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Leaf-cutter ants and La Caminata

Anna:

While walking down the Hidden Valley Nature Trail, we stumbled across a line of ants carrying bits and pieces of leaves on their backs.  The ants were following paths brushed clean of any debris, as if a gush of water had flowed through and washed 4 to 5 inches of ground clear.

Maggie and I joined the line of ants and soon came upon a huge mound, about twenty feet wide and three feet tall.  The mound was clearly the center of the leaf-cutter ant operation, with trails radiating out in all directions from their home.  I tried to follow a trail to its end, but eventually gave up --- the ants travel long distances, often running along horizontal tree limbs in addition to their cleanly swept trails.  I decided that ants must prefer certain tree species to go to so much trouble when they could use the leaves of the trees growing right out of the mound.

Costa Rica is home to two genera and several species of leaf-cutter ants, but a handy pamphlet (The Fabulous Leafcutters, by Amy Mertl) explained that the most common species is Atta cephalotes.  Just like the Azteca ants farm mealybugs, the leaf-cutter ants farm fungi --- they carry home leaves, chew the plants up to feed the fungi, then eat the gongylidia produced on the fungal strands.  When new queens leave to form their own colony, the queens carry a little bit of fungus with them, just as the first European colonists to the Americas brought along seeds of their vegetable crops.

Later, I watched another colony of leaf-cutter ants gnawing circles out of an elephant ear leaf.  An ant stood on the bit of leaf it was biting off, then reached out its legs to grab onto the main part of the leaf just in time so that excised section and ant didn't fall together to the ground.  The leaves remaining on the plant looked like someone had gone after them with a hole-puncher.

Although Maggie and I thought the leaf-cutters were unbelievably cool, farmers are less impressed.  Every day, a colony of leafcutter ants can harvest as many leaves as an adult cow, and the ants are quite fond of banana, sugar cane, and corn.  Scientists estimate that leaf-cutter ants harvest 12 to 17% of Costa Rica's total leaf production every year --- and I thought our deer problem was bad!

I did have a small run in with the leaf-cutters after months of watching their work.  During the rainy season, I often carried home plant specimens to draw indoors, but one day I noted:

Yesterday was a glorious and horrible day --- nearly more than I could bear.  It was the day of La Caminata, another of the Friends' School's impressive fundraiser ideas.  The concept --- La Caminata was a 12.5 kilometer walk up to San Geraldo Mirador, from which, on good days, one can see the Arenal Volcano.  We didn't get to see the volcano --- too misty --- but we did see the lake at its base through the mist, saw lovely new scenery, and had a ball getting there.