Wood Ant Wars the Relationship Between Aggression and Predation in the Red Wood Ant (Formica Polyctena Forst.) by

In a dune area near The Hague (The Netherlands) nest populations of Formica polyctena are common. The number of nests was counted over a period of five years. The number of the inhabited nests remained almost constant in this period (ca. 23 nests per 4 ha.). Nest populations of Formica polyctena give rise to new nests by means of nest splitting. In the study area 17 viable daughter nests were established in a period of five years. This means that nest populations merged and/or died off. Ten mergers were observed in this period (all of them between mother and daughter nests), and seven populations died off. Since there was a surplus of suitable nest sites, the question was raised as to whether the disappearance of populations could be due to intraspecific competition for food or even to intraspecific aggression. To find out whether, and if so how, aggression is related to the food requirements of a population, the aggressive behaviour as well as the food supply of nest populations were investigated. In the spring, after hibernation, increasing numbers of wood ant workers swarm out from the nest into the surrounding area. Many of them move in the direction of the most important food sources of the preceding year: trees and bushes with many aphids (Fig. 4). The observations indicated that the workers can remember throughout the winter period the direction in which major food sources were located in the preceding year. Some of the workers travel farther from the nest, especially on warm days; and consequently the frequency of meetings between workers from neighbouring nests increases. Since new nests usually arise by splitting off from existing nests, neighbouring nests are generally of the same origin. Meetings between their workers can result in voluntary transport or in an aggressive encounter. The longer two nests are isolated from each other, the more the number of transported ants declines and the number of victims increases as the result of meetings. An increasing difference in odour between the populations may be responsible for this pattern. Experiments have shown that, when given a different diet, isolated populations of the same origin behaved more aggressively toward each other the longer the separation had lasted. Locally, the number of fighting ants can increase rapidly due to storage and transfer of information about the battle: ants can remember the location of the battlefield for a long time, and they can attract the attention of other workers by means of scent substances and conspicuous behaviour. As a result, a war can develop to a point at which thousands of ants are involved. The casualties (thousands per day) are dragged to the warring nests. A few days later (marked) casualties were found on the dumping ground, i.e. a place situated