Why are chili peppers hot?

A fire alarm with no fire

Bite into a chili and your mouth seems to be on fire, but the chili isn't warm at all. The burn comes from capsaicin, the main hot ingredient in chili peppers, which switches on the nerve cells that report pain¹. In 1997, David Julius's lab at the University of California, San Francisco found the molecule it targets: an ion channel on those nerves, now called TRPV1^{1, 3}.

Here's the twist: real heat switches on the same channel once it reaches the painful range¹. Julius's team linked it to 'moderate' heat of about 43 °C². The Nobel committee puts its threshold above 40 °C, close to the point where heat starts to hurt⁴. So when capsaicin opens TRPV1, your pain nerves fire much as they would for something painfully hot. Julius shared the 2021 Nobel Prize in Physiology or Medicine for this discovery³.

The heat isn't spread evenly through the pod. In most chilies, it's made and stored in the placenta, the pale core and ribs that hold the seeds. The seeds themselves make none, though they can pick some up from the tissue around them^{11, 12}.

Why would a plant want to hurt you?

A fruit is a deal: animals get food, and in return they carry the plant's seeds away⁸. So why make the fruit painful? In 2001, researchers Joshua Tewksbury and Gary Nabhan reported that capsaicin discourages animals that destroy seeds without putting off the animals that spread them best⁵. This idea is called 'directed deterrence'^{5, 7}.

Those better spreaders are birds, and birds don't care about capsaicin's burn⁶. In 2002, Jordt and Julius found a likely reason: the chicken version of TRPV1 ignores capsaicin but still responds to heat⁶. In lab tests, swapping in one region of the rat receptor made the chicken receptor sensitive to capsaicin⁶.

Fieldwork points the same way. Researchers followed more than 400 marked fruits of two wild chili species in Bolivia and Arizona. Almost every fruit vanished during the day, when rodents are inactive, and video cameras caught only birds taking them. Most of the fruit was taken by bird species known to pass chili seeds in a condition that can still grow⁷. Earlier feeding trials had pointed the same way: rodents chewed up most of the chili seeds they ate, while birds passed them whole and able to sprout^{7, 13}.

A second enemy: fungus

Mammals may not be the whole story. Some plants of a wild Bolivian chili, Capsicum chacoense, make no capsaicin at all. In this species, the biggest killer of seeds still on the plant was a Fusarium fungus. It got into the fruit through holes made by bugs that pierce the fruit's skin to feed⁸. Seeds from hot fruits carried 45–55% less fungus than seeds from mild neighbours, and in lab dishes capsaicin and its close relative slowed the fungus's growth⁸.

Where the bugs did more damage, more of the plants were hot⁸. Across a study area about 300 km long, almost every plant was hot at one end. At the other end, more than 70% made no capsaicin⁸.

Heat has a price

If heat protects the plant, why isn't every chili hot? With plenty of water, hot and mild plants made the same number of seeds. Under drought, the hot plants made 50% fewer⁹. In Capsicum chacoense, seeds from hot plants also have slightly thinner seed coats, and in tests these seeds were damaged when they stayed a long time in a bird's gut^{8, 10}. In the wild that cost seems small, because the main bird disperser there passes most seeds quickly¹⁰. Being hot seems to pay off only where the threats are big enough⁹.