A Curiosity: Hot Peppers – Why Are They Hot?

Once in awhile I find something that piques my interest that is different from the usual WUWT fare. This was one of those. I like this fellows blog because 1) I like hot peppers 2) I enjoyed his writing style 3) Given all the hot talk on climate lately, this seemed like a good topic to cool everybody off with  – Anthony

Hot Peppers – Why Are They Hot?

From a Blog around the clock by Coturnix

Some plants do not want to get eaten. They may grow in places difficult to approach, they may look unappetizing, or they may evolve vile smells. Some have a fuzzy, hairy or sticky surface, others evolve thorns. Animals need to eat those plants to survive and plants need not be eaten by animals to survive, so a co-evolutionary arms-race leads to ever more bizzare adaptations by plants to deter the animals and ever more ingenious adaptations by animals to get around the deterrents.One of the most efficient ways for a plant to deter a herbivore is to divert one of its existing biochemical pathways to synthetise a novel chemical – something that will give the plant bad taste, induce vomiting or even pain or may be toxic enough to kill the animal.

But there are other kinds of co-evolution between plants and herbivores. Some plants need to have a part eaten – usually the seed – so they can propagate themselves. So, they evolved fruits. The seeds are enveloped in meaty, juicy, tasty packages of pure energy. Those fruits often evolve a sweet smell that can be detected from a distance. And the fruits are often advertised with bright colors – red, orange, yellow, green or purple: “Here I am! Here I am! Please eat me!”

So, the hot peppers are a real evolutionary conundrum. On one hand, they are boldly colored and sweet-smelling fruits – obvious sign of advertising to herbivores. On the other hand, once bitten into, they are far too hot and spicy to be a pleasant experience to the animal. So, what gives?

Back in 1960s, Dan Johnson had an interesting proposal he dubbed “directed deterrence” which suggested that some plants may make choices as to exactly which herbivores to attract and which to deter. Hot peppers are prime candidates for such a phenomenon. What is hot in peppers is capsaicin, a chemical that elicits a sensation of pain when it bind the vanilloid receptors in the nerve endings (usually inside the mouth) of the trigeminal nerve. As it happens, all mammals have capsaicin receptors, but it was found, relatively recently, that birds do not.

To test that hypothesis, Josh Tewksbury used two variants of hot peppers – one very hot (Capsicum annuum) and the other with a mutation that made it not hot at all (Capsicum chacoense) – and offered both as meals to rodents (packrats and cactus mice) and to birds (curve-billed thrashers).

All species ate the sweet kind about equally. When Josh offered them identically prepared meals made out of the hot stuff, the two rodents refused to eat it while the birds happily munched on it.


The study appeared in 2001 in Nature (pdf) and I saw Josh give a talk about it at that time as he was joining our department to postdoc with Dr.Nick Haddad. While my lab-buddy Chris and I gave him a lot of grief in the Q&A session on his lenient criteria of what constitutes a “hungry animal” (he needed them to be hungry for the feeding tests), still the main conclusions of the study are OK.

More importantly, it really happens in nature. Mammals avoid hot peppers out in Arizona where Josh studied them (and made videos of their behavior), but the birds gorged on peppers. When he analyzed the droppings of rodents and birds fed peppers, he saw that seeds that passed through avian intestinal tracts were fully fertile, while seeds eaten by mammals were chewed, crushed, broken or semi-digested and not fertile at all.

Additionally, the thrashers tend to spend a lot of time on fruiting shrubs of different kinds. While there, they poop. The hot pepper seeds in the droppings germinate right there and this is an ideal shady spot for them to grow.

What a great example of (co)evolutionary adaptations. Next time on this blog, the second Big Question: Why do we like to eat hot peppers?

UPDATE: I’ve added this chart of Pepper “hotness” below, to help you gauge what to eat and what not to eat (or spray). – Anthony


Source: Calbob.com and Wikipedia’s Scoville Scale


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Ironic, isn’t it, that the pepper’s “defence” against being eaten by man has turned into an advantage. We now cultivate these plants in far greater numbers than they would appear in the wild.

Ray Reynolds

Why do we like to eat hot peppers?
Years ago a pack of us macho 16,17 y/o boys were sitting around a table when one announced he could swallow a spoon full of tobasco sauce….he bellowed roughly the same question.


That’s just silly! Everyone agrees that it’s the CO2 in the peppers that makes them hot.

