Stomachs on legs are proud to present

ScienceDaily (Aug. 12, 2008) — If you're a fan of habañero salsa or like to order Thai food spiced to five stars, you owe a lot to bugs, both the crawling kind and ones you can see only with a microscope. New research shows they are the ones responsible for the heat in chili peppers.

The spiciness is a defense mechanism that some peppers develop to suppress a microbial fungus that invades through punctures made in the outer skin by insects. The fungus, from a large genus called Fusarium, destroys the plant's seeds before they can be eaten by birds and widely distributed.
"For these wild chilies the biggest danger to the seed comes before dispersal, when a large number are killed by this fungus," said Joshua Tewksbury, a University of Washington assistant professor of biology. "Both the fungus and the birds eat chilies, but the fungus never disperses seeds – it just kills them."
Fruits use sugars and lipids to attract consumers such as birds that will scatter the seeds. But insects and fungi enjoy sugars and lipids too, and in tandem they can be fatal to a pepper's progeny.
However, the researchers found that the pungency, or heat, in hot chilies acts as a unique defense mechanism. The pungency comes from capsaicinoids, the same chemicals that protect them from fungal attack by dramatically slowing microbial growth.
"Capsaicin doesn't stop the dispersal of seeds because birds don't sense the pain and so they continue to eat peppers, but the fungus that kills pepper seeds is quite sensitive to this chemical," said Tewksbury, lead author of a paper documenting the research.
"Having such a specific defense, one that doesn't harm reproduction or dispersal, is what makes chemistry so valuable to the plant, and I think it is a great example of the power of natural selection."