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Hot Peppers – Why Are They Hot?

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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!”
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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.
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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?

Related: Hot Peppers

Comments

  1. #1 Gabe
    July 21, 2006

    Insightful.

  2. #2 greensmile
    July 21, 2006

    A tidy and pleasing example of evolution’s well oiled gears turning smoothly towards and ever more optimized future. [and though maybe not understood, not news to bird fanciers who have laced their bird feeders with pepper to keep away squirrels for years now.]

    We hear of the in-optimum less often, and mostly in rebuttals of ID. Being among friends of evolution here, I dare ask: what are some of the more surprising “mistakes” nature has not yet quite managed of dispose of via their unfitness?

  3. #3 Christopher
    July 24, 2006

    Wonderful! I didn’t even know this was a mystery, but now I have a delicious bit of cocktail chatter with which to wow other geeks. Care to do one next on why giant fruits like avocados, eaten by no extant animal, exist anyway?

  4. #4 coturnix
    July 24, 2006

    Thank you, Chris.

    Well, I am still working on the “Why we like to eat hot peppers” post, but the giant fruits with no eaters is a fascinating topic. Some fruits have gone extinct because their animals went extinct. Others are in the process of getting there (some trees live hundreds of years, so the species may still be around although the humans have exterminated its fruit-eating animal a century or two ago), and others are kept alive by human agriculture, e.g., avocados.

  5. #5 Grammar Nazi
    July 24, 2006

    “mutation that MAKED it”?

  6. #6 coturnix
    July 24, 2006

    Yikes! Fixed it now. Thank you.

  7. #7 Alethea
    July 24, 2006

    Seriously cool post and comments, too.

    So what was the fruit-eating animal that went for avocados? It’s a fatty fruit, so I’d imagine lots of them, but I’m curious as to the real answer.

    Cheers!

  8. #8 coturnix
    July 24, 2006

    The Giant Sloth. There is more on Wikipedia.

  9. #9 Kim
    August 15, 2006

    Whoa – I can’t even SMELL a hot pepper; seriously, it makes me nauseated. Hubby told me to eat this “chinese vegetable” when we were going out (from his kung pao chicken), only it was a hot pepper.

    I nearly fainted from what I assume was vasodilation, with profuse sweating and profound nausea. This same situation occured with a ecchinacia/cayenne combo from a health food store, while I was shopping at the mall!

    So, my progeny will have the genes for NOT eating peppers.

    I am, however, solely responsible for the continuation of the avocado. I could live on them. Even have a HUGE tree in the backyard. Maybe I should germinate the seeds on the kitchen window so they never go extinct! : )

  10. #10 Fjarlq
    August 16, 2006

    Capsaicin could be a conditioned flavor preference, because it is usually consumed along with a lot of calories. Seth Roberts has been studying taste-calorie association recently, he believes it is tied with hunger.

  11. #11 RSG
    August 27, 2006

    The fact that birds eat peppers has long been known. Down here in Texas, the chili pequin is a very small pepper, about the size of a pea, and is about a thousand times hotter than a jalapeno, very close to habaneros in Scoville ratings. They are primarily spread by birds, which eat them with gusto. I have several plants in my back yard, all of them planted by birds, none by me. The peppers are delicious, but a little bit goes a long way, and it’s convenient that they’re so small. They pickle well, and make an excellent hot sauce when pickled whole in vinegar. This complements pinto beans better than almost anything, other than cornbread.

    If you don’t like hot peppers, start off slowly with milder peppers, and work your way up. Jalapenos are relatively mild, but shouldn’t be the first one you try. The heat becomes addictive, and you’ll eventually love it. Hot peppers are also very high in vitamin C, far higher than citrus.

  12. #12 btb
    August 30, 2006

    Nice article…

    The hottest peppers I know personally are madame Jeanette peppers, supposedly they come from Haiti, I know them as being Suriname.
    Eat a small bit and for the first half hour your mouth will feel like sulfuric acid was poured into it.
    They are very aromatic and are great for cooking though.

  13. #13 Nick
    August 30, 2006

    We like peppers because of the endorphins caused by the capsacin – it’s like being addicted to a drug or getting a runners high. The more you eat peppers, the hotter you can go. Good thing they are good for you!

