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These thorns, originally intended to ward off browsing animals, now have a totally different function. The abnormally large thorns on this acacia are hollow: providing ready-made housing for ants, and even feeding them via special glands. In return, the ants protect the miraculous tree, feeding on the buds of climbing plants and discouraging anything that might try to eat it – insects, naturally, but also herbivores as big as giraffes!

This particularly impressive relationship, in which both partners benefit, is called mutualism. Although less spectacular, some non-parasitic fungi growing on the surface of trees may be another example. They offer protection against infection by their parasitic fungi.