Data supporting Lung-to-ear sound transmission does not improve directional hearing in green treefrogs (Hyla cinerea)

Amphibians are unique among extant vertebrates in having middle ear cavities that are internally coupled to each other and to the lungs. In frogs, the lung-to-ear sound transmission pathway can influence the tympanum’s inherent directionality, but what role such effects might play in directional hearing remain unclear. In this study of the American green treefrog (Hyla cinerea), we tested the hypothesis that the lung-to-ear sound transmission pathway functions to improve directional hearing, particularly in the context of intraspecific sexual communication. Using laser vibrometry, we measured the tympanum’s vibration amplitude in females in response to a frequency modulated sweep presented from 12 sound incidence angles in azimuth. Tympanum directionality was determined across three states of lung inflation (inflated, deflated, reinflated) both for a single tympanum in the form of the vibration amplitude difference (VAD) and for binaural comparisons in the form of the interaural vibration amplitude difference (IVAD). The state of lung inflation had negligible effects (typically less than 0.5 dB) on both VADs and IVADs at frequencies emphasized in the advertisement calls produced by conspecific males (834 Hz and 2730 Hz). Directionality at the peak resonance frequency of the lungs (1558 Hz) was improved by ≅ 3 dB for a single tympanum when the lungs were inflated versus deflated, but IVADs were not impacted by the state of lung inflation. Based on these results, we reject the hypothesis that the lung-to-ear sound transmission pathway functions to improve directional hearing in frogs.

两栖类在现存脊椎动物中独具一格，其拥有的中耳腔彼此内部连通，且与肺部相连。蛙类的肺-耳传声通路可影响鼓膜(tympanum)的固有方向性，但此类效应在定向听觉中可能发挥的作用仍不明确。本研究以美洲绿树蛙（Hyla cinerea）为实验对象，验证了“肺-耳传声通路可提升定向听觉能力，尤其在种内性通讯场景中”这一假说。研究采用激光测振法(laser vibrometry)，针对雌性个体，测量其鼓膜在接收来自12个不同方位角的声源所发出的调频扫频信号时的振动幅度。我们分别针对单只鼓膜场景以振动幅度差(VAD)为指标、针对双耳对比场景以耳间振动幅度差(IVAD)为指标，在三种肺充气状态（充气、放气、复充气）下测定了鼓膜的方向性。在同种雄性广告鸣叫所侧重的频率（834 Hz与2730 Hz）下，肺充气状态对VAD与IVAD均仅产生可忽略的影响（通常小于0.5 dB）。在肺部的共振峰值频率（1558 Hz）处，当肺部处于充气状态而非放气状态时，单只鼓膜的方向性提升了约3 dB，但IVAD并未受肺充气状态的影响。基于上述实验结果，我们拒绝“肺-耳传声通路可提升蛙类定向听觉能力”这一假说。