Synchronous signalling within choruses of the same species either emerges from cooperation or competition. In our study on the katydid Mecopoda elongata, we aim to identify mechanisms driving evolution towards synchrony. The increase of signal amplitude owing to synchronous signalling and the preservation of a conspecific signal period may represent cooperative mechanisms, whereas chorus synchrony may also result from the preference of females for leading signals and the resulting competition for the leader role. We recorded the timing of signals and the resulting communal signal amplitudes in small choruses and performed female choice experiments to identify such mechanisms. Males frequently timed their signals either as leader or follower with an average time lag of about 70 ms. Females selected males in such choruses on the basis of signal order and signal duration. Two-choice experiments revealed a time lag of only 70 ms to bias mate choice in favour of the leader. Furthermore, a song model with a conspecific signal period of 2 s was more attractive than a song model with an irregular or longer and shorter than average signal period. Owing to a high degree of overlap and plasticity of signals produced in ‘four male choruses’, peak and root mean square amplitudes increased by about 7 dB relative to lone singers. Modelling active space of synchronous males and solo singing males revealed a strongly increased broadcast area of synchronous signallers, but a slightly reduced per capita mating possibility compared with lone singers. These results suggest a strong leader preference of females as the ultimate causation of inter-male competition for timing signals as leader. The emerging synchrony increases the amplitude of signals produced in a chorus and has the potential to compensate a reduction of mating advantage in a chorus. We discuss a possible fitness benefit of males gained through a beacon effect and the possibility that signalling as follower is stabilized via natural selection.
Mean time lag of males attending a chorus
The time difference of chirps produced in a small chorus consisting of 4 males. Relative time differences were calculated by taking the leader signal as reference. Data is shown in Figure 2.
Mean time lag in a chorus.xlsx
Chorus_vs_Solo Chirp period
The Chirp period (CP) of males either attending a small chorus consisting of four males and the average chirp period of the same males singing in isolation.
Signal amplitude in a Chorus
The sound recording of the central microphone in small choruses consisting of four males was evaluated using a Spike2 macro. The rms and maximum amplitude values of either solo singing males and signals produced by four males simultaneously are shown in this table. Data is shown in Fig. 4B
Signal amplitude_Chorus_vs_solo.xlsx
Correlation between mean time lag, signal amplitude and signal duration
A macro was used to evaluate the timing and signal amplitude of four males acoustically interacting in a chorus. Recording channel 2 - 5 contains the evaluation of microphone recordings of individual males. Channel 6 refers to the evaluation of the microphone positioned in the middle of the chorus. CP = chirp period. This data was used to correlate the average time difference between individual signals with the signal amplitude recorded by the central microphone (shown in figure 4C). This data was also used to calculate the correlation of the time lag of chirps produced by male individuals with the duration of their chirps (Figure 5).
DeltaT vs rmsAmplitude .xlsx
Simulation results: Active space of simulated males
A Netlogo simulation was run to simulate the broadcast area of senders by taking the hearing threshold of receivers into account (active space). The extension of the broadcast area of simultaneously active senders with the same intensity is given as the percentage of active space relative to the active space of a lone singing male. Per capita male mating possibilities were calculated by assuming three different densities of equally distributed females. Data is shown in figure 6
Simulation Active space.xlsx
Female phonotaxis in two-choice trials
Females given the possibility to approach either the left or right speaker simultaneously broadcasting different song models. Different chirp periods (CP) were presented simultaneously from different directions. Furthermore, identical song models were presented with a time lag of 70 ms and 140 ms. Data are summarized in figure 7
Phonotaxis Two Choice Trials.xlsx
Phonotaxis and female motivation: CP 1.0s vs 2.0s
Mecopoda elongata females were given the possibility to approach one of two speakers. One is broadcasting a song model with a chirp period (CP) of 1.0 s and the other a song model with a CP of 2.0 s. Data show female choices and how often females were motivated to approach either speaker. A data summary of female motivation is shown in Fig. 7B
Phonotaxis and female motivation 1s vs 2s CP.xlsx
Female motivation to perform phonotaxis in two-choice trials
Mecopoda elongata females were given the possibility to approach either of two speakers broadcasting different song models differing in chirp period (CP) or signal timing (time lag of 70 ms or 140 ms). The motivation of females to approach any speaker was evaluated and is summarized in Fig. 7B
Female motivation to perform phonotaxis.xlsx
Mate choice in four male choruses
Females were given the choice to select a male among small choruses consisting of four simultaneously singing males. Females were released in the middle of the chorus and the chirps produced by males during phonotaxis were recorded and evaluated with the help of a Spike2 macro. Signal timing and signal duration was evaluated and summarized in this table. Based on this data it was possible to figure out signal parameters of selected males. Data are shown in figure 8.
Phonotaxis in 4male choruses.xlsx
