Forager activation and food availability in harvester ants

We investigated how foragers are activated in colonies of the red harvester ant, Pogonomyrmex barbatus. Each day, a harvester ant forager makes many trips out of the nest to gather seeds and bring them back to the nest. The rate at which foragers return to the nest is linked to food availability: when food is easy to find, foragers return more quickly. We examined how the return of successful and unsuccessful foragers influences the rate at which inactive foragers are stimulated to leave the nest on subsequent trips. In field experiments, successful or unsuccessful returning foragers were removed before they reached the nest. Foraging activity in response to removals was compared to that in computer simulations modelling the effect of returning foragers on departure rate. Field experiments showed that forager departure rates were not affected by the return of unsuccessful foragers, but depended strongly on the return of successful foragers. The results corresponded to the model in which inactive foragers were instantaneously stimulated to leave the nest by the return of each successful forager, and not to three other models that required inactive foragers to remember elapsed time or events. The results of field experiments were thus consistent with a simple behavioural rule that is sufficient to adjust foraging intensity to current food availability.