Chemical Alarm Systems

When caterpillars begin eating a tree's leaves, the damaged tree releases volatile organic compounds into the air. Neighboring trees detect these airborne chemicals and preemptively produce defensive toxins before the caterpillars even reach them.

Acacia trees in Africa take this further. When giraffes browse their leaves, the trees release ethylene gas. Downwind acacias detect the gas and flood their leaves with bitter tannins within minutes. Some researchers have suggested giraffes may feed into the wind to stay ahead of the warning, though that idea is debated.