Chilli Confusion: Win for Evolution

I love chilli peppers. They go great in various foods as well as just whole in a nice glass of wine. I also think they (as well as tomatoes) have a neat method of seed propagation: they get eaten by animals and let their seeds be "distributed" when their digestive systems are done with them. This apparently works well for them, since they get (i) a nice new home in foreign climes and (ii) a toasty warm bed of fertilizer to start life off with.

However, at same time this has always confused me - while I love them cut up and mixed into my food, if I were your average grazing mammal and decided to try biting into a chunk of chilli for the first time, the intense burning sensation in my mouth would soon put an end to such a behaviour. But if they get propagated in the same way as tomatoes, why would they want to ward off their middle-men?

Obviously, there's a perfectly reasonable explanation for this, but neither my teacher at school nor the then-infantile dial-up internet of the time could provide said explanation. Revisiting the problem recently gave me the answers I craved. The heat in chillies arises from evolutionary forces that depend on two major factors that make the answer kind of obvious when you know them, and are a veritable marvel of nature (at least, I think so...).

First, there's the fact that while we perceive chillies as having a hot, burning sensation, birds don't. In humans and other mammals, the capsaicin in chillies activates the same pain receptors as direct application of heat does*, but birds don't have these and so are basically unaffected by this.

Secondly, the seeds in chillies are vulnerable to mammalian digestive tracts - they are harmed when they pass through them, but not when they pass through avian digestive tracts. Thus, a chilli's heat is how it selects its preferred "thing to be eaten by".

So that's my curiosity satisfied, and since it was such a nice evolutionary arrangement I thought I'd share.


* Though it doesn't feel quite the same as actual heat applied to the inside of your mouth (at least to me) because you don't get other consequences of heat (e.g. tissue cell damage and the resultant release of various substances with further consequences). But it really is the same pain receptors that get stimulated (these ones, if you're so inclined), just in a different way.