Les Francis

I don’t know about Arizona rodents not eating hot chillies however,
My wife is from North Sumatera Indonesia. The western, northern parts of Sumatera plus Aceh are renown for having very hot (chillli hot) food.
My wife tried to grow some very hot chillies in our garden in Melbourne Australia. The local possums not only ate the extremely hot chilli fruit off the bush but also consumed the leaves, stems and the bush structure down to ground level.

How funny you should post this. I made a fish stew for dinner that required one finely diced, red hot pepper. I wasn’t sure whether the pepper would be too hot, so I made my son taste it. He barely touched it to his mouth, then ran around the kitchen yelling, followed by massive water drinking (which doesn’t wash away the capsaicin, by the way). Nonetheless, diluted in the stew, the pepper added just the right touch.

Robert Austin

Les Francis (16:46:20) :
Perhaps with Australia being biologically isolated for so long, the marsupials do not have capsaicin receptors.

Tim McHenry

The pepper in the pic with this article looks just like what a friend of mine grows. He calls it a “Medusa” and it is a tasty little number. As a poster above noted, not all mammals are deterred by the capsaicin (certainly man is not, well, THIS man is not). The reasons he gives for the existence of hot peppers is, of course, pure speculation, but it makes me want to go home and eat some right now. I think I will!


I am very interested in hearing why we like to eat them, since I ask myself that question every time I eat them.


On the other hand, once bitten into, they are far too hot and spicy to be a pleasant experience to the animal.
which suggested that some plants may make choices as to exactly which herbivores to attract and which to deter.

Birds and rodents be damned. Target a species that will transport you across the globe and plant your seedlings in every suitable habitat. A species that will kill your competitors and predators whilst feeding you all you can eat.
Chilli – the only evidence of intelligent design.


Have these guys never had a pet parrot?


Peppers are one of the few things that I can manage to grow without killing outright. I have never seen any evidence that the birds take a fancy to them, even though I have to pick (the paltry few) tomatoes early ’cause the birds get them as soon as they turn fully red.
We have a good crop (enough to make pizza spice for a few dozen units) of cayennes which are redder than tomatoes, before I pick ’em, and don’t get eaten. One amazing crop is Aji Amarillo Picante (yellow hot peppers) from Peru. These things grow as big as carrots and have a strong, but quite different flavor compared to local peppers. The plants take two years to produce and haven’t had even one bird strike. My wife wants to plant the whole yard in them. As mower-in-chief, I say “Go for it!”

The curious thing about hot peppers (with respect to Texas and Mexico certainly) is that the chillies get hotter the closer you approach the border.
From either directions.
(Clearly, the marsupials of Aus and Oz haven’t gotten across the Sumatran border yet ….)

So, if I can make an OT analogy to tie climate change to hot peppers.
The religion of Global Warming is like the hot Capsicum annuum. That is, it contains a high heat index (analogous to Scoville heat units [SHU] indicating a large amount capsaicin annum present) but in this case, it is hot air and greenhouse gasses.
Those who imbibe on Global Warming are similar to curve-billed thrashers such as Hansen, the IPCC, politicians and the mainstream news media…who can eat the Al Gore climate change theory with no problems, depite the resulting poisonous poop output such as taxes and carbon offset that causes great harm to the economy and lifestyle of other human creatures.
However, we, the AGW skeptics are like the packrats and cactus mice. Who may not be as showy as the curve-billed thrashers…who get all the attention with their headlines and sensational appearances…but we are wise not to eat the poison fruit.

Pamela Gray

If you grow hot peppers, do NOT pick them bare handed and then pick your nose, dig sleepies out of your eyes, or scratch your privates. If you do, you will spend a very miserable 24 hours wishing you were dead. And it’s worse for women. Don’t ask me how I know this.

Frank Perdicaro

The Joseph Cerniglia Woodchuck Draft Cider is a really good
liquid to cut the hot pepper sensation. Give it a try if you are
into food. The right combination of carbon dioxide, alcohol,
tannic acid and water does the trick.
A few years back I read a piece on the chemical composition
of hot stuff. One tidbit was that the hot pepper is just one
misplaced nitrogen molecule away from ginger. Probably the
same evolutionary biology at work.

Les Francis

Robert Austin (17:02:47) :
Les Francis (16:46:20) :
Perhaps with Australia being biologically isolated for so long, the marsupials do not have capsicum receptors.

