Electronic_supplementary_data_cytb_sequence 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（middle ear ossicles）的演化是哺乳动物的一项关键创新，可提升空气传导声音的效率。哺乳动物从陆地环境向多种生境辐射演化，导致其听觉机制呈现多样化分化。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仍不明确。为明确不同生活方式对应的演化模式，并为化石类群的研究提供参考，我们采用高分辨率断层成像技术，对比了真盲缺目（eulipotyphlan）物种——包括鼹鼠、鼩鼱与刺猬——的中耳形态特征；该类群已向陆地、水生、地下等多种环境完成辐射演化。我们在基于系统发育控制的分析框架内开展了三维几何形态测量分析。研究发现，定量形态特征能显著反映物种的地下生活适应程度，而受系统发育的影响较弱。我们的分析表明，地下适应相关的特征应包括：锤骨（malleus）前突相对较短、砧骨（incus）增大、镫骨（stapes）足板扩大，以及锤骨环状突（orbicular apophysis）退化。上述特征理论上可优化低频环境下的声音传导，并抑制干扰地下动物感知空气传导声音的低频噪声。