Leaves of the Acacia trees (primarily native to Australia and Africa) are eaten by giraffes and antelope. An Australian research scientist has determined that the Acacia trees pass on an ‘alarm signal’ in the form of a gaseous chemical called Ethylene to other trees when animals browse on their leaves. The ethylene gas travels on the wind and alerts nearby downwind acacia trees to produce tannin, a toxin that makes the leaves toxic, and lethal if over-consumed.

The Scientist, Wouter Van Hoven says that 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.

Van Hoven made his discovery when asked to investigate the sudden death of some 3000 South African antelope, called kudu, on game ranches in the Transvaal. He noticed that giraffe, roaming freely, browsed only on one acacia tree in ten, avoiding those trees which were downwind. Kudu, which are fenced in on the game ranches, have little other than acacia leaves to eat during the winter months. So the antelope continue to browse until the tannin from the leaves sets off a lethal metabolic chain reaction in their bodies.

There is a similar survival drama that plays out between Pine trees and the Pine Bark Beetles in the Rocky Mountain states of the US. When we lived in Colorado, we would take drives in the mountains. We would regularly see whole mountains whose trees were defoliated and dead.

As a bark beetle attacks a pine tree, the tree releases chemical compounds called terpenoids. These discourage the beetles by interfering with their digestion of the bark. In retaliation, the beetle releases chemical messages, called pheromones into the air. These ‘aggregation’ pheromones signal other pine beetles that food is available. This results in the attraction of many bark beetles, that mass-attack in order to overcome the tree’s natural defense mechanisms. As the tree becomes too full to sustain additional offspring, the beetles switch to emitting an anti-aggregation pheromone, essentially sending a “no vacancy” message to other beetles. Scientists have successfully isolated anti-aggregation chemicals that act as naturally occurring repellents and use them to help manage forests by protecting the pines.