Not only no capsicum receptors, carnation , lily, camellia and many other flower variety as well (Although the draw the line at azaleas.) Chives, mints and many other herbs are fair game for them also.
My wife is having none of this one or two chillies in her dishes either. A handful is a minimum. Some of our western friends eyes water just looking at the food let alone tasting it.
The Indonesian way of easing a “Chilli attack” . Tomato slices, cucumber slices, Chrysanthemum tea.
Here is a typical picture of some West Sumateran Food. Note the little side dishes of extra cut up chilli to add in.

Leon Brozyna

How apropos a subject as I sit down to eat my pizza, very liberally covered with crushed red pepper. For the uninitiated, you want a slice of my pizza, bring your own roll of paper towels to sop up the tears.
REPLY: Must be a pizza called “The Red Menace” – Anthony

I can see a reason why the pepper would evolve: the fruit needs an animal to pick it, to masticate it a bit, and then spit it out so that the fruit fertilizes the ground the seeds will germinate in. Being digested would cause too much damage to the seed capsule, but the ground the plant grows in is generally either undernourished, or otherwise lacking in nutrients needed to promote speedy germination.

Mike Bryant

Robinson (16:38:28) :
“Ironic, isn’t it, that the pepper’s “defence” against being eaten by man has turned into an advantage. We now cultivate these plants in far greater numbers than they would appear in the wild.”
Any plant or creature that man loves and/or consumes will never have to worry about extinction. Can you imagine cows becoming extinct?
Of course in the new crazy world we live in, I guess cows could be taxed into extinction… and maybe man too.

Pamela Gray

Wyatts microbrewery in Albany, Oregon makes the best chilli beer. They serve it on tap. I wait till I know that the tap is getting to the bottom. For some reason, the beer is hotter when the tap is almost dry. While served cold, it will set your teeth on fire. I miss my chilli beer now that I have moved back to NE Oregon.


The “antidote” for too much pepper-capsaicin… ice cream! Not only does it offer immediate relief from the feeling of heat, the calcium in the ice cream binds to the receptor that’s stimulated by the capsaicin and helps stop the pain reaction. Some restaurants that offer super spicy food have creamsicles on hand for folks that think they can handle the supa-hot stuff, but then end up realizing the error of their macho ways.

Les Johnson

Queen1: as you state, capsaicin doesn’t dissolve in water.
However, it is slightly soluble in alcohol. Which is why good, hot mexican food goes so well with Tequila.
Or, at least, that’s what I tell my wife…

Sorry, but I cannot resist. How do you know it is worse for a woman?

Kelvin Kubala

Robert A Cook PE (17:18:33) :
The curious thing about hot peppers (with respect to Texas and Mexico certainly) is that the chillies get hotter the closer you approach the border.
From either directions.
This may be due to the fact that fungi also don’t like capsaicin, but they do like warm temperatures.


A brief synopsis of a pepper experience:
My son, age 6 at the time.
My garden, happy healthy veggies, including some Thai chilies.
Blood curdling screams, “My eyes, my eyes”
I quickly put 2 + 2 together and realized that he was in excruciating pain, but thankfully not in any real danger. (I learned the hard way with a Habenaro Pizza)
A quick Google search came up with a suggestion of flooding the eyes with milk.
So I am in the back yard, calmly talking to my son, telling him it is OK to scream, pinning him on the ground, holding his arms down with my knees, prying his eyes open and pouring a gallon jug of milk in his eyes.
I was sure a neighbor would call the police! I don’t know if the milk helped, but time did, after a few hours he was fine, but the first 30 minutes he was in PAIN.
It is kinda funny now, 4 years later, I tease him about it sometimes. It is almost as funny as when he took a small chunk out of his tongue when he licked the metal rack in the freezer. Good story, but a bit of topic, lol.

As Peruvian, land of hot peppers of every taste and colour, and where we eat them profusely, I can answer the question of why do we eat them: Because they usually contain an alkaloid, RUTINE, which causes adiction as coffe´s caffein or chocolate´s theobromine (some youngsters will remember the movie “Time of wine and roses”?)

By the way Capsaicine it is soluble in oil (as oleic, palmitic, etc acids). So you can extract it through boiling in it.

I love hot, spicy stuff! My favorite is a habenero hot sauce called Sontava, made by a Texas company, Jardine’s.
OTOH, my wife thinks Bell peppers are too spicy. I tell her if the spicy stuff doesn’t make your eyes water and your nose run, you’re not doing it right.


