Crowding increases foraging efficiency in the leaf-cutting ant Atta colombica

Many animals, including humans, organize their foraging activity along well-defined trails. Because trails are cleared of obstacles, they minimize energy expenditure and allow fast travel. In social insects such as ants, trails might also promote social contacts and allow the exchange of information between workers about the characteristics of the food. When the trail traffic is heavy, however, traffic congestion occurs and the benefits of increased social contacts for the colony can be offset by a decrease of the locomotory rate of individuals. Using a small laboratory colony of the leaf-cutting ant Atta colombica cutting a mix of leaves and Parafilm, we compared how foraging changed when the width of the bridge between the nest and their foraging area changed. We found that the rate of ants crossing a 5 cm wide bridge was more than twice as great as the rate crossing a 0.5 cm bridge, but the rate of foragers returning with loads was less than half as great. Thus, with the wide bridge, the ants had about six times lower efficiency (loads returned per forager crossing the bridge). We conclude that crowding actually increased foraging efficiency, possibly because of increased communication between laden foragers returning to the nest and out-going ants.