Why Are Chillies Hot?

The most common domesticated (grown for us to eat) chilli is Capsicum annuum, which originated in Mexico, and was probably first domesticated 6000 years ago (1,2). But what in a chilli makes it feel hot? And why did the plants put it there to start with?

The spicy part of chilli comes from a chemical called capsaicin. The more capsaicin – the hotter the chilli. The hot sensation is caused by capsaicin binding a channel, called TRPV1, on pain neurons. These receptors are also activated by heat, and by acid. When capsaicin binds this channel, it causes it to change shape, and this lets ions (charged particles) flow into the neuron. This changes the polarity of the neuron – which activates it – causing the feeling of spice (3).

So that explains why chillies feel hot in the mouth – but what benefit does this give to the plant that makes them? Chilies are effectively a fruit, and the function of all fruits, from the plant’s perspective, is to be an attractive reward for any animal that eats it. The animal gets a tasty meal, and the plant's seeds (which were inside the fruit) get dispersed once they’ve passed through the animal. However, not all animals are equal. Birds are ideal, and they are not repelled by spice in chilli like mammals are. Unfortunately, chilli seeds do not survive a journey through the mammalian digestive system.

One group of researchers showed that small mammals, such as mice, avoid eating chillies which contain lots of capsaicin, and that birds are mostly responsible for removing chillies from plants in the wild (4). So, one reason chilis are hot is to stop mammals from eating them, but that might not be the only reason. Researchers have also shown that chillies which produced more capsaicins, the chemicals that make chillies hot, had less damage to their seeds by a specific fungus, then those that produced less. This suggests that the ‘point’ of the hotness could also be to put off unwanted microbial diners (5).

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(1) Chili Peppers - An American Domestication Story - https://www.thoughtco.com/chili-peppers-an-american-domestication-story-170336#:~:text=The%20most%20common%20type%20of,the%20wild%20bird%20pepper%20(C.&text=chinense%20(yellow%20lantern%20chili%2C%20believed,northern%20lowland%20Amazonia)%2C%20C.

(2) Kraft, K. H. et al. (2014) ‘Multiple lines of evidence for the origin of domesticated chili pepper, <em>Capsicum annuum</em>, in Mexico’, Proceedings of the National Academy of Sciences, 111(17), pp. 6165 LP – 6170. doi: 10.1073/pnas.1308933111.

(3) Yang, F. and Zheng, J. (2017) ‘Understand spiciness: mechanism of TRPV1 channel activation by capsaicin’, Protein & cell. 2017/01/02. Higher Education Press, 8(3), pp. 169–177. doi: 10.1007/s13238-016-0353-7.

(4) Tewksbury, J. J. and Nabhan, G. P. (2001) ‘Seed dispersal: Directed deterrence by capsaicin in chillies’, Nature, 412(6845), pp. 403–404. doi: 10.1038/35086653.

(5) Evolutionary ecology of pungency in wild chilies’, Proceedings of the National Academy of Sciences of the United States of America, 105(33), pp. 11808–11811. doi: 10.1073/pnas.0802691105.