Adventures Among Ants - Mark W. Moffett [53]
The overkill population of tens of thousands within a raid, most doing nothing but walking around before leaving again, may represent a particularly huge reserve force, to be drawn on when the occasional megameal is encountered. If big ant nests are the mainstay of the driver ant diet, there’s a good chance that raids may go bust on some days.9 Yet swarm-raider army ants can prowl over 10,000 square meters in the course of a week. That’s equivalent to combing the length and the breadth of three football fields, an area that should contain plenty of ant colonies, large and small.
That night I sat under the brilliant Milky Way and made notes in my journal in front of the campfire. Finding windfall meals, I scribbled, requires that ants maintain the size of their raids even during periods when their take almost always ends up being small. What surprised me most about the driver ants was not the strength of the day’s raid but something else I noticed: throughout the raid, workers were constantly going home early and empty-handed, while equal numbers streamed out to replace them.
Theodore Schneirla, the dean of army ant research, concluded that army ants are inefficient. When a raid is at full steam, each worker’s transit between nest and raid can take an hour. Based on my records from that first raid and others I saw later on, I calculated that an aggregate of thirty hours’ work time was lost to the ant society every single second from all this walking to and fro by thousands of ant workers.
Why doesn’t each worker stay out in the raid until she has something to show for her efforts? Coming and going from the front lines, might workers spread among themselves the risks of the hunt? Or do workers tire out and plod home for food and rest as fresh troops stream from the nest to replace them? These notions were illogical. Rather than commute to the nest, workers could save an hour by doing their R&R within the raid itself. That’s where the food is, and indeed many workers within the raid do stand around. I have said they serve as guards, but they sometimes look more like office workers stealing a power nap at their desks in the middle of a grueling day.
Pausing in my writing to watch a burst of falling stars, I thought of another explanation: maybe the ants roaming the trails make contributions outside the raid, such as building arcades or guarding the route. However, there were times when all the ants ran between raid and nest without pause. I was confident that most ants returning from a raid hiked all the way home, accomplishing nothing along the way but cardio exercise.
Feeling the weight of dinner in my belly, I wondered if the ants heading back to the nest were transporting hidden booty.10 In the marauder ant, nest-bound workers often have abdomens bloated from drinking the syrup of overripe fruit.11 But this kind of “tanking up” would not account for the huge numbers of driver ants going home holding no visible reward in their jaws. Where would the food needed to fill so many bellies come from? Not fruit, ordinarily: though driver ants do eat certain native forest fruits, in general vegetable matter is a minor part of their diet. With so few ants hauling prey corpses, the only way the homebound ants could have full bellies would be if most of the prey were being consumed on the spot. Though raiding workers do lap tasty juices off worm