Adventures Among Ants - Mark W. Moffett [52]
Food also got snagged whenever the trail passed under leaves. Lacking the major-caste road crews of the marauders, the drivers could get past these obstructions only after relentless buffeting from the brute force passage of workers below, except at one moist site, where the ants had eschewed their bristling covers of live guards in favor of marauder-style earthen barricades.
I was lost in thoughts of road construction when two giant forest hogs, hirsute and high as my chest, ran into the streambed ahead of me. They gave me a look of evident horror, then dashed off noisily. An hour later, an impressively virile olive-colored baboon mock-charged, fangs bared, while his females walked behind him bleating nervously.
THE DRIVER ANT RAID
Back at the field station that night I saw my first driver ant raid. It was advancing at full force outside the dining room, where the savanna grasses had been chopped to create a lawn. While the primatologists remained on the porch drinking Star Beer, Caspar, Darren, and I got on our bellies to ant-watch. The raid extended from the nest of a Dorylus rubellus colony at the base of a tree 15 meters distant. That meant the ants were just getting started: driver ants surge ahead for 80 to 120 meters before retreating. Behind a front about 7 meters wide was a prodigious swarm 15 to 30 centimeters deep. Small workers rushed through to reach the front, where they slowed down to search for prey before retreating. Larger ants were mostly located in the whirlpool of activity farther back in the raid. Great numbers of them stood in guard posture all around the reticulating columns in the raid fan, while others were busy killing, dismembering, and carrying the kills.6
A swarm raid requires substantial ant power. The regiment has to be packed tightly because there’s no telling where the next kill will show up within the “net” of ants. Within this raid there were tens of thousands of ants, with two to five workers occupying each square centimeter of ground. They were so numerous that the sound of them rummaging in the litter or dropping from twigs was like the patter of rainfall. (Indeed, they did create a shower: driver ants scrounging for prey will climb up plants, but they save time by dropping to the ground instead of climbing back down.) In two hours, however, all this ruckus bagged the colony only two thimblefuls of invertebrate meat—ant-sized plant hoppers, centipedes, worms, and spiders.
Perhaps the raid was passing through an unfruitful stretch of land. Because driver ants and other army ants have been found to travel much farther during their raids than do marauder ants, I’d expected that they wouldn’t be as sensitive as marauders to local shifts in food abundance. Their unresponsiveness to food distribution would encourage army ants to continue through a barren part of the landscape until they finally found prey, or until their distance from their nest ensured diminishing returns. Such doggedness may be important when a search party is concentrated in a raid rather than spread widely, as it is in solitary-foraging ants, especially when food is scarce and scattered, as it is during Nigeria’s droughts.
Yet this seemed to be only part of the explanation, because there was prey around that the driver ants missed. The tens of thousands of ants we watched in those two hours took in less than several nearby packs of two hundred or so Pachycondyla ants that were on their evening excursions, recruited by scouts to catch termites. Grasshoppers, crickets, and Pachycondyla broke away from the trawling rubellus raid and survived, even when we increased the ants’ chances by tossing the escapees back into the swarm.
I thought about the slowness of marauder ant raids and the meticulous care the workers put into combing the raided