Adventures Among Ants - Mark W. Moffett [117]
This small leafcutter worker from a Panamanian colony of Atta sexdens is taking a tuft of white fungus to “plant” in fresh leaf mulch.
Much like vintners trimming back the branches of grape vines to maximize their yield, minor workers prune the garden surface, stimulating more edible fungus growth.28 The gardens’ brainlike fissures dramatically increase the surface area from which minors can harvest meals.29 The small workers seem to take on the primary role in distributing food. They drink sap while processing leaves and nibble on the fungus while tending the garden recesses. Then they feed their nestmates, either by regurgitation or by handing them edible wads, which they also give to the larvae scattered over the garden.
The leafcutter ant Atta vollenweideri plays an important ecological role in Paraguay, where the Caranday palms and mesquites of Chaco savannas sprout from the fertile soils of dying nests.
All along this conveyor belt, the workers defecate on the leaves. Their feces, like the manure we use in our gardens, contain ammonia and amino acids that promote garden growth. Leafcutter excrement also includes enzymes from the fungi they have eaten, which pass through their digestive systems intact and speed the breakdown of fresh substrate, helping each new tuft get better settled. Coddled and cared for, in a day the fresh fungus has sprouted what look like microbe-sized masses of cotton candy. The masses consist of swollen hyphae tips, configured to lie in easy reach of hungry ants. As seen under the microscope, they are arranged in grapelike clusters, much as the fungus garden chambers are along their underground runways, but at one-thousandth the size. Found in no other fungi and with no purpose other than to be eaten, the swellings reveal that the fungus has been selected over long periods of time by the ants to serve as food, just as plump grapes and rosy apples show generations of cultivation by human hands.
As in human industries, such large-scale operations require the support of extensive transport and distribution networks. On average, each leafcutter colony maintains a trail system 267 meters long at any given time, which requires the completion of 2.7 kilometers of roads over the course of a year. That much construction requires 11,000 ant-days of labor, during which the workers expend the energy attained from eight thousand leaf burdens. That sounds like a lot, but because a leafcutter workforce is larger than the human population that was employed to build the pyramids of Egypt, it takes a colony less than a day to fetch enough foliage to fuel a year’s worth of road building.30
I came across some prime leafcutter trail systems in Paraguay in the early 1990s while driving with a companion through the Gran Chaco, an alternatively swampy and scorchingly dry savanna region. The thermometer read 125 degrees Fahrenheit, and the heat was overpowering—literally: on our second day, my friend was catatonic from exposure, staring blankly into space for half an hour before I could rouse him. We had come in search of Atta vollenweideri, which can unearth more cubic footage than that boasted by an average New York apartment.31 The nests, with their excavated discs of pale sand, several meters across and visible to the horizon, each radiated trunk trails as wide as a human foot and more than 70 meters long, leading to the grass this species prefers, which it cuts into long, linear segments. Days later, I had a chance to view the area from a low-flying airplane. The leafcutter communities resembled highway maps of human urban centers; in fact, some leafcutter species reportedly have beltways encircling their metropolises.
The leafcutter transportation system comprises durable trunk routes with weaker side trails near their far