For ecology graduate student Joshua Tewksbury of the University of Montana in Missoula, the hot pain of biting into a chili pepper is one of life's great pleasures. He's also come to think of it as a lesson in evolutionary manipulation.
Chili plants pump their fruits full of capsaicin, a chemical that stimulates pain-sensing neurons in the mouth. After field studies in southern Arizona, Tewksbury may have discovered why the plants go to such great lengths. Mammals such as cactus mice and desert pack rats find capsaicin unpalatable-a good thing for the plant, because the animals' digestive systems would destroy the seeds within the chilies. Birds can't taste the chemical, however, so they freely eat the chilies. Chili seeds eaten and then expelled by birds are three times more likely to germinate than those that fall off the plant naturally. In lab tests, Tewksbury found that rodents greedily eat specially bred, capsaicin-free peppers. "Chilies clearly benefit from knocking mammals out of the picture," he says.