The two flower species differ in floral structure. The bills of males are more compatible with one and those of females better designed for the other. And a possible explanation emerged. The study showed that ants and acacia trees have a special alliance in which each helps the other survive. The fuzzy-looking acacia flowers are usually yellow or white, and many are fragrant, a lure to pollinating insects.
Ants live on some acacia trees and protect the trees from plant-eating insects.
The ants attack an invading beetle or aphid that plans to make a meal of acacia leaves. In return, the tree provides shelter for the ants, which also thrive on nutrients produced by the tree. A mystery surrounding the observed bond between acacias and ants is how other insects manage to pollinate the flowers when the tree is guarded by ants. P. G. Willmer of the University of St. Andrews, Fife, in the United Kingdom and G. N. Stone of Oxford University studied ants protecting African acacia trees whose flowers are pollinated mainly by solitary bees during the midday period. Like any other insect, a solitary bee is not welcome by ants guarding a tree.