The short-term regulation of foraging in harvester ants

In the seed-eating ant Pogonomyrmex barbatus, the return of successful foragers stimulates inactive foragers to leave the nest. The rate at which successful foragers return to the nest depends on food availability; the more food available, the more quickly foragers will find it and bring it back. Field experiments examined how quickly a colony can adjust to a decline in the rate of forager return, and thus to a decline in food availability, by slowing down foraging activity. In response to a brief, 3to 5-min reduction in the forager return rate, foraging activity usually decreased within 2–3 min and then recovered within 5 min. This indicates that whether an inactive forager leaves the nest on its next trip depends on its very recent experience of the rate of forager return. On some days, colonies responded more to a change in forager return rate. The rapid colony response to fluctuations in forager return rate, enabling colonies to act as risk-averse foragers, may arise from the limited interval over which an ant can track its encounters with returning foragers.