Competition and cooperation in a chorusing insect

One of the most fascinating phenomena in insect communication is the synchrony of signals in a group of individuals of a chorus or a flashing firefly aggregation. Although much research in the past was concerned with the elucidation of those proximate mechanisms leading to synchronous signalling, so far only little is known about cooperation among signallers in such aggregations. This neglects the possibility that inter-male cooperation may have significantly contributed to the evolution of synchrony. This project is dedicated to the investigation of competitive, as well as cooperative interactions in the synchronizing bushcricket Mecopoda elongata. Males of this insect species attract females with repetitive acoustic displays (chirps), which are highly synchronized among individuals and thus exhibit a high degree of signal overlap in a chorus situation. Synchrony in this species is not perfect because some males initiate their chirps first (leaders) within acoustic interactions. Chorus synchrony in M. elongata is thought to have evolved due to female preference for the leader, and the resulting competition to establish the leader role among males in a chorus. This competitive explanation of the evolution of chorus synchrony, however, does not explain why males lagging behind the leader (followers) continue signalling despite their relative unattractive role and what prevents males from developing ever faster solo chirp rates. Therefore, hypotheses will be tested addressing possible compensatory behaviours of followers, like a strategic spacing in relation to leaders, or chirp plasticity. In addition, I address two hypotheses for cooperative interactions among males: (1) Chorus synchrony may result in a mutual benefit for males due to a "beacon effect" that may lead to a higher number of attracted females calculated on a per capita basis. (2) Chorus synchrony may help to preserve a species-specific temporal song pattern that would be otherwise difficult to maintain in a noisy habitat such as the nocturnal tropical rainforest. As long as the costs associated with a seemingly unattractive follower role are lower than the expected benefits arising from a synchronous display of advertisement signals, synchrony may be the result of inter-male cooperation rather than inter-male competition.

One of the most fascinating phenomena in insect communication is the amazing synchrony of signals displayed in a group of individuals of the same species (e.g. insect chorus or a synchronously flashing firefly aggregation). Although already much is known about those mechanisms leading to synchronous signalling, so far only little is known about the contribution of inter-male competition and cooperation to the evolution of chorus synchrony. This project investigated female mate choice in the synchronizing bushcricket Mecopoda elongata to reveal those selective forces driving evolution towards synchronous signal displays.Males of this insect species attract distant females with the help of periodic acoustic signals (chirps), whereby certain males (leaders) consistently initiate their chirps first within acoustic interactions with other males. Choice experiments have shown that females prefer periodic signals over arrhythmic ones and leading signals over follower signals. Such female preferences forces males in a chorus to compete for leader signals whereby synchrony emerges as a by-product. An obvious disadvantage of becoming follower may be partially compensated by an elevated peak sound intensity arising from synchronized signals, which is known as beacon effect. Indeed, acoustic interaction of four males in a chorus caused a strong beacon effect which makes it is likely that females detect a chorus at great distance compared to solo singing males. A major reason for an elevated sound level in a chorus is a high degree of signal overlap, which is facilitated by a remarkable signal plasticity of followers.This kind of cooperation among males may not be the only mechanism that stabilises the follower role in a chorus. A still unknown parasitoid fly species homes in on M. elongata males caught in the field. These tachinid flies possess a hearing organ on the basis of the first pair of legs, which is famous of its extraordinary directionality and enables the localisation of hosts by eavesdropping calling songs of insects. A related fly species infesting field crickets showed a clear preference for the leader of identical signals that were broadcast from different directions. Therefore, it is possible that males lower their risk of becoming host of this parasitoid by signalling as follower.A further topic of this research project was dedicated to the investigation of acoustic communication in the natural habitat of Mecopoda where background noise is extremely high. Behaviour experiments demonstrated a rather high tolerance against nocturnal rainforest noise as well as against calling songs of a congeneric trilling species. This allows males the establishment of synchronous choruses under noisy conditions. When the calling song of a trilling Mecopoda species was used as masker synchronous entrainment mainly depends on a two kHz frequency component of the conspecific signal. In a neurophysiological study we revealed that songs of the trilling species improved the detection of this species-specific signal component in a fascinating manner.