Data of the tympanum and auditory sensitivity in two torrent frogs

Tympanic middle ears are important for anuran hearing on land; however, many species are observed to lose part or all of the tympanic apparatus. Some researchers have compared eared anurans' hearing sensitivity with earless species that produce calls and communicate with acoustic signals. Some anurans, however, lack both calling behaviour and well-developed tympanic membranes, and it is unclear whether they have fewer hearing perception-based selection pressures and how their hearing differs from closely related species with a developed acoustic communication system. Here, we compared the tympanic membranes of two sympatric sibling species living in noisy streams and showed that Hainan torrent frogs (Amolops hainanensis) were not only "dumb" but also had a vestigial eardrum that was smaller than that of little torrent frogs (Amolops torrentis) possessing a developed acoustic communication system. Then, we measured their auditory brainstem responses (ABRs) and compared the differences in ABR thresholds and latencies. Our results suggested that the two species exhibited significant differences in hearing sensitivity. A. hainanensis had higher thresholds and longer initial response times than A. torrentis at lower frequencies. The mechanical foundation and mechanism of such hearing changes require further study.

鼓室中耳对于无尾两栖类（anuran）的陆地听觉功能至关重要；然而，诸多物种会出现部分或完全丧失鼓膜装置的现象。已有研究者对比了具耳无尾两栖类的听觉敏感性与能够发出鸣叫声、依靠声学信号完成交流的无耳无尾类。但部分无尾两栖类既不具备鸣唱行为，也无发育完好的鼓膜，目前尚不明确它们是否承受更少的基于听觉感知的选择压力，以及其听觉系统与拥有成熟声学交流体系的近缘物种存在何种差异。本研究对比了两种栖息于嘈杂溪流中的同域姊妹物种的鼓膜结构，发现海南湍蛙（*Amolops hainanensis*）不仅‘不善鸣唱’，其退化的鼓膜也较具备成熟声学交流体系的小湍蛙（*Amolops torrentis*）更小。随后我们测定了二者的听觉脑干反应（auditory brainstem responses, ABR），并对比了ABR阈值与潜伏期的差异。研究结果显示，两个物种的听觉敏感性存在显著差异：在低频频段，海南湍蛙的听觉阈值更高，初始反应时也更长。这类听觉变化的力学基础与作用机制仍有待进一步探究。