Many woody species in humid tropical forests synchronously flush entire canopies of young red leaves. Numerous unsuccessful attempts have been made to explain the adaptive value of this visually striking phenomenon. In the humid tropics, fungal attack is a potentially important source of mortality for expanding young leaves. We propose that the anthocyanins responsible for the red coloration of young leaves may play a protective role against invasions by leaf-attacking fungal pathogens.
Fungus-growing leaf cutting ants (Atta columbica Guerin) were used in choice tests because they are known to select against leaves or chemicals containing fungicidal properties. In feeding trials with leaf discs from 20 common species, ant preference decreased significantly with increasing anthocyanin content. In feeding trials with pure anthocyanin (3,3',4',5,7-pentahydroxyflavylium chloride) presented on oat flakes, ants again showed a significant dosage dependent preference. This suggests that even low concentrations of anthocyanins may be harmful to the fungal colonies of ants. Additional work on the effects of anthocyanin on leafattacking fungi is encouraged.