How site fidelity leads to individual differences in the foraging activity of harvester ants

Abstract

We examined how differences in activity among individual foragers of the red harvester ant, Pogonomyrmex barbatus, could arise from site fidelity. Using observations of individually marked foragers, we found that each day most foragers made a few foraging trips, whereas only a few foragers made many trips. To determine whether only particular individuals are capable of high foraging activity, we removed the foragers that made the most foraging trips on 1 day and examined the frequency distribution of foraging the subsequent day. The most active foragers were replaced by other individuals. We then examined site fidelity of foragers. Though foraging trails extend up to 20 m from the nest, observations of marked individuals showed that on successive trips, a forager returns to sites within about 0.5 m. Foraging trip duration depended on search time and not on the distance from the nest of the final destination. Thus, the more food available, the shorter the search time and the shorter the trip. Because foragers return to the same site over and over within a day, a forager making many short trips to a high-quality patch can make more foraging trips per day. Thus, variation in patch quality, rather than individual variation in foraging ability, could produce the observed distribution of trip number. These results show that regulation of foraging in harvester ants does not require any individuals to show others a particular location with abundant food. Instead, a decentralized system of interactions tunes the numbers foraging to current food availability.