Department of General Zoology, University of Duisburg-Essen, Universitaetsstraße 5, 45141 Essen, Germany.
Institute of Physiology, Faculty of Medicine, University of Duisburg-Essen, 45147 Essen, Germany.
J Exp Biol. 2021 Nov 15;224(22). doi: 10.1242/jeb.243371. Epub 2021 Nov 19.
Hearing in subterranean rodents exhibits numerous peculiarities, including low sensitivity and restriction to a narrow range of comparatively low frequencies. Past studies provided two conflicting hypotheses explaining how these derived traits evolved: structural degeneration and adaptive specialization. To further elucidate this issue, we recorded auditory brainstem responses from three species of social subterranean rodents that differ in the degree of specialization to the underground habitat: the naked mole-rat (Heterocephalus glaber) and the Mashona mole-rat (Fukomys darlingi), which represent the ancient lineage of African mole-rats (Bathyergidae), and the coruro (Spalacopus cyanus), a South American rodent (Octodontidae) that adopted a subterranean lifestyle in more recent geological time. Additionally, we measured call amplitudes of social vocalizations to study auditory vocal coupling. We found elevated auditory thresholds and severe hearing range restrictions in the African mole-rats, with hearing in naked mole-rats tending to be more sensitive than in Mashona mole-rats, in which hearing notably deteriorated with increasing age. In contrast, hearing in coruros was similar to that of epigeic rodents, with its range extending into ultrasonic frequencies. However, as in the mole-rats, the coruros' region of best hearing was located at low frequencies close to 1 kHz. We argue that the auditory sensitivity of African mole-rats, although remarkably poor, has been underestimated by recent studies, whereas data on coruros conform to previous results. Considering the available evidence, we propose to be open to both degenerative and adaptive interpretations of hearing physiology in subterranean mammals, as each may provide convincing explanations for specific auditory traits observed.
地下啮齿动物的听觉表现出许多独特的特征，包括敏感性低和限制在相对较低频率的狭窄范围内。过去的研究提出了两种相互矛盾的假说，解释了这些衍生特征是如何进化的：结构退化和适应性特化。为了进一步阐明这个问题，我们从三种社会地下啮齿动物中记录了听觉脑干反应，这些动物在对地下栖息地的特化程度上有所不同：裸鼹鼠（Heterocephalus glaber）和马绍那鼹鼠（Fukomys darlingi），它们代表了非洲鼹鼠（Bathyergidae）的古老谱系，以及南美啮齿动物（Octodontidae）的 coruro（Spalacopus cyanus），它们在更近的地质时期采用了地下生活方式。此外，我们还测量了社交发声的叫声幅度，以研究听觉发声耦合。我们发现非洲鼹鼠的听觉阈值升高，听觉范围严重受限，裸鼹鼠的听觉敏感性比马绍那鼹鼠高，而马绍那鼹鼠的听觉随着年龄的增长明显恶化。相比之下，coruro 的听觉与穴居啮齿动物相似，其听觉范围延伸到超声波频率。然而，与鼹鼠一样，coruro 的最佳听觉区域位于接近 1 kHz 的低频。我们认为，非洲鼹鼠的听觉敏感性虽然明显较差，但最近的研究低估了这一点，而 coruro 的数据符合之前的结果。考虑到现有证据，我们建议对地下哺乳动物的听觉生理学持退化和适应两种解释，因为每种解释都可能为观察到的特定听觉特征提供令人信服的解释。
