Way before people started inhaling nicotine in cigarette smoke, plants were using the chemical to defend themselves from animals.  Nicotine is a poison, and an exceptionally deadly one. It targets proteins that tell our muscles to fire when they receive signals from our nerves. At high enough doses of nicotine, these proteins force muscles to constantly contract, leading to paralysis and death. And since the same proteins are found in every animal with muscles, nicotine can kill cows and caterpillars alike.

The tobacco hornworm is an exception. As a caterpillar, this moth specialises in eating tobacco leaves, because it can cope with doses of nicotine that would kill other species. It gets rid of most of the