Adventures Among Ants - Mark W. Moffett [46]
Looking closely, I could see how the heavier items would cause the ants problems. The porters tend to space themselves evenly, but the bigger a burden is, the more tightly packed they become, until they are barely far enough apart to avoid treading on each other. An earthworm, which is lighter per unit volume than the Grainut chunks and, being long and slender, offers more space for porters, could weigh more overall than a cereal chunk and still be transportable, because more ants could gather around it.
Thanks to the marauder’s skills, few foods need to be diced for carriage—this labor-intensive step can be delayed until the catch is in the protected confines of the nest. Once its flailing limbs are removed, dinner is sped away posthaste. Among marauder ants, more than 80 percent of the colony food supply is brought to the nest by groups of ants. The rest consists of small items carried by individuals.
AMATEURS AND EXPERTS
My surveys of the animal kingdom had shown that group food-carrying exists in only 40 of the 283 known genera of ants. Of the remaining species, some restrict themselves to small prey that do not require this skill (such as Acanthognathus trapjaw ants that prey on tiny springtails). Even among ants with a well-suited diet and adequate means for communicating the location of meals, there are species that fail at group transport because of poor coordination: they end up engaging in a tug-of-war, though clumsy retrieval can occur if perchance the workers pull in the same direction. Otherwise, they eat the food where it’s found or divide it into single servings and cart those away. Even that requires some cooperation, since all but one worker has to let go of each piece before it can be moved. In species adept at group transport, the workers are able to postpone dissecting and consuming the food while they coordinate as a group to move it.
In 1960, John Sudd of the University of Hull studied a British big-headed ant that performed badly in a freight-hauling group, often working at cross-purposes and dragging prey rather than carrying it. But given time, Sudd observed, the workers modified their behavior in such a way that the force they exerted on the food generally increased until they got the job done.8
It turned out the adjustments they made, such as changing the angle at which they applied force or shifting from pushing to pulling, were identical to the changes they made when hauling food alone, and these changes led, as they did for the solitary ant, to the food being moved. In other words, the British workers succeeded in group transport by behaving as if none of the others were there.
Programmed to replicate this kind of coordination, a group of simple robots was able to move a large object. European scientists even used these so-called swarm-bots to stage a mock “rescue” of a child by dragging her across the floor. Another team used tiny swarm-bots scented of cockroach to influence the roaches’ collective decision about where they would gather to hide from the light.9
In contrast to Sudd’s ants and the simple swarm-bots, marauder ants are unambiguously cooperative. Moreover, the behavior they display when moving food as a group is seen only during gang retrievals. A worker carrying a burden on her own walks on all six legs, grasping it in her jaws. If she joins a group, however, she places her forelegs on the burden, then presses her head against its surface, jaws open, but she does not use her jaws to grip unless she can hold on to a projection such as a limb. She walks with her remaining legs as she and her nestmates transport their load.
With modest colonies of a few thousand, Daceton ants in Venezuela have developed only rudimentary cooperation in the transport of food. Here two workers have pulled so persistently in conflicting directions that the moisture has been wrung out of this caterpillar. Flies sneak in to drink from the oozing meat.
What about this technique makes marauder ants excel at gang retrieval? Picture several people hefting