For a plant, fruits function like cars – they disperse a plant’s offspring and function to protect the seeds while they mature. The details of how they do this depends on the specifics of the plant species. Some plants drive Volvos – they don’t get outside the suburbs much, but there are airbags under every surface. Some plants… well, let’s not take this analogy too far. The point is that the seed dispersal strategy for a given plant species has evolved to fit its particular circumstances – the environment it’s in and the other species it interacts with. This can include what kind of fancy chemicals it packs into its fruits. Unripe fruit often tastes less than pleasant. This is because the seeds aren’t mature yet – a plant which produced tasty fruit, tempting animals to eat and disperse them, before its seeds were mature enough to sprout, would never leave any offspring behind.
So why would a chile fruit (Capsicum spp.), even ripe, burn your face off? Because you aren’t the intended disperser. Chiles contain a group of compounds called capsaicinoids, which when eaten, bind to the heat-sensing pain receptors in your mouth and throat, telling your brain something bad has just happened. In response, your heart rate increases, you break out in a sweat, and endorphins, lovely endorphins, typically a response to pain, are released. The chile plant is telling you to back off, buddy. But don’t worry, it’s not just you. Chiles have a particular disperser in mind, and it’s not silly mammals with their heat sensing pain receptors.
Capsaicinoids are quite effective at keeping away unwanted guests, like fungal pathogens and seed-damaging mammals. But they have little effect on the chilies favored fruit disperser, birds. Birds lack these pain receptors, and so can eat the chile fruits without a twinge. The bird can then fly off, pooping out viable seeds wherever it may go. Capsaicinoids can have an effect on a bird’s gut retention – that is, they can cause the bird to experience a bit of constipation. There is some evidence that particular chile species have dialed in on this: chiles with thin seed coats generally cause less constipation in the bird species that mainly feed off of them, while chiles with thicker seed coats, needing to spend a longer time in the bird’s gut to increase scarification and germination rate, cause more constipation. Natural selection is pretty clever that way.
Haak, D.C., McGinnis, L.A., Levey, D.J. & Tewksbury, J.J. (2012). Why Are Not All Chilies Hot? A Trade-Off Limits Pungency.
Proc. R. Soc. B, 279, 2012-2017 DOI: 10.1098/rspb.2011.2091
Tewksbury, Joshua J. (2008). COSTS AND BENEFITS OF CAPSAICIN-MEDIATED CONTROL OF GUT RETENTION IN DISPERSERS OF WILD CHILIES Ecology DOI: 10.1890/07-0445.1
Douglas J. Levey, Joshua J. Tewksbury, Martin L. Cipollini and Tomás A. Carlo (2006). A field test of the directed deterrence hypothesis in two species of wild chili Oecologia DOI: 10.1007/s00442-006-0496-y