Electronic_supplementary_tables from Into the dark: patterns of middle ear adaptations in subterranean eulipotyphlan mammals

Evolution of the middle ear ossicles was a key innovation for mammals, enhancing the transmission of airborne sound. Radiation into various habitats from a terrestrial environment resulted in diversification of the auditory mechanisms among mammals. However, due to the paucity of phylogenetically controlled investigations, how middle ear traits have diversified with functional specialization remains unclear. In order to identify the respective patterns for various lifestyles and to gain insights into fossil forms, we employed a high-resolution tomography technique and compared the middle ear morphology of eulipotyphlan species (moles, shrews and hedgehogs), a group that has radiated into various environments, such as terrestrial, aquatic and subterranean habitats. Three-dimensional geometric morphometric analysis was conducted within a phylogenetically controlled framework. Quantitative shapes were found to strongly reflect the degree of subterranean lifestyle and weakly involve phylogeny. Our analyses demonstrate that subterranean adaptation should include a relatively shorter anterior process of the malleus, an enlarged incus, an enlarged stapes footplate and a reduction of the orbicular apophysis. These traits arguably allow improving low-frequency sound transmission at low frequencies and inhibiting the low-frequency noise which disturbs the subterranean animals to hear airborne sounds.

中耳听小骨的演化是哺乳动物的一项关键革新，可提升空气传导声波的效率。哺乳动物从陆地环境向各类生境发生演化辐射，由此促成了听觉机制的多样化分化。然而，由于受系统发育控制的相关研究较为匮乏，中耳特征如何随功能特化发生分化，目前仍不明确。为明确不同生活方式对应的演化模式并解析化石类群的特征，我们采用高分辨率断层成像技术，对比了真盲缺目（Eulipotyphla）物种的中耳形态——该类群已向陆地、水生、地下等多种生境完成演化辐射。我们在受系统发育控制的研究框架内开展了三维几何形态测量学分析，结果显示定量形态特征与物种的地下生活适应程度高度相关，而与系统发育关系的关联度较低。我们的分析表明，适应地下生活的中耳特征应包括：锤骨（malleus）前突相对缩短、砧骨（incus）体积增大、镫骨（stapes）足板扩大以及环状突（orbicular apophysis）退化。上述特征可有效提升低频声波的传导效率，并抑制干扰地下动物感知空气传导声波的低频噪声。