Foraging wood-ants Formica aquilonia yarrow (hymenoptera: formicidae) tend to adopt the ideal free distribution

Abstract

Three colonies of red wood-ants were established in the laboratory; each colony was provided with two feeding sites, each one metre from the nest. An aqueous solution of sucrose (7% w/v) was pumped at a defined rate to each site, where it dropped on filter paper for collection by foraging ants. The distribution of foragers between sites was compared daily with the distribution of delivery rates of sucrose between sites. Ratios of delivery rates between sites were 1:1, 2:1, or 3:1; each distribution was maintained for eight days, and in the 2:1 and 3:1 conditions the delivery rates were exchanged between sites after eight days so that the richer of a pair of sites was impoverished and the poorer was enriched. Foragers came to approximate the ideal free distribution within two or three days of an exchange of delivery rates; their distribution then remained stable until the next exchange of rates. The adoption of the ideal free distribution suggests that individual ants used movement rules which maintained the maximum possible rate of feeding: this behaviour would ensure that the foraging population as a whole maintained the fastest rate of intake of food possible from a given environment.