The defensive behaviour of two Formica slave-ant species: coevolutive implications with their parasite P. rufescens (Hymenoptera, Formicidae)

The cooperative defence in ant societies is a suitable system that can keep the integrity of the colonies from threats represented by competitors, predators and parasites. This is due to the presence of a well-organized alarm communication system that induces defensive behaviours. Alarm reaction of Formica cunicularia and F. rufibarbis ant species was investigated by means of laboratory tests presenting different kinds of threatening stimuli (ants from alien colonies, homospecific and heterospecific) to colony fragments. Results show that both species react to a potential threat to the colony with an aggressive alarm instead of a panic alarm, characterized by a prompt exit of workers from the nest, attraction towards the stimulus and aggressive behaviours. Moreover they seem to feature the phenomenon of the ‘Enemy Specification’ towards their obligate parasite Polyergus rufescens. This could be considered as a result of a reciprocal adaptation between host and parasite. Furthermore, the comparison between the alarm reaction of the two studied species shows that F. rufibarbis seems to be more aggressive than F. cunicularia towards the parasite; this result could add a further clarification about the preference of the parasite towards F. cunicularia as host species.