July 26, 2001 GAINESVILLE, Fla. --- It adds the fire to chili and the hot to salsa, but what does the zing do for the pepper? As it turns out, quite a lot. Working with the ancestor of most varieties of chili pepper plants, a University of Florida researcher has shown that the plant relies on its spiciness to ensure the very survival of its species.
In an article set to appear in Nature on Thursday, Josh Tewksbury, a UF postdoctoral researcher in zoology, and co-author Gary Nabhan, an ethnobotanist at Northern Arizona University, conclude that mammals, sensitive to the chemical that makes peppers taste hot, avoid the Capsicum annuum pepper. Birds, however, are unaffected by the chemical, known as capsaicin, and they happily eat the peppers. This is essential for the plant, since birds release the seeds in their droppings ready to germinate -- whereas if mammals ate the seeds, they would crunch them up or render them infertile, the researchers report.
"The upshot is that it’s very beneficial for the pepper to have mammals avoid its fruit and have birds attracted to them," Tewksbury said.
Plants that produce apparently poisonous or undesirable fruits – the edible reproductive body of a seed plant -- have long puzzled biologists. Evolutionary theory says the main reason that plants create fruits is to encourage animals to eat them, so that the animals will disperse the plant’s seeds. Why, biologists wonder, would plants go to the trouble of making a fruit, only to use chemicals to deter an animal and potential seed distributor?
Evolutionary biologist Dan Janson proposed in the late 1960s that plants may use chemicals to deter some animals without deterring others, thus selecting only preferred seed distributors. Known as “directed deterrence,” this theory received very little attention and was never observed in nature, and it gathered dust in scholarly journals until Tewksbury and Nabhan decided to see if it might hold true in chili peppers.
The researchers did their investigation in a field in Southern Arizona about 35 miles south of Tucson, using the Chiltepine chili pepper, Capsicum annuum. The plant is the progenitor of virtually all peppers native to North America, including jalapeno, poblano and bell peppers.
Using video cameras trained on the plants, they discovered that birds -- in particular, a species known as the curve-billed thrasher -- were the only animals eating the small, red peppers. Meanwhile, pack rats and cactus mice, the dominant fruit- or seed-eating mammals in the area, avoided the peppers entirely.
That was fine as far as it went, but Tewksbury and Nabhan needed to prove that the rats and mice were avoiding the capsaicin chemical in the peppers. To do so, they found a pepper similar in size, shape and nutritional content to capscium annum -- but because of a genetic quirk, the pepper, a variety of Capsicum chacoense, completely lacks capsaicin. The researchers fed this “spiceless” pepper to packrats, mice and birds in labs. All gobbled it up. When the researchers swapped the hot pepper with the spiceless pepper, the birds continued to eat the pepper, but the rodents refused to even nibble it.
Analyzing the droppings and feces of the birds and rodents, the researchers discovered that the birds passed the seeds whole and capable of germinating. The rodents, however, chewed up most of the seeds, and any that remained were too damaged to germinate.
To cap it off, Tewksbury and Nabhan discovered that the curve-billed thrashers tended to spend a lot of time on a variety of fruiting shrubs, frequently releasing their droppings there. The peppers, they discovered, grew much better in the shade of the shrubs than the hostile open desert, which comprises the majority of habitat. The chilies also get two additional advantages: Birds are more likely to eat the pepper from chili bushes growing near the shrubs, further dispersing the seed, and an insect that kills the seeds and fruit of the pepper was much less common in the shade of the shrubs.
So not only are the birds distributing undamaged pepper seeds, they are doing so in just the places the resulting plants are most likely to thrive, Tewksbury said.
"From the pepper’s perspective, it’s very beneficial to get pooped out as a seed underneath a shrub, particularly a shrub that has fleshy fruits itself, and that’s just where the thrashers deposit the seed," he said.
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