Why are chilies hot?

If you've taken one of my classes where I talk about the scientific method and the nature of questions and explanations (i.e., most of my classes, but esp. Evolution), you've heard me tell a little personal anecdote to illustrate the difference between Proximate and Ultimate explanations. One about the question: Why are chilies hot?

If you just want the answer to the question, go here to read Bora's nice essay on the topic. If you are interested in my anecdote also, read on...

Some 11 years ago, when I was in the throes of writing my dissertation, I shared an apartment, for a short while, with Ajay Chitnis, a developmental neurobiologist then postdoc-ing at the Salk Institute (he's now at NIH). It was one of the most stimulating few months of my life, as we both enjoyed cooking, especially spicy foods (not just curries, mind you, but world cuisine!), as well as talking about all matters scientific, philosophical, literary, culinary, and cinematic, well into the wee hours of the night - ahh the intellectual ferment of those grad student days...! I don't think I have ever had such a wide-ranging ongoing discourse with anyone else before or since, so in some ways he was the best flat-mate I ever had. I learnt a great deal from Ajay about evo-devo in those early days of the field, and like to think that some of my evolutionary-ecological blatherings found some foothold in his fertile imagination.

Anyway, to the point of the anecdote: one day we had a rather intense discussion about chillies, starting with my asking the question: why are chilies hot? Ajay's immediate answer was, because they have capsaicin in them! So naturally, given the smart-ass evolutionary ecologist in me, my comeback was, but why do they have capsaicin? why are they hot, again? Why have they evolved that way? And, as we wound up talking about this for the next hour, it also dawned on us that our initial responses said something about the nature of explanations we were seeking: Ajay, with his training in medicine and molecular biology, had first come up with a proximate biochemical answer to the question, whereas my own focus was on the ultimate evolutionary answer. Obviously, both are necessary to fully answer the question, thus the pedagogic value of this anecdote.

What had triggered our discussion? It was a seminar by a grad student visiting my lab at the time - Joshua Tewksbury, who was conducting some neat field studies on the coevolution of chillies and their seed dispersers in the Arizona desert at the time. And why am I bringing this up now? Because Bora has just posted a very nice summary of some of Josh's research - he's written it so well that I have to send you to his blog for the answer to this question. And watch his blog, for he promises to follow up with an exploration of why we are the only weird mammals with a hot tooth!