Science Puzzle
Why Do Chillies Burn but Birds Don’t Mind?
Bite into a hot chilli and your mouth feels as if it's on fire, even though nothing in it is actually hot. Mice and other mammals avoid chillies. Birds gobble them up as if they were sweets.
Why do chillies burn mammals but not birds?
The Answer
The burn comes from a chemical called capsaicin, which triggers the heat-sensing nerves of mammals. Birds' heat sensors are shaped slightly differently, and capsaicin doesn't fit them, so birds feel nothing.
Your mouth has nerve endings with a sensor that switches on when something touches them hotter than about 43 °C, warning you before you're scalded. Capsaicin happens to switch on the very same sensor, so your brain gets a "burning" message although nothing is hot. In 2002 scientists showed that the bird version of this sensor differs in just the spot where capsaicin attaches, so it stays switched off.
That difference suits the chilli plant perfectly. A mouse chews up the seeds and destroys them. A bird swallows them whole, flies off and drops them somewhere else, still able to grow. The burn is aimed at the animals the plant wants to keep away. People use the trick too: some bird seed is laced with chilli to stop squirrels stealing it.
Mint plays the opposite trick: its menthol switches on a nerve sensor for cold, which is why it feels cool in your mouth.
The principle: Chemical defence aimed at one group. Capsaicin triggers the heat-sensing pain receptors of mammals but not of birds, so chillies deter seed-chewing mammals while birds spread the seeds.