laxative in birds, which helps them spread the seeds, he says.
The study may be the first well-documented instance in which a plant uses a chemical selectively, to repel animals that pose a threat without affecting others that boost its chances to reproduce. "That seems to be the first very good case where you can poison your enemies but not your [friends]," says ecologist Judith Bronstein of the University of Arizona, Tucson. "It's a great story."