Receiver discriminability drives the evolution of complex sexual signals by sexual selection

A hallmark of sexual selection by mate choice is the evolution of exaggerated traits, such as longer tails in birds and more acoustic components in the calls of birds and frogs. Trait elaboration can be opposed by costs such as increased metabolism and greater predation risk, but cognitive processes of the receiver can also put a brake on trait elaboration. For example, according to Weber's Law traits of a fixed absolute difference will be more difficult to discriminate as the absolute magnitude increases. Here we show that in the Emei music frog (Babina daunchina) increases in the fundamental frequency between successive notes in the male advertisement call, which increases the spectral complexity of the call, facilitates the female's ability to compare the number of notes between calls. These results suggest that female's discriminability provides the impetus to switch from enhancement of signaling magnitude (i.e. adding more notes into calls) to employing a new signal feature (i.e. increasing frequency among notes) to increase complexity. We suggest that increasing the spectral complexity of notes ameliorates some of the effects of Weber's Law, and highlights how perceptual and cognitive biases of choosers can have important influences on the evolution of courtship signals.

配偶选择介导的性选择，其标志性特征为夸张性状的演化，例如鸟类更长的尾羽，以及鸟类与蛙类鸣叫声中更多的声学组分。性状的复杂化会受到各类成本的制约，例如代谢消耗提升、被捕食风险升高，而接收者的认知过程同样会抑制性状的进一步复杂化。例如，根据韦伯定律（Weber's Law），当绝对刺激量升高时，固定绝对差值的性状将更难被区分。本研究以峨眉琴蛙（Babina daunchina）为对象，发现雄性广告鸣唱中连续音节间基频的提升——这会提升鸣唱的频谱复杂度——能够增强雌性对比不同鸣唱间音节数量的能力。上述结果表明，雌性的辨别能力为鸣唱信号从强化信号强度（即增加鸣唱中的音节数量）转向采用新的信号特征（即提升音节间频率）以增加复杂度提供了驱动力。我们认为，提升音节的频谱复杂度能够缓解韦伯定律带来的部分影响，并揭示了选择者的感知与认知偏误如何对求偶信号的演化产生重要影响。