Recruitment behavior, home range orientation and territoriality in harvester ants, Pogonomyrmex

1. Scouts of the harvester ant Pogonomyrmex barbatus, P. maricopa and P. rugosus which discovered a new rich foraging area recruit nestmates by laying a trail with poison gland contents from the feeding site to the nest. Laboratory experiments have shown that Pogonomyrmex workers are stimulated to follow the trail by the trail pheromone alone. 2. The biological significance of the recruitment behavior was analyzed in the mesquite-acacia desert in Arizona-New Mexico, where the three species occur sympatrically. P. maricopa recruits less efficiently to food sources than does P. barbatus and P. rugosus. Generally the recruitment activity depends on a number of parameters of the food source, such as distance to the nest, density of the seed fall and size of the grains. 3. The recruitment activity is also affected by the presence, absence or distance of hostile neighboring colonies. 4. The use of chemically and visually marked trunk trails which originate from recruitment trails, guarantees and efficient partitioning of foraging grounds. It could be demonstrated that trunk trails, used by P. barbatus and P. rugosus during foraging and homing, have the effect of avoiding aggressive confrontations between neighboring colonies of the same species. They channel the mass of foragers of hostile neighboring nests into diverging directions, before each ant pursues its individual foraging exploration. This channeling subtly partitions the foraging grounds and allows a much denser nest spacing pattern than a foraging strategy without trunk trails, such as that employed by P. maricopa. 5. The behavioral mechanisms which maintain overdispersion both within and between species of Pogonomyrmex were investigated. Aggressive confrontations at the colony level and aggressive expulsion of foundress queens from the nest territories of mature colonies play thereby a major role. Observational as well as experimental data led to the conclusion that the farther away from its nest the intruder is, the less vigorous are the aggressive confrontations with the defenders. Only when neighboring colonies are located too close together will increased aggressive interactions eventually lead to the emigration of the weaker colony. 6. P. barbatus and P. rugosus have a wide niche overlap, whereas P. maricopa seems to be more specialized in regard to food. This is consistent with the findings that interspecific territoriality between P. barbatus and P. rugosus is considerably more developed than between these species on the one side and P. maricopa on the other. 7. Although foundress queens, which venture into a territory of a conspecific mature colony are fiercely attacked, most of them are not injured, but rather dragged or carried to the territorial border and then released. 8. Nevertheless foraging areas, even of conspecific colonies, frequently overlap, but aggressive interactions there are usually less intense than at the core areas (trunk trails plus nest yards), which normally do not overlap and are vigorously defended.