To compensate for their own inability to pick up and move elsewhere, many plants produce fleshy, seed-bearing fruits to lure animals that then unwittingly do the legwork for them. Yet a number of fruits contain chemicals that vertebrate species find unpalatable. New research, published today in the journal Nature, shows that in the case of the chiltepine chili, this apparent paradox is actually an ideal adaptation.

Laboratory studies have demonstrated that capsaicin, the chemical that lends chilies their spicy heat, is toxic to mammals but not to birds. Perhaps not surprisingly then, when study authors Joshua J. Tewksbury of the University of Montana and Gary P. Nabhan of Northern Arizona University observed a native population of chiltepine chilies in southern Arizona, they found that only birds (mostly curve-billed thrashers) eat the wild chilies; the local small mammals (cactus mice and packrats) avoid them.

This selective discouragement, it turns out, is in the plant's best interest. Experiments conducted using a non-peppery cousin of the chiltepine chili (the mammals refuse to eat the hot chilies) revealed that chili seeds ingested by cactus mice and packrats failed to germinate. Consumption by the thrashers, on the other hand, resulted in germination rates comparable with those of seeds planted directly from the fruit. (Hot chilies fed to the thrashers gave similarly positive results.)

The thrashers make better dispersers for several reasons, Tewksbury and Nabhan conclude. For one thing, unlike the mammals, they pass whole seeds. But they also often deposit the seeds in prime spots: under the protective shade of other fruiting shrubs and trees. As a result, the authors write, "seedlings can be more easily established, predispersal fruit predation is lower, and seed dispersal is greater." The chili's fiery strategy, it seems, yields sweet rewards