Data from: Better than fish on land? Hearing across metamorphosis in salamanders

Early tetrapods faced an auditory challenge from the impedance mismatch between air and tissue in the transition from aquatic to terrestrial lifestyles during the Early Carboniferous (350 Ma). Consequently, tetrapods may have been deaf to airborne sounds for up to 100 Myr until tympanic middle ears evolved during the Triassic. The middle ear morphology of recent urodeles is similar to that of early ‘lepospondyl’ microsaur tetrapods, and experimental studies on their hearing capabilities are therefore useful to understand the evolutionary and functional drivers behind the shift from aquatic to aerial hearing in early tetrapods. Here, we combine imaging techniques with neurophysiological measurements to resolve how the change from aquatic larvae to terrestrial adult affects the ear morphology and sensory capabilities of salamanders. We show that air-induced pressure detection enhances underwater hearing sensitivity of salamanders at frequencies above 120 Hz, and that both terrestrial adults and fully aquatic juvenile salamanders can detect airborne sound. Collectively, these findings suggest that early atympanic tetrapods may have been pre-equipped to aerial hearing and are able to hear airborne sound better than fish on land. When selected for, this rudimentary hearing could have led to the evolution of tympanic middle ears.

早石炭世（350 Ma）时期，早期四足类（tetrapods）在从水生向陆生生活方式过渡的过程中，面临着空气与生物组织间声阻抗失配（impedance mismatch）这一听觉难题。因此，直至三叠纪（Triassic）鼓膜型中耳（tympanic middle ear）演化出现之前，四足类可能有长达1亿年（100 Myr）的时段无法感知空气传播的声音。现存有尾两栖类（urodele）的中耳形态与早期 Lepospondyl类微螈（microsaur）四足类的中耳形态高度相似，因此针对其听觉能力开展的实验研究，可为解析早期四足类从水生听觉向空气传声听觉转变的演化与功能驱动机制提供参考。本研究结合成像技术与神经生理学测量方法，探究了从水生幼体到陆生成体的发育转变如何影响蝾螈的耳部形态与感官听觉能力。研究结果显示，在120赫兹以上的频率区间内，蝾螈对空气压力的感知可提升其水下听觉灵敏度；同时，陆生成体与完全水生的幼体蝾螈均能有效检测空气传播的声音。综合上述发现，早期无鼓膜（atympanic）四足类可能已具备感知空气传声的预适应能力，且在陆地上的听觉表现优于鱼类。若经定向选择，这种初步的听觉能力或可推动鼓膜型中耳的演化进程。