Adventures Among Ants - Mark W. Moffett [130]
Saplings have less chance to sprout at the dying nests of Atta species that discard their trash—including seeds—far underground. Even so, I like to think these leafcutters still assist the reproduction of some of the vegetables they consume. During tropical downpours, a lot of booty, including seeds, is knocked from the mandibles of ants returning to the nest. On the opposite side of the earth from the leafcutter ants, on the island of Borneo, I have watched Dyak tribesmen walk along their traditional trails eating forest fruits and tossing away the pits. Like unintentional Johnny Appleseeds, both ants and people may sow the next generation of their favorite trees along their preferred paths.
Whether dropped along a trail or thrown out with the garbage, only a minute fraction of the seeds moved by leafcutters take root, whatever the plant species. But the constant loss of foliage, flower, fruit, and seed is a given among trees, for which on average one offspring in a centuries-long lifetime experiences a ripe old age. For plants such as Miconia argentea, then, a loss of foliage to leafcutters may be a small price to pay for reproduction. No relationship in nature and life is without its costs, after all: human love stories take time and energy, even couples counseling. In a similar way, “ant plants” and their resident ants form alliances beneficial to both, even when a plant’s services of food and lodging come at a very high price for the plant.30 As we saw for leafcutters and their fungi, in the diplomatic machinations of nature, there is no black and white, no good and evil. There are advantages to sleeping with the enemy.
THE FIRST FUNGUS GROWERS
Although the earliest steps of fungus growing by ants are lost to history, genetic studies have revealed many specifics about the evolution of their agriculture. The first ancestor of the leaf-cutters that developed a relationship with fungus was almost certainly a slovenly species living in the mold-prone litter of the tropical rainforest some fifty million years ago. Such ant Cro-Magnons were trash collectors, and their habits are retained today by inconspicuous relatives of the leafcutters known as the lower attines.31 These New World ants—of which there are 180 species—raise fungi not on foliage but on such ordinary food sources of fungi as moist organic debris and the manure of other insects.32 Their farming methods don’t require a complex division of labor. Colonies of lower attines are small, simply organized, and so retiring that the ants curl up and play dead when threatened rather than fight; their fungi are virtually indistinguishable from wild species and can in fact go feral.
This mode of fungus farming was the ant world’s only form of agriculture for thirty million years, until one species bred a fungus with improved edible tips, domesticating it to such an extent that its prospects for growing wild became less likely, if not almost nil. From this species arose further novelties. Some ants began to rear a spherical yeast version of the same fungus; another switched to a different kind of fungus entirely. The descendants of these two groups and their unique fungi are alive today.
Cutting fresh leaves was the third, most phenomenally successful of these evolutionary experiments, exploiting the tropics’ unlimited supply of vegetable matter, a superabundant source of solid tissues generally unavailable to ants. Leafcutters arose eight million to twelve million years ago when South America was going through a dry spell.33 Grasslands were spreading and rainforests were in retreat. In the insect version of the first hominids who left the forest to walk upright in the expanding savannas of Africa, leafcutters may have gotten their start in grasslands