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（距今3.5亿年），早期四足类（tetrapods）在从水生向陆生生活方式转型的过程中，面临着空气与软组织之间声阻抗失配所带来的听觉难题。因此，在三叠纪鼓膜中耳（tympanic middle ears）演化出现之前，四足类可能有长达1亿年的时间无法感知空气传播的声音。现生有尾类（urodeles）的中耳形态与早期壳椎类微螈类四足类高度相似，因此通过实验研究其听觉能力，有助于阐明早期四足类从水生听觉向空气听觉转变背后的演化与功能驱动机制。本研究结合成像技术与神经生理学测量手段，厘清了从水生幼体到陆生成体的转变如何影响蝾螈的中耳形态与感知能力。研究结果显示，在120赫兹以上的频率区间内，空气诱导的压力感知能够提升蝾螈的水下听觉敏感度；同时，陆生成体与完全水生的幼体蝾螈均能感知空气传播的声音。综合来看，上述发现表明，早期无鼓膜四足类（atympanic tetrapods）或许已经具备了空气听觉的预适应能力，且在陆地上的听觉表现优于鱼类。若经过自然选择的定向筛选，这种雏形听觉能力或许推动了鼓膜中耳的演化。