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end from which the ants spread out to forage.32 In canopy-foraging species, each side route lasts for a few days, during which time it grows longer because, as ants harvest foliage, a lack of space for late arrivals forces them to move out to explore other, interconnected vines and branches.33 These temporary outgrowths often cause the layout of paths for a colony to resemble a two-dimensional sketch of a shrub or tree, with its stable trunk and more ephemeral branches. This branched design is a reflection of local resources—or their absence. Every twist in the stem of a vine, for example, is a record of its responses to changes in light and support brought about by the comings and goings of the plants around it. The location and architecture of ant trails offer a similar record.

The parallels between ant highway systems and plant architecture are easiest to detect in clonal plants such as ivy that spread over surfaces with the intelligence of an ant superorganism.34 Sometimes ivy uses what is called a guerilla strategy, developing long, unbranched stems that carry it quickly (for a plant) through sectors poor in resources—such as shady spots—with a minimal investment of tissue and little searching around. Entering a sunny patch, the ivy shifts to a phalanx strategy, combing the ground by growing more branches and short stems. It can even sense nearby plants (by shifts in the wavelength of light reflected from their foliage), and so can grow away from rivals.

Many kinds of ants similarly orient their trails to the location of food while circumnavigating the competition, as we saw with marauder ants, which construct long direct trails to distant productive regions and short branching ones within food patches.35 But unlike marauder ant colonies, which employ scoutless raids that are uncertain of what lies ahead, leafcutter colonies send lone foragers to gauge conditions over several feet and lay recruitment trails to choice vegetation. Still, because the distances they search are so short, the colony as a whole—the superorganism—is effectively nearsighted and makes choices based on distorted information. A tree next to a trunk trail may be discovered no matter how far it is from the nest, whereas a sought-after specimen away from a trail will go unnoticed, no matter how close it is to home. Even if a forager were to go far enough to detect such a plant, she’d probably be unsuccessful in mounting a return expedition. As a result, leafcutters haul foliage from long distances for what may appear to us to be no good reason.

Marauder ants and certain seed-harvesting ants have trail systems that shift every few weeks, apparently to track the location of food, but leafcutters are obstinate about retaining old trunk trails and seldom start new ones. Some ground-cover plants show a similar static pattern, staying in place like sit-and-wait predators to absorb nutrients as they become available, before their more mobile plant competitors, creeping over the ground, can show up.36 Many leafcutter trunk trails may function this way, their exact location a historical artifact of their being laid out in early life while the maturing superorganism was in an adventurous state of mind, with its workers exploring farther and in novel directions. Thereafter the routes are maintained through a kind of inertia that locks a colony into certain sectors within reach of a nest, where leaf flushes can be quickly harvested as they become available. Over time, heavy use and trail maintenance crews make the routes wide, smooth, and deep, enhancing their durability. Even if such a well-made trail becomes inactive, it’s likely to remain visible for months, if not years. This physical persistence pays off by enabling a colony to revisit sites that have shown a high productivity in the long term. The greater experience of older ants has nothing to do with these choices, because the trails last far longer than the life span of the individuals. Traces of a trail are a kind of long-term memory at the superorganism level.

To keep their trunk trails operating