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Interviewee: Josh Tewksbury, University of Washington
Hot and delicious… chili peppers may scorch your tongue, but they’re hard to resist. But why are chilies hot in the first place? University of Washington biologist Josh Tewksbury says chili plants get hotter and hotter in response to invaders.
Tewksbury says they do it "to defend themselves from, essentially, unwanted consumption, things they don’t want to eat them."
Researchers have known for some time how chilies create the sensation of heat in the mouths of mammals that eat them, including us humans. As Tewksbury explains, the heat is actually a trick the chili pepper plays on our brain, using a chemical called “capsaicin.”
“We’re essentially being fooled by this chemical into thinking we’re actually being damaged, so our brain goes ‘Oh my gosh, I’m being hurt, I’m being hurt like I’m touching a hot stove, quickly, do something!’ and you get this fight or flight response.
That “fight or flight response” is basically a rush of endorphins flooding our bloodstream, which effectively kicks our body chemistry into high gear, preparing us for danger. It also creates the so-called “runner’s high” that athletes experience after strenuous exercise. This natural boost of energy is part of what attracts people who just can’t get enough of chili peppers, a food obsession which has inspired whole festivals devoted to this spicy plant.
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A chili aficionado himself, Tewksbury has long been fascinated by chili peppers and why they have developed this unique chemical response. That interest eventually led him to a line of research that included testing different species of animals for their response to eating chili peppers. In the course of that research, he came across a few varieties of chilies that were not naturally hot at all. Using these chilies as a control, he found that most mammals will eat the not-hot chilies with no problems, but once they taste a hot chili, they stay away from all varieties. But, strangely enough, birds seem to be unaffected by the heat. Further investigation by Tewksbury and other researchers around the world found that birds were different from mammals in one special way.
Tewksbury says, “Birds can sense capsaicinoids, but they don’t sense it as heat. And it turns out that there’s a slight difference in the receptor, what’s called the binding site, that triggers our brain, that tells us we’re in danger, we’re being hurt. And in birds, the capsaicin molecule doesn’t quite fit, it doesn’t quite open that site and cause the birds to say they’re in pain.”
This turned out to work in the chili pepper’s favor, because unlike many grazing mammals, birds do not chew up the seeds they eat. This allows chili seeds to leave the bird’s body undigested to spread and take root across vast stretches of their environment.
But Tewksbury was convinced that this relationship between birds and chilies wasn’t the ultimate reason for the evolution of the capsaicin response. He believed something else must have originally been at work.
Thinking the answer might be connected with the species of not-hot chilies he had used in his previous study, Tewksbury took his team on an expedition to Bolivia, where they studied smaller, undomesticated chili plants. There they found that hot and not-hot versions often grow just a few feet from each other. Comparing them revealed that the hotter chilies were more often under attack, and that there are several players in this evolutionary war.
Battle for the Chili Seeds
Tewksbury discovered that the different organisms that have helped shape the heated nature of chili peppers include the chili plants themselves, seed-eating insects, and a particularly nasty species of fungus.
As he explains, "If you go to these areas where all the chilies are hot, they’re being attacked by bugs. These are seed-eating bugs primarily, that poke the hole into the chili and every time they poke a hole in, a fungus gets in that hole, and it starts eating the seeds."
In response to this attack, the plants produce more and more capsaicin. Intrigued by these findings in the field, Tewksbury and his team brought back samples of the wild chilies and the fungus to their lab in Seattle. As he wrote in the study published in the Proceedings of the National Academy of Sciences, his lab confirmed that capsaicin defends against the seed-eating fungus.
"The hot plant is protected, partially, from this fungus. It slows down the growth of the fungus, and its seeds survive longer,” says Tewksbury.
By slowing down the growth of the fungus, the chilies remain appetizing to look at, and therefore gain a chance to be eaten by birds, which can then spread the undamaged seeds around. If the chili lives long enough to ensure its reproduction in this gambit, it wins.
“So it’s an arms race between the fungus trying to eat the chili seeds, and the capsaicin trying to stop the fungus”, says Tewksbury.
You might call it an arms race with some "tasty" results.
This research was published in the Proceedings of the National Academy of Sciences, August 2008, and was funded by The National Geographic Society.
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