Acacia trees pass on an ‘alarm signal’ to other trees when antelope browse on their leaves. It is said that acacias nibbled by antelope produce leaf tannin in quantities lethal to the browsers, and emit ethylene into the air which can travel up to 50 yards. The ethylene warns other trees of the impending danger, which then step up their own production of leaf tannin within just five to ten minutes.
I hope this helps. =^P