Behavioural contexts of sound production in pachycondyla ants (Formicidae: Ponerinae)

The friction of differentiated body regions, by rubbing a file with a scraper to emit sounds, named stridulation, is a widespread way of making acoustic signals in Insects. Stridulations are of common occurrence in a great number of ant species and have been observed in many behavioural contexts that could be extremely variable according to the species and the social organization of the colonies. Here we studied behavioural functions of the stridulations emitted in the Neotropical Pachycondyla apicalis species complex, which has been recently shown to exhibit striking temporal and frequency specificities between different cryptic species. First we examined the occurrence of stridulations in several behavioural contexts such as daily activity inside colony, nest disturbance, nest emigration, presence of potential predators and encounters between sexual individuals. Additionally, we considered worker responsiveness to homospecific and heterospecific stridulations in a playback experiment in order to understand if the differences in the signals produced by sympatric species could have evolved in response to competition between them. Our results showed that stridulations were produced in situations of nest disturbance and presence of predators. No acoustic signal was emitted during mating recognition or copulatory behaviour. However, freshly inseminated females stridulated when further males solicited mating. Finally, the playback test of homospecific and heterospecific stridulations did not trigger any behavioural responses in these ants. Thus our study emphasizes the context-dependent meaning of stridulations. Further play-back experiments considering the social context in which the acoustic signals are present are needed for a better understanding of the whole channel of acoustic communication in this group of closely-related cryptic ants.