Spatial movement optimization in Amazonian Eciton burchellii army ants

Abstract

Foraging army ants face a problem general to many animals—how best to confront resource depletion and environmental heterogeneity. Army ants have presumably evolved a nomadic lifestyle as a way to minimize re-exploitation of previously foraged areas. However, this solution creates a challenge for an army ant colony: foraging by this colony and others creates a shifting landscape of food resources, where colonies should theoretically avoid their own previous foraging paths as well as those of other colonies. Here, we examine how colonies exploit this resource mosaic, using some of the optimality arguments first proposed and tested by Franks and Fletcher (1983), but with much larger data sets in a new location in SW Amazonia. Our data supported Franks and Fletcher’s (1983) model for systematic avoidance of raided areas during the statary phase, as well as a hypothesis of distance optimization between successive statary bivouacs. We also test and find significant evidence that foraging raids turn in opposite directions from the previous day’s directional angles more frequently than what would be expected if turning angles were distributed at random, which acts to move a colony away from recently exploited areas. This implies that colonies follow a straighter line path during the nomadic phase as opposed to a curved one, which acts to maximize distance between statary bivouacs. In addition to intra-colony movement optimization, we examine evidence for inter-colony avoidance from more than 330 colony emigrations and suggest that colony-specific pheromones are not necessarily repulsive to other colonies. Lastly, we compare our results with those of similar studies carried out at Barro Colorado Island (BCI), Panama. Despite a higher density of army ants in the SW Amazon region, colonies spend less time emigrating than their counterparts at BCI, which suggests a higher prey density in SW Amazonia.