Chilli peppers embraced one of the most innovative evolutions of the plant kingdom. Most plants make their fruits as tasty as possible to lure animals to eat them and spread the seeds - but the chilli pepper takes a different approach. Their distinctive hotness or spicy taste makes most of the animals to ignore them - though mankind made a culinary culture out of this. Then why a plant has such a ridiculous strategy when it is straight against the law of nature?
Chilli peppers are widely used in many cuisines as a spice to add heat to dishes. The substances giving chilli peppers their intensity when ingested or applied topically are capsaicin and related compounds known as capsaicinoids. So these capsaicinoids are the chemicals in which we are now interested.
Flavour is the defence
Capsaicin is produced as a defence mechanism against particular microbes - a fusarium fungus. Which is carried by insects that attack certain species of chilli peppers. Without a thick and strong skin, the chilli peppers are pretty much defenceless against those insects. So evolution gave them capsaicinoids. Chilli peppers increased the quantity of capsaicin in proportion to the damage caused by fungal predation on the plant's seeds.
Then how do they spread their seeds?
Of course by birds. Capsaicin targets a specific pain receptor in mammals. But birds do not have the same sensitivity to capsaicin. So they can eat fruits just like any other. After eating the delicacy you know what they do... And help this innovative plant to spread their seeds.