Adventures Among Ants - Mark W. Moffett [21]
Mass foraging can also be a tool for mass transit, as with cellular slime molds. After they eat an area clean of bacteria, hundreds of thousands of amoeba-like cells join together to produce a sluglike creature that resembles a blob of petroleum jelly. This slug can journey far greater distances than a single amoeba and can pass over pockets of air between grains of soil that would stop the lone amoeba cold. As it goes, the slug sheds individual amoebas, which feed on the local bacteria.20 The slug is searching not for food, however, but for areas of low moisture and high illumination, where it casts off spores.
Another group, the “true” slime molds, grow by the expansion of one amoeba into a fan-shaped body called a plasmodium, which hunts for decaying matter. In high school, I kept an orange species that resembled a swarm raid shrunk to a few centimeters across. If there was little food, my pet crept over its Petri dish slowly but steadily. A sizable bonanza could bring it to a halt as it set about gorging itself; if a patch of food was more modest, part of the slug gathered to eat while the rest continued searching, its fanlike front reduced. A slime mold isn’t as dumb as its brainlessness suggests: one variety can find the shortest route through a maze.21 I admit, though, that a person must be very patient to find it interesting as a pet.
Some of the most army ant–like strategies are deployed by vegetarians. Workers of a few termite species spread out in a loose network while foraging, each walking ahead a centimeter or two and laying an exploratory trail before she retreats and another takes her place. The advance resembles the progression of a marauder ant raid, though it’s less methodical and more dispersive than cohesive.22 A forager who detects wood at a distance, likely by scent, will abandon its search and move straight to the food. Usually she explores the wood alone, then lays a recruitment trail back to the nest. Being defenseless and easily dehydrated, termites expire fast when lost. Staying in the columnar networks helps them find their way back home and hastens the construction of the galleries the termites require to survive on exposed ground.
Another vegetarian engages in mass hunts that have a protective as well as a nutritive function. Whereas an unaccompanied eastern tent caterpillar can easily lose its grip on a tree, several together will lay a silk mat that engages their feet and keeps them from falling. These leaf eaters then find meals in a procession, with the pioneers pushing ahead short distances before retreating, to be replaced by the ones behind.23 A group can follow an old silk trail or strike out over new terrain. A lone caterpillar finding satisfactory greenery will lay an especially attractive—perhaps chemically stronger—recruitment trail back to the silk tent housing the colony, in some cases drawing out the entire population.
This is where all other animals that search for food in groups differ from ants like the marauder: whether caterpillar or bird, bacterium or wolf, individuals are fully capable of moving away from the pack or flock and foraging without companions. And with rare exceptions, “alone” in these species really means alone, because few animals have the capacity to recruit assistants from a distance. A few birds and primates call one another to food: for example, in Africa chimpanzees draw others to bonanzas of fruit in trees by uttering loud hoots, and pied babblers lead their novice fledgling offspring to feeding spots with a “purr” sound.24 But such social actions are virtually unknown in most species, where signals such as the yelp of the coyote or the singing of whales more often function in maintaining appropriate spacing between individuals, in combat, courtship, or group bonding, or to keep pack members together when they are on the hunt, than in calling in the troops.
One rare exception is the naked mole rat,