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Excerpts from Jim Conrad's
Naturalist Newsletter
from the January 5, 2007 Newsletter issued from Sierra Gorda Biosphere Reserve, QUERÉTARO, MÉXICO
DISORGANIZED LEAFCUTTER ANTS
While photographing the above Poinciana fruit I noticed a pile of the tree's green leaflets heaped at the trunk's base. I knew what had left the mess, for I've seen such a thing many times before. You can see this particular mess mess below.
If you look closely at that picture you can see ants on the Poinciana's trunk and hiking across the leaf litter. Those are leafcutter ants. You can see one ant on the trunk carrying a leaflet as if it were a parasol. The ants are slowly defoliating the Poinciana, carrying leaflets one at a time to their underground chambers where fungus will grow on the compost, and the ants will eat the fungus.
But, just why have so many leaflets been dropped and not picked up? If you watch the ants for long you'll see that their behavior has such a disorganized appearance that it's easy to guess what's happened: The ants carrying the leaflets just got confused, decided to do something else, decided to turn around and go the other way, who knows? All up and down the ant line you can see individual ants behaving every way but systematically. Sometimes you even see an ant carrying a leaflet back up the tree. You decide that genius may reside in the ant colony's social structure, but individual ants are pretty scatterbrained.
That doesn't detract from the genius inherent in the colony's organization, however. For example, leafcutter ants rotate their activities among the trees they harvest -- don't ravish the same tree again and again so that it dies. Also, leaves from any one tree are only taken to one part of the nest, rather than distributed throughout it. This prevents the whole nest from being infected if a particular crop of leaves turns out to harbor a disease or be toxic to the fungus.
There are nice pictures and more info on leafcutters at http://www.richard-seaman.com/Insects/CostaRica/LeafcutterAnts/.