The secret behind the spiciness in some chili peppers and the lack of it in others resides in a long-lasting battle for the perpetuation of the species. To be more precise, the chilis become hotter as they are more exposed to the attack of a fungus that enters the chili peppers and destroys their seeds.

In a study published in Tuesday’s edition of Proceedings of the National Academy of Sciences, and partly funded by the National Geographic Society, researchers reveal chilis’ very own defense mechanism against unwanted intruders, who could interfere with the natural dispersal of their seeds.

Joshua J. Tewksbury, from the University of Washington, and his colleagues, explained that as the primary function of a fruit is to attract animals capable of dispersing viable seeds, the fruit also needs to defend itself from unwanted predators, and in this case, it’s a chemical fight.

Capsaicinoids are not only known to be the chemicals responsible for the pungency in the chilly peppers, but humans have been using them for their antimicrobial properties. They can be found within the fruit of Capsicum species, and their function is restricted only to the fruit of the plant.

Researchers have taken into consideration for this study the Capsicum chacoense, which is native to the Chaco region in Bolivia, Argentina and Paraguay. Covering a 300-km-long study area in southeastern Bolivia, they found a mixture of plants that contain capsaicinoids and plants that are similar on nutritional level but lack capsaicinoids.

They drew three major conclusions: (1) Microbial fruit pathogens have a large negative impact on non-pungent chili fruits; (2) Capsaicinoids reduce microbial damage to chili fruits and seeds; (3) Among populations, the proportion of plants producing capsaicinoids will increase as the intensity of microbial attack increases.

Microbial infections are primarily provoked by a single fungal species, Fusarium semitectum, causing discoloration, killing seeds and thus reducing chances of survivability. The fungus has easy access into the fruits through pierces made by insects. A fungus infection is very easy to spot - the surface of the fruit turns black as the infection spreads.

But this is where the chemical defense mechanism comes into play: by experimenting with pungent and non-pungent plants, the team of scientists discovered that seed-infection rates are twice as high in non-pungent fruits.

However, although pungent fruits seem to have a better defense mechanism, they also lack something non-pungent fruits have: a seed-coat thickness that better protects the seeds as they pass through the digestive track of animal dispersers.

On the other hand, as capsaicinoids are responsible for protecting the seeds and keeping the fungus away, it doesn’t seem to have the same effect on birds, which means it’s role is indeed a protection one.

The chemical response to the fungal attacks has not only captured the interest of scientist, but also the interest of humankind. People have been using spicy chili peppers for ages, and they even domesticated the species to better take advantage of its properties: antimicrobial protection and food preservation.