  14. #14 coturnix
    August 30, 2006

    Hey, that’s Nick from the HotSauceBlog! Welcome here!

  15. #15 triticale
    September 8, 2006

    The book Defying Gravity : land divers, roller coasters, gravity bums, and the human obsession with falling, by Garrett Soden, discusses an apparent genetic connection between thrill seeking and the eating of spicy foods.

  16. #16 Emily Procter
    March 15, 2007

    my comment is a question: what is the world’s hottest pepper? where can it be found? can i buy it in a store? pleaae answer these three questions in a blog, or e-mail me at emily_procter_is_cute@yahoo.com thank you.

  17. #17 Stagyar zil Doggo
    September 9, 2007

    Great post. I look forward to the second part.

    On the subject of why we eat hot peppers, I’ve read that people who live in warmer climes have a preference for spicier foods in general. This was explained by proposing that its a genetically acquired trait as spices constitute effective food preservatives. Consequently, people with a preference for spicy foods are less likely to suffer food poisoning.

    This sounds less than convincing to me because obviously, food spoils in pretty much all climates that are habitable by humans, even if faster in some. Also, amongst my acquaintances the preference for the hottest food is shown by a person of korean descent.

    I would appreciate scientifically literate comments on a) whether the preference for hot peppers (and spicier food in general) has any genetic basis and b) whether it affords (or afforded in pre-modern times) any fitness benefits – either universally or to residents of particular geographical areas.

    regards,

    PS: I really enjoy your chronobiology posts. You seem to do less of those these days.

  18. #18 coturnix
    September 9, 2007

    Thank you. I think that the preferences are cultural, not genetic. The literature is atcually quite big on this, which is why I never found enough time to do thoroughenough research to write the follow-up post. As for chronobiology posts – I have a couple in the works, but I am so busy these days!

  19. #19 Stagyar zil Doggo
    September 9, 2007

    Oh Crap! I didn’t realize that this was an old thread.

    Nevertheless, a google search failed to find the promised second part of this series, so I guess my request stands if you ever get to it.

  20. #20 Stagyar zil Doggo
    September 9, 2007

    Never mind on the Oh Crap!. I really should reload before posting. :-)

    But on the subject of old threads, I would appreciate a response to a comment I left here, at your leisure.

    regards,

  21. #21 David Harmon
    September 10, 2007

    I think that the preferences are cultural, not genetic.

    I suspect there might be a nice example of cultural evolution there, with groups whose preferred food spices deterred the local parasites or other diseases, getting the benefit thereof. I hadn’t heard so much about spices affecting spoilage — I thought that was mostly drying, salt/sugar, and occasionally fermentation.

  22. #22 MJ Memphis
    September 10, 2007

    “The book Defying Gravity : land divers, roller coasters, gravity bums, and the human obsession with falling, by Garrett Soden, discusses an apparent genetic connection between thrill seeking and the eating of spicy foods.”

    Without having read it, I’m guessing that holds true more for people who acquire a taste for spice than those that grow up with it. My wife and I are pretty conservative (personally not politically) but as a mixed Cajun/Thai household we consume a *lot* of chilis and have both done so since childhood.

    My favorite chili is the Thai chili pepper, preferably mashed in a mortar and pestle before adding to food (evens out the flavor, instead of having hot bites and mild bites).

  23. #23 chain saw
    November 8, 2007

    Brilliant study, I must admit. I also, though, wonder what constitutes a hungry animal! As far as I’m concerned, animals are always hungry. I mean, they’re quite serious about food, aren’t they?
    This is actually a fascinating example of how brilliant nature and creation is. In my opinion, plants like peppers or fruits that have opted for this route are definitely getting the upper hand over plants that simply are poison or hard to reach. This may also show that birds are attracted to color and not taste well, obviously, they must be. But the last question I would still like to see the answer to why the heck DO we eat hot peppers? I think it has something to do with insanity, personally!

  24. I think this will lead us to another research topic: What is the effect of varying amounts of capsaicin to a group of animals?

  25. #25 original oil paintings
    September 15, 2008

    I have a question: Why is it that most chili or super hot peppers are usually red in color? Does the amount of capsaicin vary directly with the redness of a particular pepper?

  26. #26 Coturnix
    September 15, 2008

    There is no correlation between color and hotness, as color is likely not targeting mammals. It is targeting birds, as a lure.

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