Network interactions, and the uses of downtime
Among harvester ants—the ants I know best—the important interactions are brief antennal contacts. An ant uses the rate at which it meets other ants to decide what to do. If you have ever watched ants closely, you have seen them touch antennae. When a harvester ant moves from tasks inside the nest to tasks outside, its odor changes, so an ant’s hydrocarbons identify its current task as well as its colony. To test how brief antennal contact influences ant behavior, my colleague Michael Greene and I presented ants with little glass beads coated with the odor of ants who are performing a particular task. Some of the beads smelled like patrollers, the first ants to go out of the nest each morning and travel around the colony’s foraging area. The safe return of the patrollers, at a rate of about ten ants per second, stimulates the first foragers to go out to search for food. When foragers meet beads bearing the hydrocarbons of patrollers, at the correct rate, they leave the nest. This experiment shows that an ant’s rate of brief antennal contact influences what the ant does next.
And what an ant does next may not be much at all. Contrary to another of our beloved myths about ants, told by Aesop, Homer, and the writer of Proverbs 6:6, many ants don’t work very hard. In a large harvester-ant colony, about a third of the ants at any time are hanging around doing nothing. As Mark Twain put it, this “will be a disappointment for the Sunday schools.” Because colony behavior is regulated by a network of interactions, inactivity might have its uses. Idle ants may act as a buffer to dampen the interaction rate when it gets too high. My colleagues and I have found that ants will move around to adjust their interaction rate—either they seek each other out when there are few ants, or they avoid each other when crowded. Sometimes interactions create positive feedback, as when ants go out to forage in response to interactions with foragers bringing food back to the nest. But eventually this could lead ants to search for food when there is none left. The colony may need some inert ants, unlikely to be stimulated by interactions, to buffer the network.