If you’ve ever taken a bite out of a habanero, you might think the purpose of its spiciness is obvious.

Eating it hurts. A lot.

So for a plant, spice is a great way to tell us to not eat that. At first, that’s what scientists thought, too: that pepper plants evolved spiciness to deter mammals like us from eating their fruits. Joke’s on them!

But careful research on wild peppers suggests that’s actually a secondary benefit at best. The real reason peppers are spicy probably has more to do with defending against much smaller dangers, like insects and mold. The main compound that makes peppers spicy is capsaicin.

It binds with special heat-sensing receptors on your nerves, which is why spicy peppers feel like they’re burning you. And while lots of people like eating spicy foods, it’s not usually because they enjoy the painful feeling that comes with biting into a raw hot pepper. For the most part, other mammals don’t like the pain, either.

Which led scientists in the 1980s to suggest that spiciness evolved for that reason—an idea known as the direct deterrent hypothesis. When this hypothesis has been put to the test, it’s largely held up. Plants generally produce fruit to encourage animals to eat them and spread the seeds around in their poop, but some animals are better seed spreaders than others.

For pepper plants, it turns out mammalian guts often reduce the viability of the seeds, while bird guts don’t. So ideally, a pepper plant wants its fruits to be eaten by birds but not by the likes of us. Birds’ heat-sensing nerves aren’t sensitive to capsaicin, so it seems like a perfect choice for chemical defense against mammals.

Some lab and field studies support the idea that spiciness evolved as a deterrent, because rodents and other mammals just don’t really seem to like eating spicy peppers. But that’s not always the case, which has led researchers to question if the direct deterrent hypothesis is right. For example, we know that spiciness doesn’t stop humans from eating hot peppers, partly because we can develop a tolerance for increasing levels of heat.

Partly because, apparently, we love pain! Or just showing that we can! We’re like, “I’ll take the extra spicy!” And then it’s the Thai restaurant and you’ve made a huge mistake...

And it turns out that other mammals can appreciate spiciness, too. In a study published in 1990, researchers had 4 rats eat spicy food for two weeks while 5 other rats didn’t. Afterwards, when given the choice to return to a non-spiced diet, 3 of the 4 rats in the first group stuck to the spiced meals.

And 1 of the 5 that didn’t have the spicy stuff went over to the spicy stuff! That’s a small study, and it’s nearly 30 years old, but it does suggest that some mammals are okay with a little heat, and maybe would even learn to eat spicy peppers if surrounded by them. But the biggest blow to the direct deterrent hypothesis comes from studies of the natural variation in pepper spiciness.

In wild populations, the spiciness of a given species varies, probably because producing capsaicin costs the plant in other ways. So if they can get away with being bland, they stop making it. When scientists have surveyed wild pepper plants in places like Bolivia, where both mild and hot peppers grow natively, they’ve found that mammals don’t tend to eat the peppers — not even the mild ones.

Plus, peppers don’t tend to be more spicy when there are more mammalian plant-eaters around to eat them. So mammals don’t seem to be a big concern, even for the non-spicy plants. Instead, spiciness is associated with local climate: spicier peppers grow in areas with more rainfall.

And scientists have found that more than 90% of wild plants have signs of fungal damage. So in the late 2000s, scientists started to think that mold, not mammals, might be the real reason for spiciness. One fungus in particular has proved devastating to wild peppers—researchers found it was the only significant cause of damage to fruits and seeds of Bolivian peppers prior to dispersal.

When insects make holes in the peppers, they create tiny scars that are associated with infection, probably because they give the fungus a foothold. But capsaicin effectively stops its growth. Where both spicy and mild varieties of the same pepper grow, spicier peppers are much less likely to be infected.

And researchers have found that the more scars you find on peppers in a given area, the spicier they tend to be — probably because they need more protection from the fungus that can grow in all those scars. Capsaicin is a decent bug repellent, too. All this suggests that spiciness really did evolve to deter insects and kill fungi — and if so, that also explains why pepper fruits get hotter as they mature.

Most fruits have gross-tasting compounds in them early on, which is thought to prevent critters from eating them before the seeds are ready to be spread. Once they’re ripe, those compounds mostly disappear, inviting seed-spreading animals to feast. Peppers exactly the opposite—they get spicier as they ripen.

And if the spice is meant to deter mold, that makes sense. There’s no point where the plant can let down its guard against mold, because the fungus can—and will—attack no matter how big or small the pepper is. And riper fruits represent more investment by the plant, so the absolute worst time for a fruit to become spoiled is right when it’s finally ready for its seeds to be dispersed.

So that increase in spiciness over time is another point in favor of the idea that it probably evolved as a defense against insects and the fungi they facilitate. Researchers think that if mammals are also discouraged from eating them, it’s probably more of a helpful side bonus. So the next time you dig into a nice plate of Szechuan or vindaloo, rest assured that that burn probably isn’t meant for you.

It’s just the lovely taste of chili pesticides. Thanks for watching this episode of SciShow! If you love eating the hottest peppers in spite of the burn, you might like our episode on how spicy food might be good for you—independent of its antimicrobial action.