The perception of airborne infrasound (sounds below 20 Hz, inaudible to humans except at very high levels) has been documented in a handful of mammals and birds. While animals that produce vocalizations with infrasonic components (e.g. elephants) present conspicuous examples of potential use of infrasound in the context of communication, the extent to which airborne infrasound perception exists among terrestrial animals is unclear. Given that most infrasound in the environment arises from geophysical sources, many of which could be ecologically relevant, communication might not be the only use of infrasound by animals. Therefore, infrasound perception could be more common than currently realized. At least three bird species, each of which do not communicate using infrasound, are capable of detecting infrasound, but the associated auditory mechanisms are not well understood. Here we combine an evaluation of hearing measurements with anatomical observations to propose and evaluate hypotheses supporting avian infrasound detection. Environmental infrasound is mixed with non‐acoustic pressure fluctuations that also occur at infrasonic frequencies. The ear can detect such non‐acoustic pressure perturbations and therefore, distinguishing responses to infrasound from responses to non‐acoustic perturbations presents a great challenge. Our review shows that infrasound could stimulate the ear through the middle ear (tympanic) route and by extratympanic routes bypassing the middle ear. While vibration velocities of the middle ear decline towards infrasonic frequencies, whole‐body vibrations – which are normally much lower amplitude than that those of the middle ear in the ‘audible’ range (i.e. >20 Hz) – do not exhibit a similar decline and therefore may reach vibration magnitudes comparable to the middle ear at infrasonic frequencies. Low stiffness in the middle and inner ear is expected to aid infrasound transmission. In the middle ear, this could be achieved by large air cavities in the skull connected to the middle ear and low stiffness of middle ear structures; in the inner ear, the stiffness of round windows and cochlear partitions are key factors. Within the inner ear, the sizes of the helicotrema and cochlear aqueduct are expected to play important roles in shunting low‐frequency vibrations away from low‐frequency hair‐cell sensors in the cochlea. The basilar papilla, the auditory organ in birds, responds to infrasound in some species, and in pigeons, infrasonic‐sensitive neurons were traced back to the apical, abneural end of the basilar papilla. Vestibular organs and the paratympanic organ, a hair cell organ outside of the inner ear, are additional untested candidates for infrasound detection in birds. In summary, this review brings together evidence to create a hypothetical framework for infrasonic hearing mechanisms in birds and other animals.

中文翻译：

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鸟类的次声听力：听力测量和假设的结构功能关系的回顾

在少数哺乳动物和鸟类中记录了空气传播的次声（低于 20 Hz 的声音，人类听不见，除非在非常高的水平）的感知。虽然产生带有次声成分的发声的动物（例如大象）是在交流中潜在使用次声的显着例子，但陆生动物中存在的空气传播次声感知的程度尚不清楚。鉴于环境中的大多数次声来自地球物理来源，其中许多可能与生态相关，因此交流可能不是动物对次声的唯一用途。因此，次声感知可能比目前意识到的更为普遍。至少三种不使用次声进行交流的鸟类能够探测到次声，但是相关的听觉机制还不是很清楚。在这里，我们将听力测量评估与解剖观察相结合，以提出和评估支持鸟类次声检测的假设。环境次声与非声学压力波动混合在一起，这些波动也发生在次声频率下。耳朵可以检测到这种非声学压力扰动，因此，区分对次声的响应和对非声学扰动的响应是一个巨大的挑战。我们的评论表明，次声可以通过中耳（鼓室）路径和绕过中耳的鼓室外路径刺激耳朵。当中耳的振动速度向次声频率下降时，全身振动——通常比“可听”范围内（即 >20 Hz）的中耳振动幅度低得多——没有表现出类似的下降，因此可能达到与中耳在次声波下相当的振动幅度频率。预计中耳和内耳的低硬度有助于次声传播。在中耳，这可以通过与中耳相连的颅骨中的大空气腔和中耳结构的低刚度来实现；在内耳中，圆窗和耳蜗隔板的刚度是关键因素。在内耳内，螺旋体和耳蜗导水管的大小有望在将低频振动从耳蜗中的低频毛细胞传感器分流方面发挥重要作用。基底乳头，鸟类的听觉器官，某些物种对次声有反应，而在鸽子中，次声敏感神经元可追溯到基底乳头的顶端、非神经末端。前庭器官和副鼓室器官（内耳外的毛细胞器官）是其他未经测试的鸟类次声检测候选器官。总之，这篇综述汇集了证据，为鸟类和其他动物的次声听觉机制创建了一个假设框架。