An evolution-based understanding of Hot Peppers - Why Are They Hot?

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).