Adventures Among Ants - Mark W. Moffett [18]
HOW RAIDS BEGIN
The only time marauder ants are motivated to leave their manicured avenues and raiding paths to strike out as independent individuals, rather than in a coordinated raid, is in the face of disaster. Whenever I trod on a trail and mangled a bunch of ants, both the workers I panicked and their dead and damaged comrades released pheromones causing widespread alarm. The agitated survivors, whom I call “patrollers,” rushed about in a frenzy, dispersing up to a third of a meter from their trunk trail. Each appeared to take her own path away from the trail rather than tracking those around her. The patrollers seemed to be in a frantic search for the source of the problem and would give my leg a serious chew if I didn’t notice them in time.
Self-portrait, after stepping on a marauder ant trail near Malacca, Malaysia.
Unless I bothered them further, all the patrollers would make their own way back to the trail within fifteen minutes. However, after stepping on marauder ant trails hundreds of times, mostly by painful accident, I noticed that on occasion a weak column would emerge from the bedlam and remain active much longer, advancing away from the trunk trail and branching here and there. Supplied with more ants pouring off the trunk trail, a minority of these columns would expand gradually into a wide, fan-shaped swarm raid that often reached 2 to 3 meters across—the largest I measured was 5 meters—and contained troops that pressed forward in concert. At this point, the ants no longer seemed to be looking for me but were again expanding out in a regimented hunt for food. So what began as a response to a footstep or possibly to a tree branch that had crashed onto a trail had transformed into mass foraging in epic proportions. Indeed, the more food the workers came upon in their journey, the more epic the raiding response seemed to become.
This was different from army ant behavior. Marauders raid both in columns and in swarms, with an occasional column expanding into a swarm raid. Army ants typically raid either in columns or in swarms, but not both. Most army ant species are column raiders, whose raids stay in narrow columns from start to finish, whereas swarm-raiding army ants spill directly from their nests in a broad swath, and the raid continues as a swarm throughout.11
What triggers the marauder ants to launch a raid? Though there is no scout to shepherd a raiding party toward any particular meal, I noticed that when a patrolling worker fell by chance upon some morsel, nearby ants converged on the site immediately—suggesting that the worker who made the discovery had released a pheromone—and with their arrival a new pathway soon formed. This episode of conventional recruitment to something tasty could escalate into a raid when an excess of ants coming to the meal continued to advance in a column beyond it, a response known as recruitment overrun.
Was food necessary to the process? Determined to get closer to the truth about how raids develop, I set up camp one weekend, coming as close as anyone has to roughing it in the Singapore Botanic Gardens. A tent would have called attention to myself, so I didn’t bring one. In any case, my intent was not to sleep, or even to move from one spot. All I needed was a camp chair and a stockpile of grub—Grainut cereal, fruit and cheese, and jerky. Stationing myself just far enough away from a 50-meter trunk trail so as not to disturb the action, I watched a 2.7-meter-long segment of the route for fifty hours straight. The midday sun left me roasted. During the second night, a storm waterlogged my notes. But it was worth it. Twice during that period I saw a raid start spontaneously, with a column of marauders streaming out from the trunk-trail throughway without food or provocation. If that had been typical for the