To compete or not to compete: bushcricket song plasticity reveals male body condition and rival distance

Males of several animals, including insects, use acoustic signals to attract a sexually receptive conspecific partner. In the orthopteran chorusing genus Poecilimon (Tettigoniidae), male signalling as well as female preference can be related to male body condition and to the social environment. Song is thought to be an honest signal of male quality, and song characteristics are therefore often important for sexual and social selection. At the same time, signal expression is plastic and this plasticity depends on the quality of the individual signaller, the acoustic components preferred by females and rivals' body condition and proximity. Using the bushcricket species Poecilimon ampliatus as a model, we investigated how both internal (body condition) and external (level of competition) factors affected the expression of temporal song characteristics. We show that both factors significantly affected acoustic signalling activity: when competing against light rivals, heavy males adjusted the characteristics of their songs to different social conditions. However, light males competing against a heavy rival showed less plasticity in their acoustic signals across social conditions. During the most escalated competition, heavier males increased their acoustic signal investment up to the maximum level, signalling with longer verses and higher duty cycles, in comparison to all other treatments. Body condition and the social environment affected male acoustic signal activity, which suggests that these factors mediate the allocation of resources for signalling and different strategies adopted in competition. The adaptive plasticity of acoustic signals in this species raises new questions about the potential role that this process could play in natural choruses, where more than two competitors are signalling simultaneously.