Adventures Among Ants - Mark W. Moffett [128]
The most noxious disease of the garden fungus is caused by another fungus, Escovopsis, whose web of filaments can slow growth or even overwhelm garden and nest. The garden fungus synthesizes an antibiotic against Escovopsis, but this chemical is of limited effectiveness. And so at least some leafcutter ants grow a filamentous bacterium on their bodies that they spread over the gardens, which in turn synthesizes a more potent fungicide that targets Escovopsis. A relative of the Streptomyces that we use to create most of the antibiotics in our own pharmacology, the ant-grown bacterium may also sterilize the harmful spores the workers gather in their oral pocket.18 Genetic analysis reveals that the relation between the bacterium and the ant has extended back as long as the ants have cultivated their fungi.19 The bacteria, like the garden fungus, has become a component of the ant superorganism.
Once in western Colombia, while waiting for the chief of an Embera Choco Indian tribe to give me permission to stay in their village, I spent an afternoon next to the mound of an Atta colombica colony. After two days getting to the village in a dugout canoe, I was glad to idly watch a stream of ant workers stagger out of the nest bearing bits of defunct fungus garden. I surmised that it’s one thing to keep gardens tidy and disease free; it’s quite another to handle the resulting trash. Most of the mid-sized leafcutter ants collect foliage and maintain trails; a few unfortunates are relegated to sanitation.
Not surprisingly, the garbage-transport and garbage-processor ants can die from handling hazardous waste.20 Why take such lethal jobs? Perhaps contact with unhealthy or old gardens forces young workers onto this career path. To add insult to injury, the janitorial staff is treated as untouchables by the other ants, who evade infection by dropping off refuse at safely located waste-transfer centers and staying away from the trash heaps themselves, and by attacking any of the sanitation squad who approach them or their gardens. With no access to plant sap or fresh fungus meals, the waste-management ants that take the trash from the transfer centers onward to the midden must either scavenge from the offal or starve. Working herself into an early, diseased grave, a sanitation worker cannot change careers to become a forager (nor can a forager take the suicidal move of employment on the dump).
Such a permanent segregation is coldly logical. With waste management, in ants as in humans, it’s important to separate the general population from its unhealthy by-products. This has often meant that human populations locate dumps in poor neighborhoods and arrange for socially ostracized laborers to deal with trash, such as the untouchables in India and the buraku in Japan. Among Americans today, sanitation workers have three times the on-the-job death rate of police and firefighters.21 In leafcutter ants, the clean-up crews are an expendable part of this process.
Newly commissioned garbage collectors of Atta colombica first take on the job of shuttling refuse to the dump. When they grow older, they settle down on the exposed midden heap, where they tear up the defunct mulch. Because this species has aboveground waste disposal, contaminants can build up near the nest. Whenever possible, the midden ants take their refuse to an overhang, where it tumbles away, and the wind and rain carry it farther. I have also seen the ants heave trash down embankments into a river—a common, if ill-advised, strategy for people as well.
In species of Atta that store trash belowground, different individuals concentrate on removing the dead, hauling fresh or old waste, and rearranging all this trash.22 By fragmenting and mixing the garbage, these last workers, along with the roaches and other insects