Why Are Chillies Hot? Not for Birds

The answer

Capsaicin burns mammals, and mammals chew up the seeds. Birds can't feel it, and the seeds pass through them whole, ready to grow.

Tewksbury & Nabhan (2001), "Directed deterrence by capsaicin in chillies", Nature 412:403–404: in Arizona, wild chiltepín (Capsicum annuum) fruits were eaten by curve-billed thrashers, while pack rats and cactus mice avoided them; seeds passed by birds stayed whole and germinated, while rodents destroyed most seeds. https://doi.org/10.1038/35086653
University of Florida news release on the paper (2001): rodents "chewed up most seeds", leaving the rest too damaged to germinate; birds "are unaffected by the chemical". https://archive.news.ufl.edu/articles/2001/07/for-peppers-hot-quite-literally-the-spice-of-life-uf-research-shows.html
Jordt & Julius (2002), "Molecular basis for species-specific sensitivity to 'hot' chili peppers", Cell 108:421–430: capsaicin activates the mammalian heat/pain receptor TRPV1, but the chicken version is "normally insensitive", which is why birds can disperse the seeds. https://doi.org/10.1016/S0092-8674(02)00637-2
Caterina et al. (1997), "The capsaicin receptor: a heat-activated ion channel in the pain pathway", Nature 389:816–824: the capsaicin receptor is also activated by noxious heat (> ~43 °C), so "heat sensors" / "thinks it's on fire" is literal. https://doi.org/10.1038/39807

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