Adventures Among Ants - Mark W. Moffett [119]
System maintenance, including trail-clearing operations, is as expansive as trail building. Removing a kilogram of debris from a trail takes 3,359 ant-hours of labor, equivalent to the energy content of four Snickers bars. For an ant, that’s a lot of kilojoules. Trail-clearing workers, most of which are larger than leaf carriers but smaller than soldiers, are present in sufficient numbers that obstacles such as litter—or tents—are quickly removed. They haul off small objects and gnaw larger ones while smoothing and widening the trail’s surface, until any traffic problem is alleviated. Minor workers have a separate role in trail maintenance: they loop back and forth under the feet of the larger, leaf-bearing ants to reinforce the trail as its pheromone markers dissipate.37 This is an especially important task when a section of trail is damaged by a falling branch or a passing animal or washed away in a storm; until the chemical signals are reinstated, commerce halts.
When foragers depart from the trails, they prefer to search on plant limbs, including fallen branches, rather than on the ground surface—which makes sense, given that the leaves the ants seek grow on twigs. Incorporated into trails, roots and branches serve the ants well: clean and smooth, they are maintenance free and suited for speed. By following them rather than moving along the forest floor, the multitudes of ants reduce their transit time in total by many thousands of hours over the course of a day.38
Some leafcutter trails are so well etched in the earth that I have gotten lost while hiking in the South American tropics when I mistook an abandoned ant roadway for a tidy human path. A well-built trail increases ant walking speed four- to tenfold.39 With scores of ants waving leaf banners the breadth of a thumbnail, their caravans can seem Olympian in pace and scope. “If we magnify the scene to human scale,” writes Edward O. Wilson, “so that an ant’s quarter-inch length grows to six feet, the forager runs along the trail for a distance of about ten miles. . . . picks up a burden of 750 pounds and speeds back toward the nest at 15 miles an hour—hence, four minute miles.”40
Yet with so few superhighways and so many ants on them, congestion still can be a problem. Unlike outbound marauder and army ants, which avoid those returning home with bulky prey by taking to the trail’s edges, forming one inbound and two outbound lanes, outbound leafcutters simply slip to the left or right of the homebound leaf carriers with their slim loads. Though full-stop head-on collisions are rare, each slight run-in jogs a leaf carrier off her path. Paradoxically, the best traffic flow occurs when outbound and returning workers pass one another in equal numbers, maximizing this interference, which spreads all the ants apart across the trail, forming no lanes at all.41 If this scattering doesn’t occur, the carriers end up too close together and the leaves bump together, impeding each ant’s progress. The small but frequent diversions therefore result in the fastest foliage retrieval overall, even though the ants are slowed individually.42
CACHING AND LETTING GO
The two that watched the garden . . . did not notice the ants who were robbing them . . . climbing the trees to cut the flowers, and gathering them from the ground at the foot of the trees. . . . Thus the ants carried, between their teeth, the flowers which they took down . . . [and] quickly they filled the four gourds with flowers.43
This passage from the Mayan creation myth Popol Vuh describes leafcutters stealing flowers from under the noses