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By Root 573 0

time for an animal to consume, and patchiness describes an uneven distribution of resources.21 Some patches are bonanzas, like a watermelon rind in one lump.22 Once the recruits arrive at such a large object, they need search but little, if at all, to harvest it completely. Other patches consist of a cluster of scattered pieces; once that type of patch is located, the ants must seek out the constituent items, such as the brood scattered within a raided ant nest. The two kinds of patches aren’t mutually exclusive. Our watermelon castoffs each represented a bonanza, but all of the pieces, scattered across the lawn, represented a patch, too. Some patches are restocked, as with the excess fruits that fall continually from a tree or are tossed daily from a dining room—these are patches that keep on giving. Ants finding a long-lasting food supply can save themselves a lot of foraging effort by returning to the site as long as visits pay off.

Shuttling individuals to resources in appropriate ways is a matter of having an appropriate transit system. For marauder ants, the grand pattern of food acquisition depends on the colony’s durable trunk trails, which are rare in swarm-raiding army ants. My time in Asia had shown me that when most of the land within range of a marauder ant colony is barren with the exception of a verdant field rich in seeds and bugs, the colony’s trunk trail will point to that field like an arrow.23 In addition to accessing patches of widely scattered food like this over weeks, branches of a trunk trail can lead to concentrated bonanzas on which the ants may feed uninterrupted for days. Because the food present in another colony’s jurisdiction is available only at the cost of war, the most stable routes also lead away from the trunk trails of other colonies. This configuration allows competing colonies to avoid each other even when their nests are close together.24

That third, moonless night in Gashaka, I set my alarm at two-hour intervals and groggily checked the dining room colony repeatedly to confirm that the ants spent until 5 A.M., well after the raid itself had ended, eating the watermelon down to its hard green casing. As a spotted eagle owl hooted in a nearby tree, I watched ants ply the same trails without pause until they devoured the feast, keeping the former raid route open, at least temporarily.

A driver ant emigration is being staged beneath this protective envelope of aggressive workers in Gabon.

Apparently, driver ants can gorge themselves at one place in much the way marauder ants do, albeit for a shorter interval. A colony Caspar studied in Kenya spent three days carving away at a colobus monkey corpse until it was reduced to fur and bone.

Driver ants also reuse abandoned trails that extend into patches rich with scattered food. I saw an example of this at the start of that third evening, when the first ants returned to the watermelon-strewn area near our kitchen. The initial sign of the impending raid was a trickle of workers crossing the lawn in a narrow file. I was puzzled, because this species should raid only in swarms, not columns. Then I realized that it was not a raid at all—instead the workers were retracing the path they had used for their raid two days earlier. The feeble vanguard turned out to be a harbinger of the swarm that arrived as a separate wave an hour later.25 Just as the chemical traces of an old route could (in theory, at least) repel future raids if the original swarm had left little to eat behind, another old route may attract raids to a locale that remains desirable over an extended period—it remains as a persistent track, or trunk trail.

While trunk trails are invaluable for accessing food patches, they have their uses no matter how food is distributed. It’s basic geometry. Marauder ant raids, for example, extend 20 meters at most from their starting point. If every raid departed from the nest, the colony would be able to hunt in a circle with only a 40-meter diameter, a region that could quickly be exhausted of food. But the trunk trails of most colonies