Chili peppers are for the birds, study suggests

By Bryn Nelson

Newsday

To the unwary, the unwise or the undeterred, it’s famous for its tear-inducing, sweat-producing, lunge-for-the-water fire. Now, researchers are discovering why the chili pepper produces its fiery flavor in the wild.

In a study published July 26 in the journal Nature, scientists from Montana and Arizona demonstrated that a chemical produced by the ancestor of most chili pepper plants selectively wards off unwanted diners while welcoming others — the first time this phenomenon has been demonstrated in the wild.

"There’s an evolutionary paradox as to why a plant would go to the trouble of making a fruit, which is thought to be made to facilitate consumption, and then fill that fruit with toxic chemicals that would stop consumers," said lead author Joshua Tewksbury, who completed the research while in the Department of Zoology at the University of Montana.

Tewksbury, now at the University of Florida, said a number of theories have been proposed to explain the paradox of fruit toxicity. One explanation, first proposed by University of Pennsylvania evolutionary biologist Dan Janson in the late 1960s, known as "directed deterrence," had never been directly observed in nature.

Directed deterrence, Tewksbury said, attempts to explain how the plant is "able to distinguish between the good guys and the bad guys" among these consumers.

"The idea is that if you would make a chemical that would only deter the bad guys, that would be good," Tewksbury said. The "bad guys," he explained, are predators that eat a plant’s fruit and seeds but destroy the seeds in the process and thus prevent their dispersal. Under the notion of directed deterrence, toxic chemicals in a plant’s fruit would keep them at bay.

But "good guys" that actually promote the dispersal of seeds by excreting them whole through droppings should suffer no ill effects of the toxic fruit.

Tewksbury said his study "is the first to show that indeed, a single chemical can completely change the evolutionary landscape in which the plant interacts with its consumers."

In this case, the plant is the Chiltepine chili pepper, Capsicum annuum, which produces the feisty capsaicin chemical of flame-throwing chili and eye-watering salsa fame, and the potential consumers are the birds and small rodents of the southern Arizona desert.

"The first question to ask was, ‘Who eats the hot chilies?’ and a related question, ‘Who does not eat the chilies because they are hot?’ " Tewksbury said. "We needed to isolate the heat as the reason that these animals were not eating it."

Tewksbury and co-author Northern Arizona University conservation biologist Gary Nabhan answered the first question by monitoring the chili plants. Surveillance demonstrated that only birds ate the chilies, primarily the curve-billed thrasher. The researchers then caught some of these grayish-brown birds and two of the main mammalian seed predators in the area, packrats and cactus mice, and fed them different fruits.

The packrats, mice and birds gobbled up the proffered fruit of the hackberry, the most abundant fruit-bearing tree in the region. And the captive curve-billed thrashers readily dined on the hot Capsicum annuum peppers, but neither the packrats nor the cactus mice even ventured a nibble.