Adventures Among Ants - Mark W. Moffett [137]
Even more unexpected, David and his colleagues discovered that the home-grown Argentine ants belong to a rich and sustainable community of ant species, among them relatives of California’s besieged harvester ants. This isn’t to say their relationships are harmonious. Just as in California, the Argentine ants wage territorial battles with other ant species that they then raid for their favorite snack, ant brood.10 But in Argentina the resident ants put up a much better fight, so all the species persist.
Indeed, the Paraná River drainage in this part of Argentina is an exceptionally cutthroat environment for ants, forcing the indigenous species to hone their battle skills. The drainage is the original home of not only the sugar ant but also other species that are invading vast swaths of the world: Pheidole obscurithorax, Pseudomyrmex gracilis, Paratrechina fulva, Solenopsis richteri, Solenopsis invicta, and Wasmannia auropunctata.11 As one friend put it, any ant colony exported from this region is like a World Cup–winning soccer team sent to suburban Ohio to compete in the junior-varsity soccer league.12
What has turned the sugar ant and these other species into unrivaled warriors? All of them harvest a broad range of foods through mass recruitment that brings great numbers of workers to a feeding site, creating intense competition. With many ant species, however, aggression between colonies declines after the combatants tussle, establish territorial borders, and then back off. Much as humans do in similar situations, the ants adjust to the other’s presence in a kind of stalemate, even going so far as to establish a strip of land in limbo between disputed areas. Though the two sides continue occasionally to test each other, mortality drops—a behavior known as the dear enemy phenomenon, observed in ant species such as the weaver ant and in vertebrates from frogs to birds.13
Argentina’s shifting floodplains allow for no such stalemate. Frequent rising waters repeatedly force colonies to high ground—even up trees—and into whatever temporary living space is available. Each time the floods drain away, the ants descend to their former homeland, where they must struggle to reestablish their territories from scratch. This training regime of agile movement and repeated and relentless marathon combat has turned the ants into expert globetrotters and efficient, cold-blooded killers. Shifting from place to place at the slightest opportunity, the Argentine ants and their floodplain adversaries move swiftly into any available living quarters and thus are able to hitchhike on ships bound for far corners of the world. Once a ship docks and the ants disembark, their aptitude for resettlement and conquest gives them control over the new landscape.
JUMPING AND BUDDING
A mastery of leapfrogging to far locations is called jump dispersal, and it is a highly developed skill among tramp species of ants. Before humans introduced oxcarts, boats, cars, and planes, the tramps had the potential to extend their ranges only when winged queens were blown by storms or colonies caught a ride on water-borne detritus. These stressful events rarely took the ants very far and had a low probability of success. The castaways often expired in transit.
A few days after our arrival, David took us to the murky Paraná. The river was so broad that the opposite bank was a barely visible green line, with masses of vegetation drifting on the currents and container ships passing in the middle distance. To the roar of howler monkeys in the trees, David led us on foot to the river bank along a route made treacherous by haphazard piles of massive driftwood and other dreck. I passed the toothy skull of a piranha being picked clean by Argentine ants. With the abundance of such resources at the water’s edge, it’s no wonder the ants often end up on flotsam. That must have been how they were