As I undertand it we delight in hot chilli because it induces a pain that stimulates the body to release natural endorphins (painkillers) which induce a ‘high’ in the same way a long distance runner gets high. That’s why chilli is addictive, we get addicted to the high.
BTW regards the possums. If you feed your possums stuff they like – like fresh apples etc they will leave your garden alone.


Why do humans like hot peppers?
A possible answer is humans have evolved to like peppers because eating them removes parasites from the gut.
BTW, this is conventional wisdom in some cultures, but a quick google didn’t show any scientific studies to support the theory.
Also if true. I would expect people descended from areas where peppers are native to be much more tolerant of hot peppers than others.
However, this wouldn’t explain how a caucasian living in Texas or California would love hot chili.


A dozen years ago I was planning to give my 4 year old son a lesson in spicy hot versus CO2 (I mean temperature) hot.
The test object was a yellow hungarian wax pepper, and the plan was for him to touch it to his lips. This specific pepper was one which easily transferred to capsaicin by touch.
The test didn’t work as planned and he decided to take a bite and he was not very happy.


Capsaicin retards food spoilage and appears to have a strong anti-biotic effect against certain inimical organisms, e.g. http://www.annmicro.unimi.it/full/55/zeyrek_55_125-127.pdf .
There is also research showing that it reduces salmonella in poultry.
So maybe the excruciating pain is for our health?


Les Johnson:
“However, it is slightly soluble in alcohol. Which is why good, hot mexican food goes so well with Tequila.
Or, at least, that’s what I tell my wife”
LOL. One of my earliest “appreciations” of chemistry, sixth grade, was the discovery that water did nothing, only fat (milk, meat, especially meat juices) KILLS the horrible pain that I loved so well! 🙂 (The chemical is Capsaicin (8-methyl-N-vanillyl-6-nonenamide, (CH3)2CHCH=CH(CH2)4CONHCH2C6H3-4-(OH)-3-(OCH3), courtesy Wicki) is fat soluble and varies moooocho with different peppers: Here’s the Scoville Scale: http://ushotstuff.com/Heat.Scale.htm


I used to grow cayenne peppers. Late in the season I would let some dry on the bush. One day I discovered my cat eating one of the dried peppers off the bush. He ate the whole thing with a great deal of deliberation. From then on, he ate dried cayennes frequently. He liked watermelon, too. I got that on video. Other than some of his eating habits, he seemed like a pretty normal cat. I wonder how that would fit into the study.


The right combination of carbon dioxide, alcohol,
tannic acid and water does the trick.

Put the still under the oak tree? OK.

Claude Harvey

The true test of pepper “hotness” is not to be found on the Scoville rating. The Holy Grail of pepper hotness is to be found on the Relknie scale which measures not the degree of discomfort the pepper inflicts as it enters the digestive track, but rather gauges the discomfort inflicted as the product leaves said alimentary canal. The Relknie scale ranges from “1” to “10”. It ain’t a “10” unless a “911” call and paramedics become involved.

Pamela Gray

No silly. Put it next to the temperature sensor. Have you not learned anything?

Pamela Gray

Recently that scale has been expanded to include the more sensitive areas on the female body. Basically, your 10 multiplied by my 10. You see, it equates to 10 to the power of 2. Logarithmic scale. Beyond birthing. Police are called. Fire trucks. Hazmat. Emergency crews dress in asbestos removal garb. They use tongs, the kind used in blacksmith shops. Double gloved and gas masked is mandatory. Neutralizing showers for all personnel and equipment if used to move the victim. The injury and mortality rate among said personnel is very high.

Mike Bryant

I had always heard that in the past, peppers were used to cover the bad taste of meat that was just a little to old. I guess it makes sense that meat helps to put out the fire, then. Also, to hear that capsaicin retards food spoilage and appears to have a strong anti-biotic effect against certain inimical organisms seems to fit.
I will not comment on how that might tie in to your comments Pam.

Jeff Alberts

Food should not be painful, coming or going. Of course humans do a lot of stupid things, why should eating things that cause severe discomfort be any different?
I never understood the macho BS of having “hot sauce contests”. Must go along with other stupid human tricks of smoking and drinking.


Interesting post. Also interesting to note that with (at least) four different nerve cells responding to specific levels of heat, only level 3 also responds to capsaicin:
Easy to see the survival value for the plant here, but I’m not sure what advantage accrues to the mammal. Does co-evolution imply mutual advantage, or does any opportunistic adaption by one organism in response to another also count? As you can tell, I’m still at the basic terminology stage of my education here.


