Adventures among Ants. Mark W. Moffett

colony size, and then looked through the remainder of the bag for any queens. My colonies each contained one queen and from 64,000 to 127,000 workers. But I also found a nest of a few thousand workers and twenty-three wingless queens that clustered together amicably. The founding of a nest by a gathering of queens is called pleometrosis. With multiple queens laying eggs, the worker population no doubt increases rapidly, perhaps giving the colony a head start in foraging as a group.¹⁹

      The diversus queens that I kept together likewise showed no aggression toward each other, though why each foundress attaches her eggs to her own body remains unclear. All the marauder colonies I dug up contained only one queen; if pleometrosis is common in this species, and in silenus, too, the number of queens must decline with time.

      In contrast to marauder ants, army ant workers cull their queens before they mate. Typically, they raise several new queens, and when half their number depart with the chosen one to form a new colony, and the old queen goes her own way with the other half, the excess queens, blockaded by the workers, are left behind to die.

      It seems the marauder ant workers likewise do the deed of disposing of excess queens, but in their case this occurs much later in the life of the colony. I learned this at the Botanic Gardens as I tallied workers who were repairing a damaged thoroughfare. At one point I noticed a group dragging a dusky object out of the nest and along their trail. Extracting it from them, I found in my hand a wingless queen with the worn mandibles and the near-black pigmentation of an aged animal. She was very much alive but had apparently outlived her usefulness to the colony and was being evicted. Twice more I saw the same event at nests sizable enough to suggest that marauder ant societies can retain more than one queen for a long time. Allowing these workers to finish their job, I watched them abandon the struggling queen at the side of their trail or in the garbage heap.

      Calling the female reproductive ant a queen is a poor metaphor because ant royalty does not lead, and unlike human monarchs, they sire their minions. Nevertheless, the two varieties of queens share a characteristic: with royalty comes favor but also great peril.

      5 group transport

      It was late in 1983. For the final leg of my doctoral fieldwork, after traveling without a break for twenty-nine months, I had ended up in the Philippines. I had just arrived at the base of Mt. Apo, on the southern island of Mindanao, at almost 3,000 meters the highest mountain in the Philippines and cloaked with forests. On exiting the bus, I found José, a self-proclaimed guide who, an hour into our walk, broke his silence to speak of the need for revolution while patting what he claimed to be a gun in his waistband.

      On my way to Mindanao, I had faced riots against the government outside the Manila airport. I now recalled a U.S. government advisory that Mindanao was the center of the Communist movement and unsafe for travelers. It occurred to me for the first time since I had left Massachusetts nearly two and a half years before that it might be pleasant to experience Christmas at home again—or even be reminded of what the holiday celebrated.

      Mist and tree ferns gave each vista of the flanks of Mt. Apo a Jurassic Park flavor. While José talked, I located my old friend the marauder ant. Among their legions, fifteen minor workers—the chief food-delivery caste—carried a centimeter-long blue sphere through dense brush. When I picked it up, more ants poured from a crack in the sphere to expose the remains of a bird embryo.

      Whether it’s political rebels or ants fetching home a bird egg, a successful social operation requires the coordination of individuals. The orchestration of a marauder ant raid is an obvious example of how organization emerges from collective masses within a superorganism. By moving in closer to watch the individuals as if they were tissues within organs, I could document equally compelling examples of social integration, involving smaller ant groups within a swarm.

      Marauder ants are successful at “marauding” in part because workers can work together to haul provisions along their superhighways—or expel a rejected queen from the nest. Group transport is the carrying, dragging, lifting, rolling, or burying of a burden by multiple individuals.¹ We’ve all seen group transport, if only when a few ants pilfer crumbs at our picnics. But the average interloper at a picnic represents only a crude example of the sophisticated group transport accomplished by the marauder.

      GROUP TRANSPORT AMONG ANIMALS

      From watching the marauder ants for months while in Asia, I had become fascinated with group transport and began to investigate which other creatures could accomplish this basic task. I discovered that group transport is as scarce in the animal kingdom as using tools or hunting in a group. Such task-oriented cooperation is particularly rare in nonhuman primates, in part due to the overwhelming drive of each individual to keep food for himself and in part because they seldom deal with large objects in nature, although captive chimpanzees will reach food by carrying a branch together to use as a ladder or by group-dragging a box.

      Even for species that cooperate to kill prey, jointly moving a cumbersome meal requires a delay in gratification that few can tolerate. Litter mates of rodents such as rats will sometimes jointly convey food they had initially fought over. Lions and also hyenas, wolves, jackals, African wild dogs, and other dog-family members typically feed in a free-for-all or along dominance-hierarchy lines, but occasionally they may jointly move meat to a shady spot or protected den. Even then, they tend to act like competitors who just happen to be pulling their dinner in the same direction.²

      Nonhuman mammal societies usually contain one or two dozen individuals, with a few dozen at most. It’s often simpler for such a small group, for example a pack of wolves, to travel to the kill site than to take a large carcass to a more desirable setting. Similarly, in the few ants with small colonies that eat big prey—such as New Guinea’s Myopopone castanea, which feeds on blubbery insect larvae—the entire society may up and move to the food after it’s been killed.³

      Outside of ants, the best examples of group transport of food are found among other arthropods. Certain spiders gang together to build a web and even capture prey and care for their young; in some species, the spiders jointly drag prey to the protected interior of their web, where all feed. Then there are mated couples that procure an item of food too massive for one of them to manage alone. Pairs of some dung beetles roll and bury a dung ball on which the female lays eggs. A male and a female carrion beetle join forces to bury the corpse of a small vertebrate such as a pigeon or squirrel, which becomes food for their larvae.

      But ants reign supreme at this form of altruism. Why is this so? For one thing, there is little antagonism between nestmates over food, reflecting how well ants work together generally. Also, as central-place foragers, ants take food back to a busy nest where much of the fare is consumed by the growing larvae in their protected nurseries.

      Beyond that, the availability of a large labor force and the use of chemical trails make it practical for large ant societies to assemble transport crews and thereafter coordinate the direction in which they move the food. At the same time, the scale of activities in these species puts a premium on workers handling heavy items efficiently. Compared to lions, who take large prey every few days, a marauder ant colony may bring down thousands of food items bigger than the workers in the course of a single day.

      Aspects of ant anatomy also simplify group transport. Their forward-directed mandibles are more effective in lifting burdens cooperatively than are the jaws and limbs of most other social animals. (Humans are a significant exception: with our upright posture and opposable thumbs, we are experts at group transport.) An ant’s center of gravity is also low relative to that of large mammals, providing easier balance in group retrieval.

      Portability is the minimum requirement for group transport of food. Ideal objects for transport, such as seeds or prey, come in solid packets just a few times heavier than their carrier—neither so small that lone individuals could carry them nor too large, soft, crumbly, or mushy to lift. It’s always possible to cut up an item that is too large, as long as the material can be carved into portions the right size for a group.⁴ Even marauders are unable to pick up rotten or soft fruit, which they ingest on the spot. (As a tool-based alternative to conveying liquidy meals, a slender New England ant sops them up with dried bits