Data from: Competition and cooperation in a synchronous bushcricket chorus

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.

同物种鸣虫合唱群内的同步信号传递，其起源可分为合作机制与竞争机制两类。本研究以长翅鸣螽（Mecopoda elongata）为实验对象，旨在探究驱动同步性演化的内在机制。同步信号传递所带来的信号幅值提升，以及同种信号周期的保留，可被视为合作性演化机制；而合唱群的同步性，也可能源于雌虫对领唱信号的偏好，进而引发雄性间争夺领唱角色的竞争。我们通过记录小型合唱群内的信号时序与对应的群体信号幅值，开展雌虫选择实验，以验证上述潜在机制。雄性个体常以领唱者或追随者的时序模式发出信号，平均时滞约为70毫秒。雌虫会根据合唱群内雄性的信号时序与信号时长选择配偶。双选实验结果显示，仅70毫秒的时滞就会使雌虫的配偶选择偏向领唱雄性。此外，信号周期为2秒的同种鸣唱模型，其吸引力显著高于信号周期不规则、过长或过短的鸣唱模型。在"四只雄性合唱"场景中，由于信号高度重叠且可塑性强，群体峰值幅值与均方根幅值相较于单个雄性鸣唱时提升了约7分贝。通过对同步鸣唱雄性与单独鸣唱雄性的有效传播空间进行建模，结果显示同步鸣唱个体的广播区域大幅扩张，但相较于单独鸣唱个体，其单只雄性的交配概率略有降低。上述结果表明，雌虫对领唱信号的强烈偏好，是雄性间争夺领唱时序地位的演化竞争的终极动因。同步鸣唱可提升合唱群整体的信号幅值，有望弥补群体内雄性个体交配优势的损失。我们还讨论了雄性个体通过信标效应（beacon effect）获得的适合度收益，以及追随者的信号模式可通过自然选择得以稳定的潜在可能。