Supplementary Material for: Effects of Frequency on the Directional Auditory Sensitivity of Northern Saw-Whet Owls (<b><i>Aegolius acadicus</i></b>)

Many animals use sound as a medium for detecting or locating potential prey items or predation threats. Northern saw-whet owls (<i>Aegolius acadicus</i>) are particularly interesting in this regard, as they primarily rely on sound for hunting in darkness, but are also subject to predation pressure from larger raptors. We hypothesized that these opposing tasks should favor sensitivity to low-frequency sounds arriving from many locations (potential predators) and high-frequency sounds below the animal (ground-dwelling prey items). Furthermore, based on the morphology of the saw-whet owl skull and the head-related transfer functions of related species, we expected that the magnitude of changes in sensitivity across spatial locations would be greater for higher frequencies than low frequencies (i.e., more “directional” at high frequencies). We used auditory-evoked potentials to investigate the frequency-specific directional sensitivity of Northern saw-whet owls to acoustic signals. We found some support for our hypothesis, with smaller-magnitude changes in sensitivity across spatial locations at lower frequencies and larger-magnitude changes at higher frequencies. In general, owls were most sensitive to sounds originating in front of and above their heads, but at 8 kHz there was also an area of high sensitivity below the animals. Our results suggest that the directional hearing of saw-whet owls should allow for both predator and prey detection.诸多动物以声音作为媒介，用以探测或定位潜在猎物，或是辨识捕食威胁。北锯鸮（Northern saw-whet owl，学名：Aegolius acadicus）在此方面尤为值得关注：它们主要依靠声音在黑暗中捕猎，同时自身也面临着来自更大体型猛禽的捕食压力。我们提出假设：这两项对立的任务（捕猎与躲避天敌）应会促使其进化出对多方位传来的低频声波（对应潜在捕食者）以及动物头部下方高频声波（对应地面栖息猎物）的感知敏感性。此外，基于锯鸮的颅骨形态以及近缘物种的头相关传递函数（head-related transfer function），我们预期：相较于低频声波，高频声波的感知敏感性随空间位置变化的幅度会更大，即高频下的听觉定向性更强。我们采用听觉诱发电位（auditory-evoked potentials）技术，探究北锯鸮对声信号的频率特异性定向感知敏感性。我们的研究结果部分支持了上述假设：低频段下，感知敏感性随空间位置变化的幅度较小；而高频段下则变化幅度更大。总体而言，北锯鸮对来自头部前方及上方的声波感知最为敏感，但在8 kHz频段下，其对头部下方的声波也存在一处高感知敏感区域。本研究结果表明，北锯鸮的定向听觉能力可使其同时完成捕食者侦测与猎物定位两项任务。