Adventures Among Ants - Mark W. Moffett [115]
HOW DOES YOUR GARDEN GROW?
Leafcutters are almost unique among ants in their total dependence on vegetable nutrition. True, many other species rely to some degree on plant matter, notably in the canopy, but vegetation contains meager protein. Hence, treetop ants like the weaver ant are omnivores, the workers using the energy they get from carbohydrate-rich plant food to procure animal flesh for their protein-greedy larvae. (Only the dwellers of a few ant plants can afford to be strict vegetarians; their plants secrete protein-rich food bodies in payment for the ants’ protective services.) Leafcutters have a similar strategy.13 The adult workers drink sap from leaf fragments (their primary energy source), energizing themselves to process the foliage that serves as compost for their larvae’s sole food: a protein-rich fungus that is related to the button mushroom sold in supermarkets. Leafcutter colonies have come to depend on the fungus so much that even the adult ants’ digestive tracts lack key enzymes for breaking down some proteins.14 The fungus does that for them, and also removes from the plant tissues any insecticides that would stop most herbivores in their tracks.15
Most people know fungi as mushrooms, but mushrooms are the substantial yet fleeting reproductive outgrowths of microscopic strands called hyphae that spread in a latticework to infiltrate and ingest soil, decomposing matter, even rock. In the leafcutter nest, this latticework spreads through the foliage that the workers have mulched in order to free its contents for absorption, a necessary step given that the fungus likely can’t digest cellulose and only takes nutrients in solution.16 The hyphae and its vegetable substrate fill the majority of leafcutter nest chambers with what is called a fungus garden, a mass of featherweight, fissured gray matter that looks like a human brain and can reach a similar volume. A garden is given its cerebral shape by workers that continually add fresh leaf matter to its top and sides while dismantling and disposing of the bottom, older half.
Leafcutters grow and harvest their fungi using farming techniques no less complex than ours. Along with the fungus-growing termites I saw attacked by driver ants in Africa, they are among the only animals besides humans that can be considered agricultural.17 The invention of agriculture has enabled societies of humans and leafcutters, which were farming long before people, to support massive populations. These massive populations then give rise to massive structures—for shelter, food rearing, and so on. The most enormous nest I have come across was in the dense forests of the Kaw Mountains of French Guiana, with soil mounds rising chest high over an area 14 meters wide and about 160 square meters total. Trails initiating from the nest’s far corners led into the forest in each cardinal direction. Scaled to human size, the space occupied by such a nest would exceed the dimensions of the Empire State Building. Such a colony might easily contain several million workers.
A small fungus garden of Atta colombica in Paraguay: the white fuzz is fungus, which is tended by the workers. Hiding in the nooks are winged queens.
A leafcutter colony’s chambers and tunnels can require the excavation of 40 tons of soil, as they must house not only queen, brood, and workers, which even in the millions occupy only a tiny fraction of the space, but also fungus gardens in the hundreds or even thousands. The garden chambers are distributed along tunnels in a pattern that can resemble grapes on a stem—with the garden-containing “grapes” the size of soccer balls and “stems” as wide as a child’s arm, which must give the jostling leaf-bearing ants elbow room aplenty.18
Cramming gardens and millions of ants together underground produces air pollution, and too much heat or too little oxygen will slow garden growth. In the Kaw Mountains, I took time off from swatting mosquitoes to peer into a nest entrance that thrust from the earth like a volcanic cone. The metabolic heat of fungus and