Adventures Among Ants - Mark W. Moffett [44]
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 of plant material that the ants slice from nearby debris, drop on the food, and then take to their nest. Feeding from these items must be like squeezing soup from a sponge.)5
There are occasional examples among animals of altruistic transport of objects other than meals—most often other members of one’s own species. Dolphins and gray whales will hold a weakened or injured companion at the surface to breathe. Elephants will work together to lift a fallen comrade to its feet. These mammals will also crowd around to help a disabled individual walk or swim from place to place, though the stricken beast often moves in part under its own power. This is a behavior I have never seen among worker ants; even if she is one of the relatively valuable soldiers, a wounded worker is left to hobble on as best she can.
Although workers of the Arizona honeypot ant, Myrmecocystus mimicus, group-transport other workers, they do so with a less friendly intent: in this species certain individuals are repletes, which have abdomens swollen to the size of a small grape with honey that they regurgitate like living spigots to nestmates. After a battle, groups of ants from the victorious colony will drag the vanquished repletes to their nest, where, hanging from the ceilings of the chambers, they are condemned to a life of slavery.6
During nest emergencies such as floods, ant workers rescue the brood, which can’t escape danger on their own; they group together to move the cumbersome ones. Workers