Supplementary Material for: Convergent anuran middle ear loss lacks a universal, adaptive explanation

Shared selection pressures often explain convergent trait loss, yet anurans (frogs and toads) have lost their middle ears at least 38 times with no obvious shared selection pressures unifying ‘earless’ taxa. Anuran tympanic middle ear loss is especially perplexing because acoustic communication is dominant within Anura and tympanic middle ears enhance airborne hearing in most tetrapods. Here we examine whether particular geographic ranges, microhabitats, activity patterns, or aspects of acoustic communication are associated with anuran tympanic middle ear loss. Although we find modest differences between the geographic ranges of eared and earless species and increased diurnality in earless species, we find no universal adaptive explanation for the many instances of anuran tympanic middle ear loss. The puzzling lack of shared selection pressures motivates discussion of alternative hypotheses, including genetic or developmental constraints, and the possibility that tympanic middle ear loss is maladaptive.

趋同性状丢失通常可通过共同选择压力予以解释，但无尾目（Anura，蛙类与蟾蜍类）的鼓膜中耳（tympanic middle ear）丢失事件至少已独立发生38次，且尚无明确的共同选择压力可将这些‘无耳’类群统一起来。无尾目鼓膜中耳的丢失尤为令人费解，因为声学通讯（acoustic communication）在无尾目中占据主导地位，且鼓膜中耳在多数四足动物（tetrapods）中均可提升空气传导听觉的效能。本研究旨在探究特定地理分布范围、微生境（microhabitats）、活动模式（activity patterns）或声学通讯特征是否与无尾目鼓膜中耳丢失存在关联。尽管我们发现有耳类与无耳类物种的地理分布范围存在细微差异，且无耳类物种的昼行性（diurnality）显著提升，但我们并未发现可普适性解释无尾目鼓膜中耳多次丢失事件的适应性机制。这一缺乏共同选择压力的难解现象，促使我们探讨各类替代假说，包括遗传或发育约束（genetic or developmental constraints），以及鼓膜中耳丢失可能属于适应不良（maladaptive）的可能性。