Impact of the dominant competitor on the aggressiveness of host ants toward slave-makers

Abstract Slave-making ants might exert strong pressure on their host populations by pillaging their nests and sometimes destroying the whole colonies. Consequently, host ants have evolved many adaptations reducing the probability of being attacked, resisting an assault or, as a last line of defence, minimising the losses associated with a successful slave-maker invasion. This paper presents the results of a study on a common Palearctic ant species, Formica fusca , which falls victim to slave raids organised by the congeneric F. sanguinea . Slave-making ants were placed close to the nest entrance of the host ants, and the subsequent instances of aggression were recorded. The study was carried out in the context of the influence of the dominant competitor by selecting host colonies located within and outside the territory of the wood ants ( F. rufa and F. polyctena ). This approach enabled the potential co-evolutionary interplay among three species performing different roles in mutual ecological interactions. The F. fusca ants reacted with aggression to the slave-making ants but not to the dominant competitors. Moreover, the avoidance of the aggression of host ants toward wood ants entails an environment-induced component, besides the presumably inherited one. The impact of the dominant competitor as a territory holder manifests itself in the reduced promptness of F. fusca ants to defend their colonies against slave-makers. The results suggest that this effect is mediated by the negative impact of wood ants on F. fusca colony size.