If animals and plants can’t defend themselves, they often form partnerships with bodyguards. Wasps use zombified caterpillars. Corals recruit goby fish. And acacia trees hire ants. The ants defend the trees against hungry mouths by biting and stinging any invading plant-eaters. Some are so ferocious that they can deter elephants. In return, the trees pay their bodyguards by providing shelter in the form of swollen thorns, and food in the form of nectar or nutritious parcels called “food bodies”.

This alliance between ants and acacias is a staple of textbooks, but it’s even more intimate than anyone suspected. Some acacias don’t just supply their ants with ...