Animal Training by Plants

If holly leaves are being nibbled by deer, the plant switches its internal code to make its leaves spiky when they regrow. On taller holly trees where deer are present, the upper leaves (which are out of reach) will often have smooth edges, while the lower leaves will be prickly.

The same phenomenon can be seen in East Africa.  Where large predators are common, and impala numbers are reduced, acacia trees are thorn-free.  But where predators are rare, acacia trees grow thorns to keep impala and other antelopes at bay.