Most of us are quite familiar with the intense pain and profuse sweating that occurs when we consume a particularly spicy pepper. This discomfort, often accompanied by a runny nose and exclamations of pain, is avoided by some (Midwesterners) and sought out by others (most everyone else). Whether the consumer is accidentally or purposefully consuming the chili pepper, it has been well established that the resulting physical reactions are due to our body’s response to a particular alkaloid compound found in these peppers. This molecule, capsaicin, is known to bind to a non-specific, heat-activated ion channel in our tongues (TRPVR1). This binding event initiates a signaling pathway that ultimately results in sensations of burning and pain, as well as a bit of a natural high, thanks to the release of endorphins. This special combination of pain and pleasure, so often at the core of “things humans like,” has fueled a fascination and worldwide demand for spice (see: growth of the underground “chilihead” movement to breed and eat super-hot chilies).
Although these spicy chilies provide us with flavorful food, their purpose on earth is not to entertain and challenge human taste buds. So, then, why are some peppers spicy?
Before we answer this question of spice, we need to first ask, why do plants produce fruit? Since plants are stationary organisms, they rely on a variety of methods to disperse their seeds, including wind, gravity, water, ballistics, and animals. Plants that use animals often produce juicy and delicious fruit to attract animal consumers and provide a reward to dispersing their seeds. However, not all consumers are created equally: fruit not only attracts organisms that can effectively disperse their seeds, but also organisms that destroy their seeds. To avoid consumption by these seed predators, some plants use directed deterrence, often chemically-mediated, to discourage these predators without deterring seed dispersers.
After observing in both labs and natural populations that peppers with high levels of capsaicin were readily consumed by birds and not mammals, it was hypothesized that capsaicin may be involved in such a mechanism of directed deterrence. In a paper published in Nature in 2001, researchers studied a population of chilies in Arizona and found that avoidance of chili peppers by small mammals was directly related to the capsaicin content of the chilies. Birds, however, readily consumed the peppers regardless of how spicy they were. While we might hypothesize that birds just love the spice, it turns out the spice taste doesn’t even register with birds: the bird homolog of the receptor TRPVR1 is not activated by capsaicin and thus birds don’t feel the heat. The researchers in Arizona also found that consumption of peppers by the small mammals resulted in zero germination of the consumed seeds (the seeds get destroyed in mammalian guts). Consumption by birds, however, resulted in germination rates similar to control seeds directly planted from the fruit. Thus, by producing a chemical that targets mammalian heat receptors, capsaicin plays a role in deterring mammalian seed predators (such as us humans) while not deterring beneficial seed dispersers. So, in other words, the pain we feel when consuming hot chilies maybe tied to the fact that we, like most mammals, don’t s&#$ out viable pepper seeds that can grow into new plants.
At this point, you may think that macro-fauna with seed-destroying-guts are the main threats to chili peppers. However, the most widespread enemy to all fruits and the seeds within them may actually be microscopic. Many fruits are thought to contain bitter, distasteful, and sometimes toxic chemicals as a method of microbial deterrence. Indeed, capsaicin has been found to be broadly antimicrobial in addition to selectively deterring mammalian consumers.
Interestingly, this anti-microbial activity of capsaicin may be connected to our culinary obsession with spicy foods: food scientists, ethnopharmacologists, and evolutionary biologists have all postulated that humans may have domesticated chili plants to harness capsaicin’s antimicrobial activity for food preservation. Microbial contamination was a major source of illness and death before the advent of refrigeration and many traditional methods of food preservation are intimately tied to efforts to control our microbial friends and foes. An study published in 1998 in the Quarterly Review of Biology called “Antimicrobial functions of spices: why some like it hot” quantified what many of us who are familiar with world cuisine know: the use of chili as a spice is more prevalent in warmer climates (India, Thailand, Central America) than in colder climates (Scandinavia, Ireland, other areas with noticeably spice-lacking food cultures), where the chance of spoilage is less likely. Humans thus may have learned to accept (and even love!) the burning and pain that comes with consuming peppers in order to reap benefits of health and longevity that comes with killing microbial pathogens in our foods.
Our story of capsaicin’s role in peppers extends from the chemical ecology of plant seed-dispersal strategies to the clever use of chilies in food. The human use of a secondary compound to serve a purpose thats mirrors the evolutionary role of this compound in nature, however, is not unique to capsaicin. We’re excited to explore the many compounds have we co-opted from the environment for our own culinary, medicinal, or otherwise “other” uses!