I seem to have jumped the gun to question 2 – why do people like hot peppers?
Anyway, I have a better theory as to why we like them, which I will save for the appropriate post, but it involves climate and predicts chili pepper sales will increase with global warming.

George M

The mockingbirds love chiltepins, and as a result, there are wild bushes of them everywhere in S. Texas. I watch the birds eat from the one outside my office window, where only the red ripe fruit is consumed.
A story I like to recall from my early employment. A Hispanic bio-technician had a small garden just outside one of the lab buildings, where he grew various varieties of hot peppers. In a conversation with him, he stated his goal, he was working toward a pepper so hot that when it ripened, it went up in a blast of flame and a puff of smoke.
As far as I know he never quite succeeded.

Pamela Gray

Anything that produces a hot, sweaty sensation should be dealt with carefully…and often.


Not that fully believe in intelligent design, nor do I believe fully in evolution… but that is the downfall of evolution. How can their “evolution” be explained? As a scientific I have a hard time with the idea that plants need to “adapt” or even worse, to “defend” themselves from this or that animal… PLANTS DON”T THINK!


How do you know plants don’t “think”, bro? Can you prove that?
After working with them for many years….they have SOME kind of “adaptive” force. Whether or not it is just a biological reaction…or maybe something a little more…either way…they are fascinating organisms.


A friend brought us a party gift, a hanaero plant with peppers on it.
After a few drinks, my wife asked if they were sweet or hot. I said with a straight face, “sweet” and she popped a whole on in her mouth.
She still gets angry about that.

The answer to the question “why do we like to eat hot peppers?” is the same as the answer to every question about human tastes: not all of us do.
Why do we like one food rather than another, or one sport rather than another, or one smell rather than another, or one style of clothing rather than another, or the company of one gender in the bedroom rather than the other, or one style of painting rather than another, or one genre of novel rather than another … and so the list goes on. We are all different and have different tastes.
I think the reason I enjoy very hot food is that it makes me sweat and clears out my flabby pores, the choice is a stonkingly hot curry or a trip to the gym. No contest really.
Some years ago I first entertained a group of American friends at FatBigot Towers and it seemed a good idea to entertain them to a traditional English meal, curry, at an excellent local Indian restaurant. I ordered a range of dishes from very mild to “please have a fire extinguisher nearby”. Most of them had not had curry before and one was tempted by the green beans garnishing the surface of the Lamb Jalfrezi, so he scooped them up in his fork and wolfed down the lot. They were, of course, long thin green chillies. His wife spent the next day nursing him back to health.


Before I moved to Texas [FROM VERMONT, for God’s Sake] almost 30 years ago, one of my fraternity brothers from Houston made me a pot of chili (using CANNED Old El Paso Jalapenos) to get me acclimated.
At the time I thought it was the hottest thing that a body could possibly eat.
Now I realize that canned Jalapenos are literally kids’ stuff; I can’t eat an ordinary hamburger anymore without a nice thick layer of them.
Many of my colleagues always carry a mini bottle of Tabasco with them everywhere, JUST IN CASE.
The secret is the hot weather; 96 degrees F day in and day out (and sauna-like humidty to boot; we say “it’s great for the complexion!”) dulls the taste buds something fierce. Something muy picante is almost a necessity.
Some Tunisian engineers of my acquaintance used to invite me over for dinner; their cuisine is like Italian-meets-the Fires-Of-Hell (not surprising, considering that historic Carthage north of Tunis is a short ferry ride from Syracusa Sicilia). I had their version of Chicken Cacciatore, and, while it was DELICIOUS, my entire GI tract (upper, lower, and “both distal ends”) wasn’t right for a week.
Now for HOT, we got your habaneros or those scotch bonnet peppers. What Ms. Gray said about “sensitive tissues” is doubly true for those puppies.
In fairness, it’s mostly what you’re used-to; almost anybody can be trained to enjoy a good helping of capiscum. The only consistent exceptions to this that I’ve seen are fair-skinned Englishmen.
I had a pasty English boss about 25 years ago who would break out in a torrential sweat if a drop of Tabasco touched his lips. Naturally, we bought him a gallon of the stuff as a nice parting gift on his return to the Auld Sod. [Once I was with him in Dehradun, India, and he ordered a HAMBURGER for dinner so as to avoid the spicy Indian fare. I’ve never really recovered my respect for the English since then.]

John H

Why Are They Hot?”
Because if they were cold they would be called Cold Peppers?