Adventures Among Ants - Mark W. Moffett [113]
Perhaps it is not the differences in colony organization but rather the seasonality of the temperate zones that is conducive to ant slavery. The production of brood in temperate-zone ants has an annual cycle, and slave raids of the Amazon ant take place during the few summer weeks when the free-living Formica nests contain workers in the coveted pupae stage of development. Earlier in the year, while the Formica colonies are still rearing their larvae, the Amazons are full- time stay-at-home loafers, their slaves tending the Amazon queen’s brood that will mature into a fresh cadre of warriors by the time they are needed, in the raiding season. Seasonal production of brood is less pronounced in the tropics. It’s also been proposed that seasonally cool temperatures might dull the ability of ants in the temperate regions to recognize the alien queens when they infiltrate their colonies, a necessary step in the evolution of many slavery species.16
I believe there is another possibility. Slavemakers show a rare forbearance in giving up the immediate gain of eating food (stolen brood) for the potentially greater long-term benefit of having slaves.17 Such delayed gratification can be especially advantageous in temperate environments due to the hardships arising from predictable changes with season, as well as more extreme and unexpected cold or warm spells.18 In essence, slavemakers such as the Amazon ant have chosen to hoard not meat but slaves, whose efforts help tide them over in lean times. Hoarding in the tropics is less often a life-and-death matter, because animals are more likely to procure a steady-enough food supply that they can eat meals when they find them or shortly thereafter.19 For Amazons, the payoff is a comfortable lifestyle in which workers avoid work outside of the raiding season, leaving their slaves to toil, come rain or shine.
A leafcutter ant, Acromyrmex octospinosus, slicing a leaf in Guadeloupe.
14 a fungus farmer’s life
In a pasture near Botucatu, Brazil, two cows were staring at me with heat-addled eyes, when all hell broke loose. Luiz, a laborer we had hired to help us, gave a shout of pain as part of the trench he was digging in collapsed. In front of him was a gash wide enough to hold a treasure chest, from which spilled not gold but a porridge-like material. Despite Luiz’s screams, my heart started beating with the same excitement a prospector must have felt when he struck a vein of gold. I ran over and lowered myself into the trench.
We were digging for leaf-cutting ants. These fall into two genera, Acromyrmex and Atta, with a total of thirty-nine species, all from the New World.1Atta, the most impressive and ecologically important of these ants and my focus in this chapter, are most prevalent in the tropics, though Atta texana range through east Texas and west Louisiana, and Atta mexicana cross into Arizona. Their popular name derives from the medium-sized members’ habit of cutting foliage. They hold the pieces aloft like little green parasols, then stream across the ground to their immense lairs, where they use the plant material as a substrate for underground farming.
In four ten-hour days, our ten-person team—led by Virgilio Pereira da Silva of São Paulo State University—had so far only scratched the surface of a labyrinth of chambers that seemed to go on forever. It was thrilling to think we might finally be reaching the heart of the nest. I now stood in one of two 7-meter-long corridors we had dug, both deeper than we were tall. Everywhere along their walls were bisected galleries. A leafcutter nest can extend 7 meters into the earth and contain nearly eight thousand chambers. The biggest hold the ant’s trash, buried—with greater thoroughness than humans use in handling nuclear waste—as deep as the ants can go, sometimes as far as the water table. In this colony the largest cavities were still below us. But we were getting close. The gash Luiz had just opened revealed several kilograms of this refuse. Passing my hand through